Heat Shock Treatment: Effect on the Quality and Shelf Life of Minimally Processed Vegetables

1. Introduction

Recently, there has been a rapidly growing demand for minimally processed vegetables and fruit. In terms of freshness and naturalness, they should be similar to intact vegetables and fruit. In the same way as whole vegetables, they respire and transpire, as they are metabolically active and undergo biochemical and physiological changes. Light processing operations, such as peeling, trimming, cutting, shredding, and slicing, are destructive to living tissue and lead to rapid wilting, discoloration, and rotting. Contamination with dangerous human pathogens poses a particular threat, explaining why research has aimed to improve the safety of cut vegetables. Other key characteristics of fresh-cut plant products include appearance, taste, texture, and nutritional value [1]. Effective storage conditions include low temperature (0°C) and high relative humidity (100%), but the temperature is usually around 10°C and the humidity is lower than optimal during transport [2]. Store shelves also maintain higher than recommended temperatures and lower humidity levels. Heat treatment (HT) has been investigated for the postharvest storage of horticultural commodities. This is an environmentally friendly solution that can be applied by immersion in hot water (HWT) or exposure to hot vapor (HVT) or hot dry air (HAT). HT of horticultural products is performed to improve their shelf life and enable better management during storage. HWT is already a well-known technology used for intact vegetables and fruit, such as tomatoes, peppers, sweet corn, melon, mango, and citrus fruit [3]. Water is the preferred medium because it is more efficient at transferring heat than air [4].

Many studies have focused on sanitary safety, as HWT at temperatures above 50°C for several minutes reduces contamination by fungal pathogens [5,6,7,8,9,10]. Heat shock (HS) reduces storage disorders, such as weight loss, softening, and discoloration, and generally extends the shelf life. Plant tissue responds to HS by altering cellular metabolism, including the protein synthesis pathway. In the cytoplasm and organelles, the synthesis and accumulation of proteins called heat shock proteins (HSPs) are increased, protecting tissues from stress [11,12].

Minimal processing induces the activity of polyamine ammonia lyase (PAL), which is the key enzyme involved in the primary phenolic pathway, leading to the synthesis and accumulation of phenolic compounds that undergo oxidation and polymerization to become brown pigments. According to Saltveit [13], Campos-Vargas et al. [14], He and Luo [15], Wulfkuehler et al. [16], and Hagele et al. [17], the redirection of protein synthesis (as a result of HS) causes HSP production instead of PAL; thus, HS prevents browning at the edges of fresh-cut products.

Changes in molecular and biochemical pathways that occur in plant tissues after HT may also increase the content of health-promoting components in these products. Some authors claim that HT increases the antioxidant capacity of plant material by promoting the synthesis of carotenoids, ascorbic acid, and phenols [18,19,20,21].

However, HS can also cause negative changes and result in the accumulation of toxic substances, such as reactive oxygen species, intensifying oxidation processes [11]. The reaction of plant material depends on the treatment, temperature, and exposure duration. Using a temperature that is too high or an exposure time that is too long leads to excess heat damage [12]. Different vegetable species modify their metabolism in different ways in response to HS. Other factors, such as growing location, soil type, production practices, ripeness at harvest time and storage conditions, also play important roles in these processes [4,12].

The aim of this review was to summarize the progress made in determining the impact of HT on the quality and shelf life of fresh-cut vegetables. As an environmentally safe method, it can be useful in reducing the quantitative and qualitative losses and extending storability and marketing. This review also discusses the effects of HT in combination with other processing methods on fresh-cut vegetables. Research covering groups of vegetables is presented below in descriptive and tabular forms.

2. Materials and Methods

This review is based on reports available in the literature, dating no earlier than 2000. The focus is on presenting the methods and their parameters (temperature and exposure time) that the authors considered to be the best among those they tested on a given vegetable crop.

3. Studies Using HS in Individual Vegetable Groups

3.1. Leafy Greens

Leafy greens constitute the largest group of minimally processed vegetables. In addition to yellowing and rotting, rapid wilting and browning of the cut surface contributes to quality loss and shortens the shelf life.

Among leafy vegetables, iceberg lettuce is a popular fresh vegetable. According to Loaiza-Valarde and Saltveit [22], HWT at 50°C for 90 s protected leaf pieces from browning and loss of green color during storage at 0°C. However, the treatment was effective when applied no later than 36 h after cutting with the maximum effect achieved approximately 6 h before cutting. The recommended application time is between 24 h before and 12 h after cutting. The treatment is effective in inhibiting phenolic compound accumulation and suppressing PAL enzyme activity. HWT at 50°C for a shorter time (60 s) significantly inhibited PAL and peroxidase (POD) induction and phenol accumulation, preventing the browning of cut iceberg lettuce during cold storage. In addition, the organoleptic estimation of treated lettuce was better than that of the control [23,24]. Pre-washing whole heads at 50°C for 60 s prior to shredding prevented browning and reduced the initial population of aerobic microorganisms [25]. HWT at 45°C for 120 s is also suitable for retaining the vitality and freshness of iceberg lettuce and to reduce PAL accumulation and microbial growth during storage at 4°C [26]. Paillart et al. [27] found that the optimal HWT temperature for inhibiting pinking in cut lettuce was 45°C for 180 s. However, PAL activity was most reduced after HWT at 47.5°C for 180 s.

HWT of fresh-cut iceberg lettuce at 47°C for 2 min, performed before irradiation at 0.5 and 1 kGy reduced the antioxidant content in plant material. HWT alone reduced browning and maintained a better overall quality compared to control samples [28]. Immersion for 1 min in a calcium lactate solution (1.5%) heated to 50°C maintained a high cell tension and reduced the shrinkage of lettuce tissue cells, preserving crispness for 12 d at 4°C [29].

Fresh-cut romaine lettuce positively responded to HWT at 50°C for 120 s, retaining a better texture and color and lower microbial contamination during storage at 5°C. It also retained a high vitamin C content and antioxidant activity [30]. PAL activity was significantly inhibited, and browning was delayed, maintaining a high quality for 4 d at 5–7°C [31].

Fresh-cut red oak leaf lettuce subjected to HWT (45°C for 120 s) before or after cutting showed a significant reduction in the PAL activity and cut edge browning. Microbial development was also inhibited during storage at 4°C for up to 12 d [16].

Rocket leaves immersed in water at 50°C for 20–40 s showed delayed yellowing when stored at 8°C. However, HWT at 50°C for 30 s was recommended as the maximum HS for rocket leaves, which does not cause damage to plant tissue. This treatment accelerated ethylene production during the first 2 d, followed by a sharp decline, and the shelf life was extended from 5 to 10 d. HWT inhibited the senescence of rocket tissue and reduced its sensitivity to ethylene in the atmosphere. HT of rocket leaves did not significantly affect the soluble solids, nitrates, phenols, or ascorbic acid (AA) content or radical scavenging activity (DPPH) [2].

Spinach leaves subjected to HWT at 37–43°C for 3.5 min showed delayed aging when stored at 23°C. This treatment protected against the excessive leakage of dissolved substances and, to some extent, against chlorophyll loss [32,33]. According to Glowacz et al. [33], immersion at 45°C for 60 s effectively maintains the carotenoid concentration and protects AA during the storage of spinach leaves at 0°C. However, this is the maximum temperature for HWT that can be applied to spinach, as more intense heating of the leaves causes tissue damage. In a previous report, Glowacz et al. [34] concluded that HWT has limited commercial potential in terms of spinach leaf quality after harvest.

Immersing endive slices in a water bath at 46°C for 120 s effectively prevents the accumulation of PAL1 mRNA. According to Salman et al. [35], this protein is responsible for the enzymatic activity and red discoloration in cut endive. Similar treatment (45°C for 120 s) also reduces PAL activity and enables endive to retain its fresh appearance for longer. In addition, it effectively reduces the microbial load during storage at 4°C [26].

Minimally processed green onions, i.e., fresh-cut pseudostems (5 mm long), responded to HWT (55°C for 2 min) by inhibiting the elongation of internal leaves (“telescoping”). In combination with a controlled atmosphere, there was a significant extension of the shelf life during storage at 5°C [36]. HWT at 52.5–55°C and exposure durations of 2–4 min also significantly reduced telescoping and the microbial load of minimally processed green onions. These treatments preserved the visual quality, inhibited discoloration, and did not accelerate nutrient decomposition [37]. Minimally processed green onions in bunches subjected to two consecutive HWT treatments (55°C for 60 s) with an edible alginate coating showed less spoilage and better firmness during storage than untreated samples. The shelf life was extended from 9 to 15 d at 4°C [38].

Fresh-cut Chinese cabbage positively responded to brief immersion in water at 53 and 55°C. Browning of the cut surface was inhibited during storage, and the cabbage retained its good appearance and sensory quality for longer at 0, 5, and 15°C [39]. Steam treatment (50°C for 3 min) of shredded white cabbage prior to inoculation with Listeria increased the growth of Listeria spp., including L. monocytogenes. However, the treatment delayed senescence, and cabbage retained its good appearance for more than 10 d at 5°C [40]. A sequential treatment involving rinsing with plasma-activated water followed by HT at 60°C improved the microbiological quality of sliced and salted Chinese cabbage. Inactivation was observed for the natural microbiota and inoculated foodborne pathogens Listeria monocytogenes and Staphylococcus aureus [41].

Table 1. Beneficial effects of thermal shock on the quality and shelf life of minimally processed leafy vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Iceberg lettuce	both: heads prior cutting, cut pieces (2×2 cm)	HWT 50°C – 90 s	- protected against browning - helped retained greenness	[22]
Iceberg lettuce (Sharpshooter)	cut pieces	HWT 47°C – 120 s	- reduced browning	[28]
Iceberg lettuce	midrib segments	HWT 50°C – 90 s	- extended shelf life - prevented browning - inhibited PAL induction - inhibited phenol accumulation	[23]
Iceberg lettuce (Lerinas)	cut pieces	Pre-cut HWT 50°C – 60 s	- inhibited cut edge browning - reduced the initial microorganism population	[25]
Iceberg lettuce	cut pieces	HWT 50°C – 60 s	Combined with lactate calcium solution (1.5%) - prevented crispness loss	[29]
Iceberg lettuce (Gartago)	strips	HWT 45°C – 120 s	- lowered PAL activity - lowered POD activity - inhibited microbial growth	[26]
Iceberg lettuce	cut pieces	HWT 45°C – 180 s	- reduced pinking	[27]
Iceberg lettuce	cut midrib	HWT 50°C – 60 s	- suppressed PAL activity - suppressed POD activity - delayed browning	[24]
Romaine lettuce (Longifoliar)	strips	HWT 45°C – 120 s 50°C – 120 s	- reduced PAL activity - inhibited browning	[14,31]
Romaine lettuce	leaf pieces	HWT 50°C – 120 s	- preserved texture - preserved color - inhibited microbial growth - preserved vitamin C - preserved antioxidant activity	[30]
Red oak leaf lettuce	strips	HWT 45°C – 120 s	- reduced PAL activity - inhibited browning - lowered microbial growth	[16]
Endive	slices	HWT 46°C – 120s	- prevented red discoloration	[35]
Endive (Flora)	strips	HWT 45°C – 120 s	- lowered PAL activity - lowered PPO activity	[26]
Rocket - Eruca sativa M.	leaves	HWT 50°C – 30 s	- delayed yellowing - extended shelf life	[2]
Spinach (Bison)	leaves	HWT 40°C – 210 s	- delayed senescence - reduced chlorophyll degradation - decreased solute leakage - prevented vitamin C degradation	[32]
Spinach (Toucan)	leaves	HWT 40°C – 60 s	- preserved nutritional quality	[33]
Green onion	plants without roots and stem plate	HWT 55°C – 120 s	- controlled internal leaf growth - inhibited discoloration Combined with control atmosphere (0.2% O2 + 7.5% CO2; 21% O2 + 15% CO2) - extended shelf life	[36,37]
Green onion	pieces of green leaves	HWT 50°C – 20 s	- lowered the microbial load	[42]
Bunching green onion	bunching of plans with peeled bulbs and trimmed leaves	HWT 55°C – 60 s	Combined with an edible alginate coating - prevented rotting - preserved firmness	[38]
Chinese cabbage (Bilko)	strips	HWT 53–55°C – 3 s	- inhibited discoloration - improved sensory attributes	[39]

Table 1. Beneficial effects of thermal shock on the quality and shelf life of minimally processed leafy vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Iceberg lettuce	both: heads prior cutting, cut pieces (2×2 cm)	HWT 50°C – 90 s	- protected against browning - helped retained greenness	[22]
Iceberg lettuce (Sharpshooter)	cut pieces	HWT 47°C – 120 s	- reduced browning	[28]
Iceberg lettuce	midrib segments	HWT 50°C – 90 s	- extended shelf life - prevented browning - inhibited PAL induction - inhibited phenol accumulation	[23]
Iceberg lettuce (Lerinas)	cut pieces	Pre-cut HWT 50°C – 60 s	- inhibited cut edge browning - reduced the initial microorganism population	[25]
Iceberg lettuce	cut pieces	HWT 50°C – 60 s	Combined with lactate calcium solution (1.5%) - prevented crispness loss	[29]
Iceberg lettuce (Gartago)	strips	HWT 45°C – 120 s	- lowered PAL activity - lowered POD activity - inhibited microbial growth	[26]
Iceberg lettuce	cut pieces	HWT 45°C – 180 s	- reduced pinking	[27]
Iceberg lettuce	cut midrib	HWT 50°C – 60 s	- suppressed PAL activity - suppressed POD activity - delayed browning	[24]
Romaine lettuce (Longifoliar)	strips	HWT 45°C – 120 s 50°C – 120 s	- reduced PAL activity - inhibited browning	[14,31]
Romaine lettuce	leaf pieces	HWT 50°C – 120 s	- preserved texture - preserved color - inhibited microbial growth - preserved vitamin C - preserved antioxidant activity	[30]
Red oak leaf lettuce	strips	HWT 45°C – 120 s	- reduced PAL activity - inhibited browning - lowered microbial growth	[16]
Endive	slices	HWT 46°C – 120s	- prevented red discoloration	[35]
Endive (Flora)	strips	HWT 45°C – 120 s	- lowered PAL activity - lowered PPO activity	[26]
Rocket - Eruca sativa M.	leaves	HWT 50°C – 30 s	- delayed yellowing - extended shelf life	[2]
Spinach (Bison)	leaves	HWT 40°C – 210 s	- delayed senescence - reduced chlorophyll degradation - decreased solute leakage - prevented vitamin C degradation	[32]
Spinach (Toucan)	leaves	HWT 40°C – 60 s	- preserved nutritional quality	[33]
Green onion	plants without roots and stem plate	HWT 55°C – 120 s	- controlled internal leaf growth - inhibited discoloration Combined with control atmosphere (0.2% O2 + 7.5% CO2; 21% O2 + 15% CO2) - extended shelf life	[36,37]
Green onion	pieces of green leaves	HWT 50°C – 20 s	- lowered the microbial load	[42]
Bunching green onion	bunching of plans with peeled bulbs and trimmed leaves	HWT 55°C – 60 s	Combined with an edible alginate coating - prevented rotting - preserved firmness	[38]
Chinese cabbage (Bilko)	strips	HWT 53–55°C – 3 s	- inhibited discoloration - improved sensory attributes	[39]

3.2. Root Vegetables

In the root vegetable group, most research on HS of minimally processed products has focused on carrots. Although it can be eaten after cooking, it has a lower glycemic index and is richer in nutrients when is eaten as a fresh vegetable.

Subjecting whole carrot roots to HWT at 100°C for 45 s immediately before slicing reduced bacterial growth during storage. The effect of microbial reduction was higher than that attained by chlorinated water [43]. In addition, the respiratory rate decreased, a high sensory quality was maintained for a longer period, and the shelf life at 5°C was extended [44,45]. This effect can be enhanced by additional UV-C irradiation of the entire roots [44] and packaging fresh-cut carrots in microperforated film [45].

Treating carrot slices with HW (60°C for 1 min) with additives, namely 2% sodium chloride, 2% citric acid (CA), or 10% lime juice, improved the shelf life when stored at 4°C. A decrease in the respiration rate, delayed softening, and discoloration of carrot slices were observed. Microbiological contamination was also lower in heat-treated carrots than in untreated ones. The longest shelf life was observed for carrots treated with a heated CA solution (up to 12 d), followed by carrots treated with an unheated NaCl solution (up to 8 d) [46]. Treating carrot slices with a 1.5% calcium lactate solution heated to 50°C for 1 min maintained the turgor of the cortex tissue and reduced the lignification of catting-edge areas [47]. Additionally, immersion in a 0.5% AA solution heated to 50°C for 5 min contributed more than HWT alone to reducing PAL and POD activities and inhibiting lignin synthesis on the cut surface of carrot discs. Additionally, lower microbiological contamination and better stability were observed during storage at 4°C [48].

In a study on sliced radishes, HWT (50°C for 1 min) followed by immersion in CA (0.3%) slowed the loss of fresh color on the cut surface of slices. Another combination of HWT (50°C for 1.5 min) and CA solution (2%) limited the growth of microorganisms and preserved high levels of phenols and flavonoids [18,49].

HWT application at 55°C for 45 s delayed the browning of sliced lotus roots and effectively inhibited the growth of mesophilic microorganisms during storage at 4°C. Therefore, the organoleptic quality of the treated lotus roots was better than that of untreated ones [50].

Table 2. Beneficial effects of thermal shock on the quality and shelf life of minimally processed root vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Carrot	slices	HWT 50°C – 60 s	Combined with calcium lactate solution (1.5%) - inhibited softening - reduced lignification of the cut surface	[47]
Carrot (Nantes)	slices	pre-cut HWT 100°C – 45 s	- reduced bacterial proliferation - improved bioactive quality - preserved sensorial characteristics	[43,45]
Carrot (Nantes)	slices	pre-cut HWT 100°C – 45 s	Combined with UV-C irradiation - improved storage ability - increased phenolic content - increased carotenoid content - reduced POD activity	[44]
Carrot	slices	microwave treatment 750 W, 45 or 60 s	- prevented whitening - increased total phenols - increased the antioxidant capacity	[51]
Carrot	slices	HWT 60°C – 60 s	Combined with sodium chloride (2%), citric acid (2%) and lime juice (10%) solutions - reduced the respiration intensity - delayed softening - delayed discoloration - reduced microbial contamination - extended the shelf life	[46]
Carrots	cubes	HWT 50°C – 5 min	Combined with ascorbic acid solution (0.5%) - reduced PAL activity - reduced POD activity - inhibited lignin synthesis - lowered microbial counts	[48]
Radish	slices	HWT 50°C – 60 s	Combined with earlier immersion in citric acid solution (0.3%) - reduced browning	[49]
Radish	slices	HWT 50°C – 90 s	Combined with earlier immersion in citric acid solution (2.0%) - increased total phenolic - increased the flavonoid content - increased the antioxidant capacity	[18]
Lotus	slices	HWT 55°C – 45 s	- reduced browning - inhibited microbial growth - inhibited organoleptic deterioration	[50]

Table 2. Beneficial effects of thermal shock on the quality and shelf life of minimally processed root vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Carrot	slices	HWT 50°C – 60 s	Combined with calcium lactate solution (1.5%) - inhibited softening - reduced lignification of the cut surface	[47]
Carrot (Nantes)	slices	pre-cut HWT 100°C – 45 s	- reduced bacterial proliferation - improved bioactive quality - preserved sensorial characteristics	[43,45]
Carrot (Nantes)	slices	pre-cut HWT 100°C – 45 s	Combined with UV-C irradiation - improved storage ability - increased phenolic content - increased carotenoid content - reduced POD activity	[44]
Carrot	slices	microwave treatment 750 W, 45 or 60 s	- prevented whitening - increased total phenols - increased the antioxidant capacity	[51]
Carrot	slices	HWT 60°C – 60 s	Combined with sodium chloride (2%), citric acid (2%) and lime juice (10%) solutions - reduced the respiration intensity - delayed softening - delayed discoloration - reduced microbial contamination - extended the shelf life	[46]
Carrots	cubes	HWT 50°C – 5 min	Combined with ascorbic acid solution (0.5%) - reduced PAL activity - reduced POD activity - inhibited lignin synthesis - lowered microbial counts	[48]
Radish	slices	HWT 50°C – 60 s	Combined with earlier immersion in citric acid solution (0.3%) - reduced browning	[49]
Radish	slices	HWT 50°C – 90 s	Combined with earlier immersion in citric acid solution (2.0%) - increased total phenolic - increased the flavonoid content - increased the antioxidant capacity	[18]
Lotus	slices	HWT 55°C – 45 s	- reduced browning - inhibited microbial growth - inhibited organoleptic deterioration	[50]

3.3. Bulb Vegetables

Bulb vegetables are important sources of vitamins, minerals, essential oils, and other health-promoting substances. They are essential for seasoning dishes and have versatile culinary uses. The use of technology that improves the shelf life of fresh bulb vegetables prepared for direct consumption, without compromising their health properties, is essential for better production and consumer supply.

In the case of leeks, cut pseudostems are becoming an increasingly popular form of sale. The cleaned, white part, without roots or a green rosette is suitable for use in its entirety in meal preparation. Tsouvaltzis et al. [52] found that exposing the whole plant to water at 50°C for 40–60 min, 52.5°C for 25–35 min, 55°C for 17.5–20 min, or 57.5°C for 10–15 min immediately before cutting the pseudostem effectively inhibited the elongation of inner leaves during storage at 4°C. HWT at 55°C for 17.5 min reduced the dry matter, soluble solids, soluble phenol, and thiosulfinate contents and the antioxidant capacity during 7 d of storage at 10°C [53].

In a study of fresh-cut onion, 3-mm-thick slices were treated with HW at 50–60°C for 1 min. The highest antioxidant properties, best color, and lowest weight loss over 21 days at 4°C were observed in slices treated with HWT at 50°C [54].

Table 3. Beneficial effects of thermal shock on the quality and shelf life of minimally processed bulb vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Leek (Gigante)	pseudostems (22 cm long)	pre-cut HWT 52.5°C – 25–35 min 55°C – 17.5–20 min, 57.5°C – 10–15 min	- inhibited the elongation of inner leaves	[52]
Onion	slices	HWT 50–60°C – 60 s 50°C – 60 s	- increased total phenolic content - protected color - protected antioxidant capacity	[54]

Table 3. Beneficial effects of thermal shock on the quality and shelf life of minimally processed bulb vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Leek (Gigante)	pseudostems (22 cm long)	pre-cut HWT 52.5°C – 25–35 min 55°C – 17.5–20 min, 57.5°C – 10–15 min	- inhibited the elongation of inner leaves	[52]
Onion	slices	HWT 50–60°C – 60 s 50°C – 60 s	- increased total phenolic content - protected color - protected antioxidant capacity	[54]

3.4. Stem Vegetables

Stem vegetables, which are rich in dietary fiber, aid in digestion and promote a feeling of fullness. They are often used as versatile ingredients in soups, salads, and other dishes. Celery is in this group of vegetables. When preparing it for sale, the roots and leaf blades are trimmed, leaving only the fleshy petioles.

Saleh et al. [55] found that celery petioles (20 cm long) subjected to HWT at 45 and 50°C for 60 s showed an improved shelf life when stored at 0°C for 16 d. The petioles responded particularly well to treatment at 45°C. Following treatment, neither browning of the cut surface nor any signs of yellowing were observed during the storage period. In addition, microbial growth was inhibited, and celery showed no signs of rotting. In another study [56], using HWT (50°C for 90 s and 55°C for 30 s) and hot air (48°C for 60 min and 50°C for 20 min) reduced the degree of browning of the cut surfaces of fresh-cut celery petioles. This treatment delayed the decomposition of ascorbic acid during short-time storage. When comparing the effects of HWT at 45–55°C and exposure times of 10–480 s on the shelf life of petiole segments, treatment at 50°C for 90 s was found to be the most effective. A significant decrease in damage-induced PAL levels and the greatest inhibition of browning at the cut surface were observed. The storage life was extended for up to 4 weeks at 0°C [57]. Similar conclusions were reached by Vina and Chaves [56], confirming that the optimal HAT is 50°C for 90 s.

Immersing asparagus spears in water at 45°C for 4 min and at 50°C for 2.9 min increased their resistance to low temperatures during storage at 2.5°C. Ion loss and cell membrane damage were reduced, and negative geotropic curvature (bending) of the shoots was prevented. However, according to Saltveit [58], the treatment parameters should be adjusted to the diameter of the spears, as thinner ones require a shorter exposure time than thicker ones. HT at 55°C for 2 or 3 min had a beneficial effect on white asparagus, as it inhibited anthocyanin synthesis, preventing the purple discoloration of spears tips [59,60]. The use of HWT in combination with MAP improved the shelf life at 3°C for both peeled and unpeeled spears [60,61].

Table 4. Beneficial effects of thermal shock on the quality and shelf life of minimally processed stem vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Celery	segments of petioles	HWT 50°C – 90 s	- reduced the rise of PAL - inhibited browning of cut surface - extended shelf life	[57]
Celery (Golden Boy)	sticks	HWT 50°C – 90 s HAT 48°C – 1 h	- reduced cut surface browning - retained ascorbic acid for longer time	[56]
Celery (Royal Crown)	petioles	HWT 45°C – 60 s	- prevented the loss of green color - inhibited microorganism growth - maintained an excellent appearance	[55]
Asparagus (Atlas)	spears	HWT 55°C – 3 or 2 min	- prevented the tips of the spears from turning purple - helped maintain good visual quality during storage	[59]
Asparagus (Dariana)	peeled spears	HWT 55°C – 3 or 2 min	- inhibited discoloration Combined with MAP - suppressed respiration intensity - delayed softening - reduced microbial growth	[60]
Asparagus	spears	HWT 45°C – 4 min 50°C – 2.9 min	- reduced ion leakage - prevented chilling injury - prevented negative geotropic curvature (bending) in spears	[58]

Table 4. Beneficial effects of thermal shock on the quality and shelf life of minimally processed stem vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Celery	segments of petioles	HWT 50°C – 90 s	- reduced the rise of PAL - inhibited browning of cut surface - extended shelf life	[57]
Celery (Golden Boy)	sticks	HWT 50°C – 90 s HAT 48°C – 1 h	- reduced cut surface browning - retained ascorbic acid for longer time	[56]
Celery (Royal Crown)	petioles	HWT 45°C – 60 s	- prevented the loss of green color - inhibited microorganism growth - maintained an excellent appearance	[55]
Asparagus (Atlas)	spears	HWT 55°C – 3 or 2 min	- prevented the tips of the spears from turning purple - helped maintain good visual quality during storage	[59]
Asparagus (Dariana)	peeled spears	HWT 55°C – 3 or 2 min	- inhibited discoloration Combined with MAP - suppressed respiration intensity - delayed softening - reduced microbial growth	[60]
Asparagus	spears	HWT 45°C – 4 min 50°C – 2.9 min	- reduced ion leakage - prevented chilling injury - prevented negative geotropic curvature (bending) in spears	[58]

3.5. Flower Vegetables

The main flower vegetables are broccoli and cauliflower. They are popular all over the world and are important sources of vitamins and minerals in the human diet.

Immersing broccoli florets in a water bath at 50°C for 1.5 min reduced weight loss, delayed browning of the cut stem surface, and delayed yellowing of the florets. This treatment also limited the growth of yeasts, molds, and other pathogenic and spoilage microorganisms [5,62]. HWT (52°C for 90 s) improved the initial color, reduced the development of enzymatic browning, and delayed the spoilage process caused by yeast and mold. However, it did not delay the development of an off-odor or reduce the total microbial count [5]. HT at 50°C for 3 min maintained the green color and appearance of broccoli and effectively reduced the growth of Escherichia coli and Salmonella spp. in broccoli florets. However, Salmonella showed greater resistance to HT than E. coli. Although the initial abundance decreased, E. coli and Salmonella still multiplied during storage at 4°C [6].

HAT at 48°C for 3 h also proved beneficial for broccoli florets, delaying yellowing during 21 d of storage at 0°C due to increased chlorophyll retention. It also contributed to better tissue integrity due to reduced electrolyte leakage. However, HAT reduced the phenolic compound content, lowering the antioxidant capacity of the florets [93]. HWT has enhanced the beneficial effects of edible chitosan coatings. The samples retained their good sensory properties for 11 d at 5°C [62]. Combining HWT (50°C for 3 min) with ultrasonic treatment (7.5 min) and immersion in CA solution (1.5%) preserved the green color and nutritional value, ensured microbial control, and extended the shelf life of broccoli florets by up to 14 d during storage at 5°C [64]. The combination of HWT (45°C for 1 min) and immersion in a sodium chlorate solution (300 ppm) for 1 min reduced the total number of aerobic bacteria, coliform bacteria, yeast, and mold of broccoli florets during storage at 4°C for 12 d. No negative effects of this combined treatment were observed on the color of the florets or the AA or chlorophyll content [65].

Fresh broccoli stalks are often underestimated in terms of raw consumption, even though they are no less valuable than the florets as a source of vitamins and minerals. Sticks measuring 8 × (50–100) mm (width × length) were immersed in water at 55°C for 1 min as well as immersed in HWT with the following additives: 1% calcium ascorbate (Ca Asc), 5% trehalose (TREH), and 1% CaAsc + 5% TREH. HWT alone reduced browning and inhibited microbial development and the respiration intensity. After 11 d at 5°C, sticks treated only with HW received the highest overall consumer acceptance rating among all evaluated samples. HWT with CaAsc increased the ascorbic acid content in broccoli stalk sticks, increasing their antioxidant activity. In turn, the combination of HWT and TREAH methods increased the number of microorganisms as a result of increased dissolution and nutrient availability [66].

Green cauliflower florets, which were packed in airtight polyethylene bags and then treated with hot air (48°C for 180 minutes), showed greater weight loss but significantly less discoloration during storage at 4°C than samples that were not heat-treated. As a result, HAT extended the shelf life to 3 weeks at 4°C [67]. Combining a 10-min treatment with a 2% CaCl₂ solution heated to 40°C maintained firmness and extended the shelf life of cauliflower florets at 4°C. This treatment inhibited the degradation of AA and glucosinolate and counteracted the increased polygalacturonase and lipoxygenase activities [68].

Table 5. Beneficial effects of thermal shock on the quality and shelf life of minimally processed flower vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Broccoli (Marathon)	florets	HWT 52°C – 90 s	- improved initial color - reduced development of enzymatic browning - controlled spoilage	[5]
Broccoli (Cicco)	florets	HAT 48°C – 3 h	- delayed yellowing and chlorophyll degradation - lowered electrolyte leakage - inhibited the decrease in total sugar and soluble protein contents	[63]
Broccoli	florets	HWT 50°C – 180 s	- delayed yellowing - reduced the growth of E. coli and Salmonella spp.	[6]
Broccoli	florets	HWT 50°C – 90 s	- reduced weight loss - reduced enzymatic browning of florets and stems - delayed floret yellowing	[62]
Broccoli	florets	HWT 50°C – 180 s	Combined with ultrasonic treatment (7.5 min) and citric acid dipping (1.5%) - inhibited the loss of green color - improved microbial safety - prevented chlorophyll degradation - prevented ascorbic acid degradation - inhibited the loss of sensory quality - extended the shelf life	[64]
Broccoli	florets	HWT 45°C – 60 s	Combined with dipping in sodium chlorite solution (300 ppm) for 1 min - reduced microbial contamination - preserved the antioxidant capacity	[65]
Broccoli (Parthenon)	sticks of stem	HWT 55°C – 60 s	- reduced of browning - reduced respiration intensity - controlled microorganism growth - extended the shelf life	[66]
Green cauliflower	florets	HAT 48°C – 180 min	Combined with packaging in PE bags - reduced color changes - extended the shelf life	[67]
Cauliflower (Huebai)	florets	40°C – 10 min	Combined with heated CaCl2 solution - delayed senescence - extended the shelf life	[68]

Table 5. Beneficial effects of thermal shock on the quality and shelf life of minimally processed flower vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Broccoli (Marathon)	florets	HWT 52°C – 90 s	- improved initial color - reduced development of enzymatic browning - controlled spoilage	[5]
Broccoli (Cicco)	florets	HAT 48°C – 3 h	- delayed yellowing and chlorophyll degradation - lowered electrolyte leakage - inhibited the decrease in total sugar and soluble protein contents	[63]
Broccoli	florets	HWT 50°C – 180 s	- delayed yellowing - reduced the growth of E. coli and Salmonella spp.	[6]
Broccoli	florets	HWT 50°C – 90 s	- reduced weight loss - reduced enzymatic browning of florets and stems - delayed floret yellowing	[62]
Broccoli	florets	HWT 50°C – 180 s	Combined with ultrasonic treatment (7.5 min) and citric acid dipping (1.5%) - inhibited the loss of green color - improved microbial safety - prevented chlorophyll degradation - prevented ascorbic acid degradation - inhibited the loss of sensory quality - extended the shelf life	[64]
Broccoli	florets	HWT 45°C – 60 s	Combined with dipping in sodium chlorite solution (300 ppm) for 1 min - reduced microbial contamination - preserved the antioxidant capacity	[65]
Broccoli (Parthenon)	sticks of stem	HWT 55°C – 60 s	- reduced of browning - reduced respiration intensity - controlled microorganism growth - extended the shelf life	[66]
Green cauliflower	florets	HAT 48°C – 180 min	Combined with packaging in PE bags - reduced color changes - extended the shelf life	[67]
Cauliflower (Huebai)	florets	40°C – 10 min	Combined with heated CaCl2 solution - delayed senescence - extended the shelf life	[68]

3.6. Fruit Vegetables

For some vegetable species, the edible part of the plant is the fruit. Similar to typical fruits, melon and watermelon have a sweet taste and are used in desserts. Other vegetables, such as tomatoes, bell peppers, and zucchini, have a savory or neutral taste, making them better suited for savory dishes than for desserts. These vegetables are generally low in calories but rich in fiber, phytochemicals, and vitamins and are consumed in large quantities around the world. Minimal processing expands the range of products available on the market and leads to increased consumption.

Pre-cutting HWT at 55 or 60°C for 180 s had a positive effect on the shelf life of fresh-cut green peppers when stored at 4°C. The pepper sticks retained their good appearance, firmness, and aroma for longer than that of non-treated samples. HT also reduced cell juice leakage during storage [69]. Different reactions to HWT were observed when the strips of organic ‘Jaen’ peppers, harvested at the green and red ripeness stages, were subjected to HWT at 45, 50, and 55°C for 1, 3, and 5 min. Fresh-cut green peppers positively reacted to treatment at 45 (1, 3, and 5 min) and 50°C (1 and 3 min). Red peppers only positively reacted to treatment at 45°C at all exposure times. HWT at 45°C for 3 min the most effectively controlled weight loss, soft rot, and color changes at 4°C for 12 d [70]. Significant differences were also observed when examining the responses of two pepper varieties harvested at full maturity to HWT. Fresh-cut red bell pepper ‘Yekla’ positively responded to HWT at 50°C for 12 s. This treatment protected the strips from softening and thus losing quality during short-term storage. In the case of fresh-cut creamy ‘Blondi’ peppers, HWT at 53°C for 3 min and 50°C for 5 min effectively protected against discoloration of the cut surface and loss of a good appearance during storage at 3, 5, and 8°C [71]. The combination of HWT (50°C for 120 s) with calcium ascorbate (6%) and calcium chloride (1%) reduced the number of aerobic bacteria and improved the shelf life during the storage of fresh-cut peppers at 5°C [72].

Dipping tomato slices in HW (55°C for 60 s) with the addition of 0.1% ascorbic acid inhibited browning of the cut surface and maintained sensory acceptance at 5°C for 10 d [73]. HAT (35°C for 6 h) and storage at 2°C under a controlled atmosphere (2.5% O₂ and 5% CO₂) reduced ethylene production and microbial contamination and retained good sensory properties longer compared to non-HW-treated sliced tomato [74]. In addition, HW immersion at 55°C for 5 min before cutting and storing tomato slices in control atmosphere (5% O₂ + 10% CO₂) decreased ethylene generation, the respiratory rate, and firmness losses. Treated slices maintained good quality for 9 d at 5°C [7].

Pre-cut HWT at 45°C for 30 min inhibited browning and polyphenol oxidase (PPO) activity in fresh-cut eggplants. This treatment had also a positive effect on soluble solids content, weight loss, and shelf life during storage at 2°C [76]. Immersing eggplant slices in a solution containing 1% ascorbic acid, followed by HWT treatment (50°C for 60 s) increased the phenolic content, reduced browning and extending the shelf life up to 8 d at 4°C [77].

HWT at 75°C for 20 s has been used for whole Galia melons intended for processing into fresh-cut products. After 4 d of storing sliced melons at 8°C, the number of E. coli on the treated material was 20% lower than that on untreated material. According to Fallik et al. [79], maintaining the cold chain during storage, distribution, and retail sale is essential. Pre-cutting immersion in water at 50°C for 60 min also reduced the microbial load of freshly diced melon during storage at 10°C. Furthermore, HS reduced the respiration rate and water loss, and the melon cubes retained desirable characteristics, such as a fruity flavor and sweet aroma, for a longer period [80]. Similarly, Supapvanich et al. [81] found that treating melon fruit with HWT at 50°C for 60 min prior to cutting delayed the softening process of fresh-cut fruit and maintained a higher total phenolic content and antioxidant capacity.

Immersing intact fruit in a water bath (50°C for 30 min) and then cutting them and packaging in polylactic film with a gas mixture consisting of 2–3% O₂ and 7–8% CO₂ improved the storage life of fresh-cut melon. The respiratory rate and damage decreased, enabling melon cubes to maintain satisfactory quality for 11 d at 6°C [82]. Heating whole cantaloupe melons in water at 76°C for 3 min reduced the number of microorganisms both on the surface of intact fruit and on fresh-cut cubes during storage at 4°C [83]. Additional treatment of whole fruit with a low dose of radiation (0.5 kGy) resulted in a greater reduction in the number of microorganisms after cutting than when using either of these methods alone. The combination of these methods extended the shelf life of fresh-cut cantaloupe melons [84]. Ukuku et al. [85] recommended treating the entire melon immediately before cutting with HWT (97°C for 60 s) or with a heated 5% hydrogen peroxide solution (70°C for 60 s). These treatments reduced the population of natural microflora and inoculated Salmonella bacteria on the fruit’s surface and eliminated the risk of pathogenic bacteria being transferred from the fruit’s surface to the internal tissues during minimal processing. According to Lamicanra and Watson [86], adding calcium lactate (1%) to HWT (60°C) and immersing melons in this solution for 60 min prior to cutting did not extend the shelf life of fresh-cut melons compared to melons treated with HWT alone. Treating fresh-cut melon with a 1% calcium chloride solution heated to 60°C for 60 min maintained better firmness and reduced the microbial load during storage at 5°C for 8 d. The melon pieces retained better sensory quality compared to pieces treated with calcium chloride solution at 5°C [87]. Subjecting fresh-cut Galia melons to two consecutive treatments, namely HW (60°C for 90 or 120 s) and immersion in a peracetic acid solution (80 mg L⁻¹ at 5°C for 60 s), reduced the metabolic activity and microbial contamination during storage at 5°C for 10 d. Combined treatments also increased the polyamine content and maintained cell membrane integrity [88].

HWT (48–55°C for 5–10 min) significantly reduced the population of coliform bacteria, molds, and yeasts on fresh-cut cucumbers. Nutrient depletion during storage was reduced, and the shelf life was extended to 9 d at 5°C. However, HWT at 55°C for 5 min maintained the best quality in sliced cucumbers during subsequent cold storage [89].

Table 6. Beneficial effects of thermal shock on the quality and shelf life of minimally processed fruit vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Bell peppers (Festos)	sticks	pre-cut HWT 55, 60°C – 180 s	- prevented firmness and aroma losses - reduced leakage of juice - extended shelf life	[69]
Bell peppers	pieces	HWT 50°C – 120 s	Combined with 1% CaCl2 and 6% calcium ascorbate solutions - lowered the microbial load - extended the shelf life	[72]
Bell peppers (Jaen)	sticks of unripe pepper	HWT – 45–50°C – 180 s	- delayed soft rot - delayed shriveling - delayed softening - decreased weight loss - prevented the rise in electrolyte leakage - decreased the respiration rate - extended the shelf life	[70]
Bell peppers (Jaen)	sticks of ripe pepper	HWT 45°C – 180 s	- delayed soft rot - delayed shriveling - delayed softening - decreased weight loss - prevented the rise of electrolyte leakage - decreased the respiration rate - extended the shelf life	[70]
Bel pepper (Yecla)	strips.	HWT 55°C – 12 s	- inhibited softening - improved the storage ability	[71]
Bel pepper (Blondi)	strips.	HWT 53°C – 180 s 50°C – 300 s	- inhibited cut surface browning - inhibited softening - extended the shelf life	[71]
Tomato (Solarset)	slices	HAT 35°C – 6 h	Combined with control atmosphere storage (2.5% O2 and 5% CO2) - limited ethylene production - limited fungal infections - maintained high taste ratings - inhibited quality deterioration	[74]
Tomato (Solarset)	slices	pre-cut HWT 55°C – 300 s	Combined with modified atmosphere (5% O2 – 10% CO2) - reduced ethylene production - reduced the respiration rate - prevented of firmness loss - extended the shelf life	[75]
Eggplants	strips	pre-cut HWT 45°C – 30 min	- inhibited browning - extended the shelf life	[48]
Eggplants	slices	HWT 50°C – 60 s	Combined with 1% ascorbic acid solution - controlled browning - reduced PPO and POD activities - increased the phenolic content - extended the shelf life	[77,78]
Cantaloupe melon	cubes	pre-cut HWT 97°C – 60 s	Combined with 5% hydrogen peroxide solution at 70°C - diminished the transfer of pathogenic bacteria to the interior tissue during cutting	[85]
Cantaloupe melon	cubes	pre-cut HWT 50°C – 60 min	- reduced the respiration intensity - reduced the transpiration intensity - limited bacterial growth - prevented taste and aroma loss	[80]
Cantaloupe melon (Acclaim)	cubes	pre-cut HWT 76°C – 180 s	Combined with irradiation after cutting (0.5 kGr) - reduced microorganism load - extended the shelf life	[84]
Cantaloupe melon	cubes	pre-cut HWT 76°C – 180 s	- reduced microbial populations	[83]
Galia Melon	pieces	pre-cut HWRB 75°C – 20 s	- reduced microbial counts	[79]
Melon (Amarillo)	trapezoid pieces	HWT 60°C – 60 s	Combined with 0.5% Ca solution - maintained better firmness - reduced the microbial load	[87]
Galia melon (Cyro)	cubes	HWT 60°C – 90 or 120 s	Combined with immersion in a peracetic acid solution (80 mg L−1) for 60 s at 5°C - lowered the metabolic activity - controlled the microbial load - increased the polyamine content	[88]
Melon (Charentais)	pieces	pre-cut HWT 50°C – 30 min	Combined with active packaging - reduced the respiration rate - increased the beta-carotene content - inhibited decay development - reduced the shelf life	[82]
Muskmelon (Honeyworld)	cubes	pre-cut HWT 50°C – 60 min	- delayed softening - maintained a high total phenolic content - maintained a high antioxidant capacity	[81]
Cucumber	slices	HAT 48–49°C – 10 min	- reduced the microbial load - lowered nutrient degradation - prevented firmness - improved storage ability	[89]

Table 6. Beneficial effects of thermal shock on the quality and shelf life of minimally processed fruit vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Bell peppers (Festos)	sticks	pre-cut HWT 55, 60°C – 180 s	- prevented firmness and aroma losses - reduced leakage of juice - extended shelf life	[69]
Bell peppers	pieces	HWT 50°C – 120 s	Combined with 1% CaCl2 and 6% calcium ascorbate solutions - lowered the microbial load - extended the shelf life	[72]
Bell peppers (Jaen)	sticks of unripe pepper	HWT – 45–50°C – 180 s	- delayed soft rot - delayed shriveling - delayed softening - decreased weight loss - prevented the rise in electrolyte leakage - decreased the respiration rate - extended the shelf life	[70]
Bell peppers (Jaen)	sticks of ripe pepper	HWT 45°C – 180 s	- delayed soft rot - delayed shriveling - delayed softening - decreased weight loss - prevented the rise of electrolyte leakage - decreased the respiration rate - extended the shelf life	[70]
Bel pepper (Yecla)	strips.	HWT 55°C – 12 s	- inhibited softening - improved the storage ability	[71]
Bel pepper (Blondi)	strips.	HWT 53°C – 180 s 50°C – 300 s	- inhibited cut surface browning - inhibited softening - extended the shelf life	[71]
Tomato (Solarset)	slices	HAT 35°C – 6 h	Combined with control atmosphere storage (2.5% O2 and 5% CO2) - limited ethylene production - limited fungal infections - maintained high taste ratings - inhibited quality deterioration	[74]
Tomato (Solarset)	slices	pre-cut HWT 55°C – 300 s	Combined with modified atmosphere (5% O2 – 10% CO2) - reduced ethylene production - reduced the respiration rate - prevented of firmness loss - extended the shelf life	[75]
Eggplants	strips	pre-cut HWT 45°C – 30 min	- inhibited browning - extended the shelf life	[48]
Eggplants	slices	HWT 50°C – 60 s	Combined with 1% ascorbic acid solution - controlled browning - reduced PPO and POD activities - increased the phenolic content - extended the shelf life	[77,78]
Cantaloupe melon	cubes	pre-cut HWT 97°C – 60 s	Combined with 5% hydrogen peroxide solution at 70°C - diminished the transfer of pathogenic bacteria to the interior tissue during cutting	[85]
Cantaloupe melon	cubes	pre-cut HWT 50°C – 60 min	- reduced the respiration intensity - reduced the transpiration intensity - limited bacterial growth - prevented taste and aroma loss	[80]
Cantaloupe melon (Acclaim)	cubes	pre-cut HWT 76°C – 180 s	Combined with irradiation after cutting (0.5 kGr) - reduced microorganism load - extended the shelf life	[84]
Cantaloupe melon	cubes	pre-cut HWT 76°C – 180 s	- reduced microbial populations	[83]
Galia Melon	pieces	pre-cut HWRB 75°C – 20 s	- reduced microbial counts	[79]
Melon (Amarillo)	trapezoid pieces	HWT 60°C – 60 s	Combined with 0.5% Ca solution - maintained better firmness - reduced the microbial load	[87]
Galia melon (Cyro)	cubes	HWT 60°C – 90 or 120 s	Combined with immersion in a peracetic acid solution (80 mg L−1) for 60 s at 5°C - lowered the metabolic activity - controlled the microbial load - increased the polyamine content	[88]
Melon (Charentais)	pieces	pre-cut HWT 50°C – 30 min	Combined with active packaging - reduced the respiration rate - increased the beta-carotene content - inhibited decay development - reduced the shelf life	[82]
Muskmelon (Honeyworld)	cubes	pre-cut HWT 50°C – 60 min	- delayed softening - maintained a high total phenolic content - maintained a high antioxidant capacity	[81]
Cucumber	slices	HAT 48–49°C – 10 min	- reduced the microbial load - lowered nutrient degradation - prevented firmness - improved storage ability	[89]

3.7. Pod Vegetables

Pod vegetables include species whose seeds are enclosed in a double-walled, edible or inedible pod. Those with edible pods, such as green beans, are harvested when they are fully grown but still firm and crisp.

Subjecting green beans to HWT at 52°C for 90 s after harvest enhanced their initial color and proved effective in preventing spoilage caused by yeast and mold, resulting in the maintenance of high sensory scores for up to 11 d at 7 and 10°C. However, it did not delay the onset of off-odors or reduce the total microbial count [5]. HWT also did not prevent the growth of Listeria monocytogenes, Escherichia coli O157:H7, or Bacillus cereus inoculated onto bean pods after HT [5]. Snap bean pods treated with HW at 35°C for 10 min showed increased resistance to chilling injury during storage at 4°C due to energy regulation, soluble sugar content, and the metabolism of cell walls and phenols [90].

Table 7. Beneficial effects of thermal shock on the quality and shelf life of minimally processed pod vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Green beans (Kenyan)	pods with the tips removed	HWT 52°C – 90 s	- enhanced initial color - reduced enzymatic browning development - controlled yeast and mold	[5]
Snap beans	pods	HWT 35°C – 10 min	- inhibited chilling injury - increased sucrose accumulation - prevented cell wall degradation - decreased PPO activity	[90]

Table 7. Beneficial effects of thermal shock on the quality and shelf life of minimally processed pod vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Green beans (Kenyan)	pods with the tips removed	HWT 52°C – 90 s	- enhanced initial color - reduced enzymatic browning development - controlled yeast and mold	[5]
Snap beans	pods	HWT 35°C – 10 min	- inhibited chilling injury - increased sucrose accumulation - prevented cell wall degradation - decreased PPO activity	[90]

3.8. Tuber Vegetables

Tuber vegetables form distinctive underground storage organs that accumulate starch and other nutrients. The most popular of these is potato, which is classified as a horticulture crop in some countries and as an agriculture crop in others.

Immersing whole potato tubers in water at 55°C for 10–20 min, storing them at 20°C for 1 d, and subjecting them to minimal processing delayed browning of the cut surfaces of peeled potato slices. However, no correlation was observed between browning and polyphenol oxidase activity in potato slices [91]. Subjecting tubers to catalytic HT with infrared radiation (55°C for 10 min) and delaying cutting (after 3 d) delayed browning of the slices during storage at 5°C. This is due to the inhibition of the enzymatic activity of PPO, POD, and PAL, the increase in the antioxidant activity, and decrease in cell membrane permeability [92]. HAT of whole potato tubers at 30°C for 24 h before peeling effectively delayed browning and the formation of an unpleasant odor during storage. Additional immersion of potatoes in a solution of citric acid (300 mg L−1) and sodium chloride (100 mg L−1) preserved the color and texture, and peeled potatoes maintained a high, acceptable quality for up to 5 d at 10°C [93].

Dipping sliced potato in HW (55°C for 60 s) with 0.1% AA inhibited the browning of cut surfaces and maintained sensory acceptance during storage [73]. Potato cubes blanched at 90°C for 30 or 45 s significant decreased the browning index during storage at an ambient temperature. Repeating the treatment for up to six cycles effectively decreased the PPO activity in potato tissue [94].

Sunchoke tubers immersed in HWT at 50°C for 20–25 min, sliced, and stored at 5°C, retained good visual quality for up to 12 d. Post-cutting treatment at 50°C for 6–8 min or at 55°C for 3–4 min significantly reduced discoloration, which did not appear on the slices during 8 d of storage [95].

Table 8. Beneficial effects of thermal shock on the quality and shelf life of minimally processed tuber vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Potato (Yopung)	peeled tubers	pre-cut HAT 30°C – 24 h	- delayed browning and off-odor development	[93]
Potato (Russet Burbank)	slices	pre-cut HWT 55°C – 10 min	- reduced browning and phenolic synthesis	[91]
Potato	slices	HWT 55°C – 60 s	Combined with AA solution (0.1%) - reduced browning - maintained better sensory acceptance	[73]
Potato (Yunschu 304)	slices	pre-cut catalytic infrared heating 55°C – 10 min	- delayed browning	[92]
Sunchoke	slices	pre-cut HWT 50°C – 20–25 min	- reduced red discoloration - extended the shelf life	[95]
Sunchoke	slices	HWT 50°C – 6–8 min 55°C – 3–4 min	- reduced red discoloration - extended the shelf life	[95]

Table 8. Beneficial effects of thermal shock on the quality and shelf life of minimally processed tuber vegetables.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Potato (Yopung)	peeled tubers	pre-cut HAT 30°C – 24 h	- delayed browning and off-odor development	[93]
Potato (Russet Burbank)	slices	pre-cut HWT 55°C – 10 min	- reduced browning and phenolic synthesis	[91]
Potato	slices	HWT 55°C – 60 s	Combined with AA solution (0.1%) - reduced browning - maintained better sensory acceptance	[73]
Potato (Yunschu 304)	slices	pre-cut catalytic infrared heating 55°C – 10 min	- delayed browning	[92]
Sunchoke	slices	pre-cut HWT 50°C – 20–25 min	- reduced red discoloration - extended the shelf life	[95]
Sunchoke	slices	HWT 50°C – 6–8 min 55°C – 3–4 min	- reduced red discoloration - extended the shelf life	[95]

3.9. Other Vegetables

Sweet corn is a highly perishable vegetable, whose edible part consists of the kernels on the cobs. It is harvested during the “milk stage” when the kernels have a high sugar and moisture content and a tender texture. HW of peeled cobs at 50°C for 5 min extended their shelf life at 20°C. This treatment reduced weight loss, protected cell membranes, and maintained high antioxidant levels [96].

Table 9. Beneficial effects of thermal shock on the quality and shelf life of minimally processed sweet corn.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Sweet corn (Crispy and Sweed 321)	peeled cobs	HWT 50°C – 300 s	- reduced weight loss - extended the shelf life	[96]

Table 9. Beneficial effects of thermal shock on the quality and shelf life of minimally processed sweet corn.

Species (cultivar)	Minimally processed pieces	Heat shock method and parameters	Effects	References
Sweet corn (Crispy and Sweed 321)	peeled cobs	HWT 50°C – 300 s	- reduced weight loss - extended the shelf life	[96]

4. Conclusions

The technology for HT of minimally processed vegetables has not yet found widespread practical application. Each fresh-cut vegetable has different properties, resulting in different reactions to thermal shock. The results of previous studies indicate that it is difficult to develop standard recommendations for fresh-cut vegetables and that it is more feasible for individual vegetable species. Treatments can be developed empirically by trial and error, but effective treatment requires the use of appropriate HT parameters, such as temperature and duration. Exposing plant material to excessively low temperatures may have no effect, whereas excessively high temperatures can cause thermal damage, such as discoloration, softening and texture loss. Some species, such as lettuce and melon, have been extensively tested, but many others, such as kale and watermelon, have not yet been subjected to any research in this area. In addition, future research should combine treatment methods to achieve synergistic effects resulting from different mechanisms of action to improve the nutritional, sensory, and microbiological quality and the storage life of minimally processed vegetables.