β-Glucosidase Activity of Lactiplantibacillus Plantarum: A Key Player in Food Fermentation and Human Health

1. Introduction

β-glucosidases are a class of exoglucosidase enzymes capable of acting on terminal non-reducing β-D-glucosyl residues by hydrolyzing the β-1,4 glycosidic bond of different glycoconjugates including glucosides, oligosaccharides, and 1-O-glucosyl esters, with release of β-D-glucose [1]. On the basis of their amino acid sequences, they are classified in families and clans that share a conserved catalytic mechanism, structure, and active site residues, but may vary in substrate specificity [2,3]. These enzymes are ubiquitous in nature and are found in all domains of living organisms, Archaea, Eubacteria, and Eukaryotes (fungi, plants, and animals, including humans) [4]. In these organisms, β-glucosidases play a significant role in various biological processes and functions including nutritional acquisition and ecological associations. However, most organisms utilize this enzyme for the hydrolysis of oligosaccharides to glucose, the most usable form of carbon. The β-glucosidases fall in the enzyme class EC 3.2.1.21. At present, 133 glycoside hydrolase (GH) families are listed in the frequently updated Carbohydrate Active enZYme (CAZY) database (http://www.cazy.org) [5]. So far, they have been classified into GH1, GH3, GH5, GH9, and GH30. Family GH1 includes β-glucosidases from archaebacteria, plants and mammals, and family GH3 comprises β-glucosidases of some bacterial, mold and yeast origin [6,7,8,9].

In plants, β-glucosidases perform a wide range of biological functions such as pathogen and insect resistance, microbial interactions, lignification, phytohormones activation, signaling mechanisms, cleavage of glycosylated flavonoids, fruit ripening and pigment metabolism [4,10,11,12,13,14].

In humans, three native β-glucosidase enzymes have been identified: glucocerebrosidase, deficiency of which causes Gaucher’s disease; lactase phlorizin hydrolase, deficiency of which causes lactose intolerance; β-glucosidase, a cytosolic enzyme of broad specificity that is abundant in the kidney, liver and small intestine of mammals and plays a crucial role in the transport and/or digestion of dietary sugars [15,16,17].

In insects, β-glucosidases are mainly involved in cellobiose digestion, the breakdown of glucosinolates (glucosylated specialized metabolites) sequestered from host plants to form a dual-component defense system, and communication and recognition among sexual or social interactions. These functions have been found in different groups of insects and they adapt to the system based on the plants they feed on [18,19].

In bacteria and fungi, β-glucosidase is a crucial element of the microbial cellulose multienzyme complex since it is responsible for the regulation of the entire cellulose hydrolysis process by easing cellobiose-mediated suppression and producing the final product glucose [20,21].

The genetic diversity and expression of β-glucosidase-producing microorganisms were studied in different habitats, including food, soil, cow dung and compost, and marine environments [21,22,23,24,25,26,27]. In particular, the β-glucosidase has been identified, purified and characterized by several species belonging to the group of Lactic Acid Bacteria (LAB) [28] and plays an important role in the performance of these microorganisms in food fermentations or during the interaction with their hosts. For example, they have the potential to improve flavor and aroma of alcoholic (e.g. wine and beer) and non-alcoholic beverages (e.g. teas and juices) by releasing aromatic compounds from flavorless glycosides. Microbial β-glucosidase is also used to hydrolyze isoflavonic glycosides (e.g. soybean products) and to reduce bitterness (e.g. juices, table olives) [6].

During the interaction with their hosts, microbes can provide β-glucosidase activities to increase the bioavailability of metabolites that improve the host health, such as plant phenolic compounds, which are usually glycosylated in their dietary format and therefore are less bioavailable than the aglycone forms [29,30,31]. A known example includes the soy isoflavones, which can be released from their glycosylated precursors by some LAB β-glucosidases during the soy fermentation [32]. The β-glucosidase activities from LAB may also have implications for food security. Cassava contains high concentrations of the toxic cyanogenic glucoside linamarin, and LAB contribute to the degradation of linamarin by β-glucosidase activities [6].

On the other hand, it is known that the mycotoxin deoxynivalenol is not toxic in its glycosylated form (deoxynivalenol-3-glucoside) but it can be activated by a LAB β-glucosidase [33].

Given the importance of LAB β-glucosidases, considerable efforts have been focused to increase our knowledge on these enzymes, which usually show a broad specificity [28].

Among the LAB able to produce β-glycosidases, Lactiplantibacillus plantarum (formerly Lactobacillus plantarum) represents an important member. This heterofermentative species is also known for its high adaptability to many different conditions, since it has been isolated from various ecological niches including milk, fruit, cereal crops, vegetables, bee bread, fresh meat [34,35,36,37,38], as well as fermented foods [39,40]. Moreover, this species is widely diffused into the gastro-intestinal tract of animals; several studies, in fact, showed that it colonizes the digestive system of insects [41,42,43], fish (Iorizzo et al., 2021), and mammals, including humans [45]. The inclusion of L. plantarum in both QPS (Qualified Presumption of Safety) and GRAS (Generally Recognised as Safe) lists [46,47], together with the many intrinsic properties of this species, led to the proposal of numerous L. plantarum strains as animal and human probiotics [48,49].

L. plantarum is widely used as a starter culture in the fermentation of raw materials of plant and animal origin, where it contributes to enhancing the sensorial quality and shelf life of fermented products. Some L. plantarum strains also increase the functional properties of various fermented foods by producing a variety of bioactive compounds [50,51].

The present review aims to provide an overview of the role of the activity of β-glycosidase produced by L. plantarum as a valid biotechnological resource in different food sectors.

2. Fermented Plant-Based Foods

The naturally chemicals of fruit includes volatile compounds, both in free and bound form, which occur primarily as glycoconjugates of sugar and an aglycone [52,53]. The sugar moiety includes glucose or a disaccharide, while the aglycone part of glycosides is often represented by monoterpenes, C13-norisoprenoids, benzene derivatives, and long-chain aliphatic alcohols [54,55,56,57]. Therefore, the hydrolysis of odorless glycosylated compounds can make an important contribution to improving the flavor of fruit juices and derived beverages [52]. β-glucosidases are an important class of enzymes because they contribute to the hydrolysis of different types of glycosidic bonds and promotes the formation of free volatile compounds.

Although endogenous β-glucosidases are present in fruits such as grapes, their activity is insufficient due to low stability under juice processing and winemaking conditions. In fact, the optimal pH levels at which plant glucosidases are most active generally range from 4.0 to 6.0; therefore, in the low pH of fruit juices, only limited activity of most glycosidases has been observed [58,59].

Due to this limited action of plant endogenous glycosidases, a large proportion of the aroma compounds in juices remain inactive, in glycosidically-bound form [60]. Research has therefore focused on finding exogenous sources of glucoside hydrolases, which can be used in the production of juices and wines [60,61].

Several procedures can be used to enhance wine aroma by releasing aroma compounds from glycosidic precursors, including acid or enzymatic hydrolysis. Acid hydrolysis causes rearrangements in the aglycone structure with the formation of undesirable flavors, while enzymatic hydrolysis specifically cleaves the glycosidic linkage without altering the aglycone structure [62].

Glycosidic precursors in fruits can be found as D-glucopyranosides in which the volatile aglycone is linked to a single D-glucopyranose by a β-glycoside bond. They can also occur as disaccharides, in which the D-glucopyranose is combined with a second sugar molecule such as α-L-arabinofuranose, α-L-rhamnopyranose or α-L-apiofuranose (Figure 1).

The enzymatic hydrolysis of glycosidically bound aroma compounds occurs in two steps and involves different exoglycosidases depending on the sugar moieties of the substrates. For example, in the presence of rhamnose or apiose: first, a α-L-rhamnosidase or a β-D-apiofuranosidase cleaves the (1-6)-glycosidic linkage, and then, the flavor compounds are liberated from the monoglucosides by the action of a β-glucosidase. A hydrolysis scheme of glycosidic aroma precursors is shown in Figure 1.

Therefore, much attention has been attracted in flavor enhancement of juices or wines through the hydrolysis of the glycoside aroma precursors using microbial β-glucosidases from mould, yeast and LAB [63,64,65,66]

Phenolics, including flavonoids, widely distributed in plants, have received much attention and were recognized as the most abundant antioxidants in the human diet [67]. Increased antioxidative activity in fermented plant-based foods is primarily due to an increase in the amounts of phenolic compounds and flavonoid aglycones during fermentation, which is the result of a microbial hydrolysis activity [68].

Flavonoids are the largest class of polyphenols that can be further categorized into several subgroups including flavonols and anthocyanins, both of which are naturally distributed in plant foods as glycosides containing single or multiple sugar moieties. Flavonoid aglycones are generally more bioavailable than their respective glycosides [69].

Several studies have shown that flavonoid aglycones content in plant-based foods can increase after fermentation due to the microbial β-glucosidase. Therefore, fermentation by LAB possessing this specific enzymatic activity is an effective strategy to increase the bioavailability of natural antioxidants present in fermented plant-based products [70].

2.1. Soymilk and Soybeen Products

Isoflavones, which are produced almost exclusively by plants of the family Fabaceae, most often occur as glycosyl groups in plants. These compounds are found in plant source mainly as O-glycosides, frequently bound to glucose, but also to other sugars such as galactose, rhamnose, arabinose and xylose [71].

The biological activity of isoflavones has been well reporte [72,73]. These compounds, known as phytoestrogens, are known to reduce the incidence of hormone-dependent steroid cancers such as breast, prostate, and colon cancer [74]. In addition, isoflavones have been shown to help prevent and treat several aging-related dysfunctions and diseases, including neurodegenerative disorders, osteoporosis, metabolic and cardiovascular diseases, and menopausal symptoms [75].

Soybeans (Glycine max) are important polyphenol sources in the diet because of their high levels of isoflavones [76]. Major isoflavones in soybean consisting of three aglycones (daidzein, glycitein, genistein) and their β-glycosides (daidzin, glycitin, genistin), acetyl and malonyl-conjugated β-glycosides (6’’-O-acetyl daidzin, acetyl glycitin, acetyl genistin; 6’’O-malonyl daidzin, malonyl glycitin, malonyl genistin) [77]. The O-β-glycosidic bonds of isoflavones are partially hydrolyzed in the gut primarily by microbial β-glycosidases to their aglycones, daidzein, genistein, and glycitein, respectively (Figure 2). Glucoside isoflavones are very poorly absorbed in the small intestine as compared with their aglycones, because of their greater molecular weight and higher hydrophilicity of the glucosides [78,79]. Furthermore, the isoflavone glucosides are known to be less bioactive than their respective aglycones [80].

Several studies have demonstrated that the content of aglycones in soy product was increased after microbial fermentation by LAB, which may be due to the changes of β-glucosidase activity [81,82,83]. Therefore, the use of these bacteria as starters, with the aforementioned enzymatic activity, in soy milk fermentation could contribute to increasing bioavailable isoflavones [84,85], thereby increasing the nutritional values and health benefits of fermented soy products [75].

Aglycones release is due to β-glucosidase activity on β-glucosides, which in most cases are also present as malonylated and acetylated forms. In these latter cases, β-glucosidase is part of a two-step process, which also requires esterase enzyme to remove acetylation (Figure 2).

It has been shown that L. plantarum LP 95 was able to efficiently bio-transform glycosides to their bioactive aglycones, thus could be used as a functional starter culture to increase the antioxidant activity of fermented soymilk products [32,86]. Other studies have confirmed that several L. plantarum strains have great potential to enrich bioactive isoflavones in fermented soy milk products [84,87,88,89]. In recent studies it has been shown that soy milk fermented from L. plantarum 200655 and L. plantarum KU210152 can be used as a prophylactic functional food with neuroprotective effects against oxidative stress [90,91]. Consistently, another study highlighted the increased antioxidant capacity of the L. plantarum Y16 fermented soybean milk, with respect to unfermented one, whose ethanol and water extracts were able to protect HepG2 cells against ABAP oxidative damage; this was reported to be dependent on the activation of Nrf2/Keap1 signaling pathway and the up-regulation of antioxidant systems expression as heme oxygenase-1, superoxide dismutase, catalase, and glutathione peroxidase [92].

Therefore, increased availability of aglycones found in soy milk fermented with L. plantarum may be useful for designing new functional foods.

Moreover, use of selected L. plantarum strains which are more effective in increasing product bioactivity can also significantly increases the quality of a soy-waste product as okara. Ultrasonic treatment L. plantarum BCRC 10357 was applied to induce a biological stress response resulting in a 100 % increase in β-glucosidase activity, with this latter responsible for the biotransformation of isoflavones glycosides to bioactive aglycones (daidzein and genistein) in okara [93]. In addition, the fermentation of enzymatically hydrolyzed okara by L. plantarum UFG169 strain was reported to increase the content of both aglycone isoflavones and vitamin B2, as well as a reduction in off-flavors, thus improving both nutrition and digestibility of this product [94].

2.2. Cassava

Cassava (Manihot esculenta Crantz), also nown as yucca, manioc or mandioca, is a perennial and herbaceous shrub that belongs to the class Malpighiales and Family Euphorbiceae. This crop has great social value and cultural identity and it is now extensively cultivated throughout tropical and subtropical regions, mainly for its edible tubers as a source of carbohydrates, flavonoids, fiber, vitamin C and Minerals [95,96,97,98].

According to Food and Agriculture Organization (FAO), cassava ranks fourth as a food crop in the developing countries, after rice, maize and wheat [99]. Despite the advantages coming from its starchy tubers, other organs of cassava plant, as leaves, can be also used for edible purposes. However, these less noble parts are characterized by a low protein content, rapid post-harvest deterioration and the presence of cyanogenic glucosides as major drawbacks which strongly limit its utilisation as a food [100].

Consumption of improperly processed cassava may constitute a health problem in rural areas of sub-saharan African countries where cassava derived products provide a high percentage of the daily calory intake [101]. In severe cases this may result in acute cyanide intoxication and in chronic paralytic diseases such as konzo and neurological disorders [102,103]. Moreover, cyanogenic glucosides are spread in all parts of the cassava plant, with the highest amounts in the leaves and the root cortex (skin layer) and are present in bound form, mostly 2-(β-D-glucopyranosyloxyl)isobutyronitrile (linamarin) and, to a lesser extent, its derivative 2-(β-D-glucopyranosyloxyl)methylbutyronitrile (lotaustralin) (Figure 3) [104].

These cyanogenic glucosides are not toxic as such because they are absorbed in the gastrointestinal tract and eliminated as such through urination. However, cyanogenic glycosides are hydrolyzed into acetone cyanohydrin by the glycosidases of gut microbiota. The acetone cyanohydrin was degraded spontaneously in the small intestine in which it had alkaline pH conditions. This degradation releases hydrocyanic acid (HCN), which bound to methemoglobin (Figure 3) [105,106] and, as known, exerts its toxicity by inhibiting the cytochrome oxidase, the complex IV of mitochondrial respiratory chain, thus preventing cellular utilization of oxygen [107]. The presence of these cyanogenic glucosides is the major limiting factor to direct utilisation, thereby necessitating its processing prior to consumption. The introduction of new processing methods has helped to reduce cassava’s cyanogenic content and, therefore, exposure levels to its cyanogenic compounds. Cassava is traditionally processed by a wide range of methods, which reduce their toxicity, improve palatability and convert the perishable fresh root into stable products. In most countries, cassava can be processed by boiling, roasting, drying, cold water leaching, or fermentation. The boiling or drying process alone may not detoxify the cyanogenic glycoside. However, fermentation is a decisive step in the detoxification process [97]. Thus, most cassava-based artisanal or marketed products in Africa (e.g. gari, fufu, pupuru, apu, attieke, Ikivunde, chikwangue, Lafun, etc) and in Latin America (e.g. puba, almidon agrio, polvilho azedo) are obtained through spontaneous fermentation. During fermentation, the roots are softened and there is disintegration of the tissue structure which causes linamarin to come into contact with endogenous linamerase, which is found in the cell wall, and microbial linamerase. These enzymatic activities result in subsequent hydrolysis into glucose and acetone cyanohydrin which are easily broken down into acetone and HCN. During the natural drying phase, free HCN evaporates easily having a boiling point of 26 °C.

However, in spontaneous cassava fermentation the activity of β-glycosidase is often not sufficient to break down all cyanogenic glycosides [108]. Moreover, the linamarase elaborated by both cassava plant tissues and fermenting microorganisms has been found to be unstable under high acidic conditions characteristic of the latter part of natural fermentation. Therefore, it is important to review the detoxification methods of cassava and improve their effectiveness for greater consumption of cassava-based foods [109].

The use of exogenous β-glucosidases from microbial sources is suggested, which hydrolyse these cyanogenic glycosides at an elevated level [110,111].

The cassava fermentation process can be carried out naturally (spontaneous fermentation) by relying on the native microbial populations present in the raw materials and in the environment. Fermentation is important for improving the flavor and aroma of the product, as well as safety, especially by reducing its toxic cyanogenic glycosides.

However, the wide range and complexity of the microbiota of spontaneous cassava fermentation are the main factors responsible for the lack of homogeneity and low product quality [112].

L. plantarum and other lactic acid bacteria (LAB) have been reported as the prevalent microorganisms associated with the spontaneous fermentation of cassava [98,113,114,115,116,117,118,119,120]. Some studies have shown that it is possible to significantly degrade cyanogenic glycosides and reduce free HCN in cassava through fermentation using L. plantarum as a single starter or in co-culture with other microorganisms [112,120,121,122,123,124,125,126,127].

Therefore, further studies are desirable for the establishment of new starter cultures that can contribute to the standardization of cassava fermentation conditions, thus ensuring higher quality products and consumer acceptability.

Especially, the selection and use of L. plantarum as a starter may be an effective biotechnological strategy that may allow for greater preservation, flavor enhancement, cyanide reduction, and improved functional properties of fermented cassava-based products [97].

2.3. Olive

Another emerging field of interest for L. plantarum β-glucosidases application is represented by olive production. One of the main problem of olive industry, in fact, is the bitterness of olives which is principally due to the main representative of olive polyphenol glucosides, namely oleuropein [128]. Oleuropein is an O-glycosylated compound constituted by a D-glucose β (1-4) bound to aglycone, which can be hydrolyzed by the β-glucosidase enzyme resulting in D-glucose and aglycone production [129]. At present, the widely used method to debitter olive consists in the alkalyne treatment of drupes by means of NaOH solution; however this methods pones a series of concerns related to both the consumers (treatment-dependent reduction in nutrients) and the environment (wastewaters entriched in toxic NaOH) [130]. At the same time, the olive debittering represents a committed step in the production of table olives giving rise to the seek for alternative NaOH-free methods [131]; in this regard, the activity of microbial β-glucosidases proved to be able in hydrolying oleuropein, thus producing low-molecular-weight phenolic compounds such as hydroxytyrosol and tyrosol [132,133]. Moreover, several papers showed the capability of L. plantarum to hydrolize oleuropein, as well as the occurrence of this microorganism among the spontaneously fermenting species of table olives [40,134,135]. For these reasons, several strains of L. plantarum have been proposed as microbial cultures for table olive fermentation, since their adaptability to fermentation conditions, as well as high β-glucosidase activity makes this species particularly useful in olive debittering [136,137,138,139,140]. More recently, an elegant study [141] proposed three different mechanisms for the coversion of oleuropein into the active compound hydroxytyrosol, which seems to depend on the L. plantarum strain and need, besides β-glucosidases, also the action of esterase activities.

In addition to olive debittering, L. plantarum fermentation has been suggested as a potential approach also for the recovery of valuable bioactive compounds, as hydroxytyrosol and tyrosol, from olive mill wastewater. This species, in fact, shares with the yest Wickerhamomyces anomalus the ability to increase the content of hydroxytyrosol in wastewater phenolic extract, with both microoorganisms proved to be more efficient, than the commercial enzyme, in 2h bioconversion tests [142]. In addition, a very recent study showed other functional properties of L. plantarum present in olive mill wastewater, as the notable acidification capability and the production of antibacterial compounds [143]. These results strongly candidates this species, and its β-glucosidase activity, as a powerful tool in the management of waste and by-products from olive industry.

3. Fermented Beverages

3.1. Alcoholic Beverages

3.1.1. Wine

The wine LAB, naturally present in grape juice, play a significant role in winemaking by guiding a biological process known as malolactic fermentation (MLF).

This process involves the conversion of L-malic acid to L-lactic acid via malate decarboxylase, resulting in a reduction of wine acidity, providing microbiological stabilization and modifications of wine aroma [144].

In the last decades, various papers have shown that LAB metabolism also involves a large array of secondary enzymatic activities capable of generating many volatile secondary compounds [145]. Several studies have demonstrated the presence of β-glycosidase activity in wine LAB, leading to the release of free volatile compounds as terpenes [146,147].

Oenococcus oeni is the main bacterial species responsible for malolactic fermentation, however, in the last two decades, it has been highlighted that other LAB associated with MLF have enormous potential to influence the composition of wine [145,148].

Among all the species, L. plantarum is frequently found on grapes and in wine and is considered as a new generation of MLF starter due to its ability of high ethanol tolerance and good enological characteristics [39,149,150,151,152,153,154,155,156,157]. In addition, L. plantarum has a wide range of enzymes, including β-glucosidase, which can also contribute significantly to the formation of wine aroma during the winemaking process [50,158,159,160].

It is because of these characteristics that some commercial starters belonging to L. plantarum species have been released in the last decade [161].

The hydrolysis of glycosides, previously reported during the malolactic fermentation through selected L. plantarum strains, may be considered as an interesting option to improve the sensorial characteristics of the wines.

Iorizzo et al highlighted that some L. plantarum strains, candidates for MLF, were able to release specific terpenes from odourless grape glycosidic precursors [158]. In another study, L. plantarum M10, used as a malolactic starter after the alcoholic fermentation of Fiano grape juice, caused a significantly higher concentration of linalool in the wine [157].

Other authors found a significant increase in β-citronellol and 2-phenylethyl alcohol amounts after MLF with L. plantarum UNQLp 11 [162]. β-Citronellol is an alcoholic monoterpene that is most abundant in musts and wines and is often found as an odourless glyco-conjugated compound [60]. Thus, β-glucosidase of L. plantarum UNQLp hydrolyzes the monoterpenyl glucoside and releases β-Citronellol. In addition, it has been hypothesized that β-glucosidase activity could also explain the increase in 2-phenylethyl alcohol (an aromatic alcohol that contributes to sweet floral attributes) in wine fermented with UNQLp 11, as previously described for other LAB strains [163].

Several studies have demonstrated that the β-glucosidase activity is mainly affected by pH, temperature, ethanol and sugars [164,165,166,167].

Therefore, screening of L. plantarum strains for their glycosidase activities is important and should be done based on the substrate to be fermented [163,168,169,170].

In another study, the β-glucosidase activity of L. plantarum USC1 was stable between pH 4.5 and 7.5 and with a maximum activity at pH 5.0 and was completely inactivated at pH values below 4.0. The optimum temperature was 45 °C and the enzyme was active against a wide range of aryl b-glucosides and b-linked disaccharides [171].

Brizuela et al analyzed the amount of 1-octanol (mg/mL) obtained by the hydrolysis of the precursor octyl β-D-glucopyranoside in sterile Pinot Noir wine containing 14.5% v/v of ethanol, at different pH values (3.2, 3.5, and 3.8); the results of this study showed that the activity of β-glucosidase is reduced at low pH, but induced in the presence of high ethanol content [172].

3.1.2. Beer

Sour beer is traditionally produced through spontaneous fermentations, involving complex microbial consortia, and is characterized by higher concentrations of organic acids.

While the production of conventional beer is usually limited to yeasts fermentation, the traditional production methods for sour beer, such as Lambic and Geuze beers, originate from Belgium, involve a spontaneous fermentation by multiple microorganisms, including yeasts and bacteria [173,174,175,176,177].

Interest in sour beer has increased substantially in recent decades and research is underway on both spontaneous fermentations and alternative production techniques [178]. Pure-culture fermentations with strains of L. plantarum and S. cerevisiae, in conjunction with the careful application of processing steps, offer a valid alternative to facilitate the production of sour beer. This approach provides a higher level of process control and more rapid fermentation compared to traditional methods [179,180,181].

In a recent study, the co-inoculation of L. plantarum CECT 9567 and S. cerevisiae was applied for the production of a probiotic beer [182]. The authors, as already highlighted by others, attribute the higher polyphenol content observed in beers brewed with co-inoculation to two phenomena: hydrolysis of bound polyphenols and increased free polyphenols. These phenomena are significantly related to β-glucosidase activity [183,184].

Polyphenols are the most important class of antioxidant compounds in beer. These phenolic compounds have several functional properties in beer, influencing its colloidal stability, flavour, aging parameters and colour [185,186].

3.2. Non-Alcoholic Fermented Fruit Products

Plant-based foods, including fruits and vegetables are naturally rich in minerals, vitamins, dietary fibers, antioxidants, and many other beneficial nutrients that make them essential components of a healthy and balanced diet. Due to new healthy trends, consumption of fruit and vegetable juices have increased in recent years [187].

Two predominant fermentation pathways have been identified in the production of fruit juices: the alcoholic pathway, which involves the utilization of yeast, and the non-alcoholic pathway, which relies on the action of LAB.

Being a traditional food biotechnology, fermentation by LAB is widely used for fruit and vegetable fermentation to convert bioactive components, enhance beneficial properties, extend shelf-life and improve sensory characteristics of final products [188,189].

Fermentation by LAB increasing the content of functional nutrients including polyphenols, flavonoids, organic acids, polysaccharides, amino acids, vitamins, minerals, and other efficacious components, giving the fruit excellent antioxidant, antibacterial, anti-inflammatory, and gut microbiota modulation activities [190,191,192].

Moreover, LAB fermentation can impart distinctive fruity and floral aromas to fruits through the production of esters, ketones, alcohols, terpenes, etc. [193,194,195].

Among the LAB, L. plantarum is quite interesting, as far as its application in the fermentation of a wide range of plant-based substrates is concerned, such as vegetables and fruit juices, since it has genome plasticity and high versatility and flexibility [188].

Several studies have shown that L. plantarum, used as a starter culture, facilitates the enhancement of flavor and aroma in fermented fruit juices through its β-glucosidase activity. Pomegranate juices fermented by L. plantarum POM1 and LP09 were characterized by high levels of terpenes, such as limonene, β-myrcene, γ-terpinene, α-terpinene, α-terpinolene, and p-cymene [196]. Monoterpenes are present in pomegranate juice as either free or glycosidically conjugated precursors and the release of glycosidically bound aromatic compounds has been shown to result in modification or enhancement of the characteristic flavor [197]. In a recent study, it has been found that Sabre mango juice fermented with L. plantarum L75 produced higher levels of β-myrcene [198]. A similar increase in β-myrcene was reported in L. plantarum POM1- and LP09-fermented pomegranate juices [196].

The main biological properties recognized to β-myrcene are the following: anxiolytic, antioxidant, anti-aging, anti-inflammatory and analgesic [199].

A recent study showed that the nutritional quality and flavor characteristics of apricot juice can be improved by L. plantarum LP56 fermentation; specifically, after 6 hours, there was a significant increase in the content of volatile compounds, including α-terpineol, nerol, β-pinene, and terpinene, reaching its maximum level [200].

In another study, L. plantarum NCU116 fermentation had a beneficial effect on the physico-chemical properties, bioactive compounds, antioxidant property, and improved the aroma profile of Momordica charantia juice. Among the terpenoids, myrthenol was the main aromatic compound of fermented juice [201].

Myrtenol is a volatile compound belonging to the terpenoid family of monocyclic monoterpenes and contributed to the woody, pine, balsam, sweet and mint notes. In addition, several reports demonstrated the pharmacological properties of myrtenol, including its antioxidant, antibacterial, antifungal, antidiabetic, anxiolytic, and gastroprotective activities [202].

Ricci et al detected, among the terpene and norisoprenoid class, an increase in limonene, β-linalool, β-damascenon, and eugenol in elderberry juice fermented by L. plantarum 285 [203].

The increase of these compounds could be related with the ability of L. plantarum to produce β-glucosidase [158,194,196].

Fruit beverages fermented by L. plantarum, not only are characterized by a pleasant aroma and taste, but show many health-promoting benefits due to their content of metabolites such as vitamins, organic acids, and phenolic compounds [204,205,206,207].

Several studies have shown that the bioavailability of phenolic compounds is enhanced by different LAB after fermentation of different fruit products [193,208,209,210].

According to several studies, L. plantarum produces enzymes such as β-glucosidase during the fermentation process, which is able to hydrolyze phenolic glycosides to the corresponding aglycones, which have radical scavenging properties [184,211,212]. This process results in an increase in the antioxidant activity of the fermented product [213].

The antioxidant activity of phenolics is related to their chemical structure. In general, flavonoid compounds present a stronger antioxidant activity than non-flavonoids, and combined forms such as glycosides present a lower activity than the free forms [214].

In a study conducted by Meng et al. the effect of different L. plantarum strains on the physicochemical characteristics and antioxidant activities of loquat juice was investigated. Results showed that nerolidol production was significantly upregulated in loquat juices fermented by L. plantarum LP2 [215].

Nerolidol, a terpenoid has good anti-inflammatory, antioxidant, neuroprotective and cardioprotective activities [216,217]. Furthermore, after fermentation by L. plantarum LP2 the antioxidant activity and the total flavonoid content in loquat juice significantly increased.

Landete et al. showed that deglycosylation by L. plantarum CECT 748 transformed food aryl glycosides (phloridzin, esculin, daidzin, and salicin) into their corresponding aryl aglycones (phloretin, esculetin, daidzein, and saligenin). Therefore, in addition to the improvement of their bioavailability, the deglycosylation of specific aryl glycosides by L. plantarum CECT 748 increase the antioxidant activity of glycosylated phenolic compounds [169].

Table 1 shows the main positive effects of the enzymatic activity of L. plantarum, mainly attributable to β-glucosidase, in different fermented fruit products. The data refer to articles published in the last 10 years. However, the impact of fermentation on phenolic compounds seems to depend heavily on the bacterial strain used and the starting material.

Li et al reported that LAB fermentation with L. plantarum 90 significantly increased the total phenolic content, while decreasing the total flavonoid content in fermented jujube juices [228].

The same author had found that the fermentation of apple juice by L. plantarum ATCC14917 caused an increase in antioxidant activity while decreasing the total content of phenols and flavonoids [218], attributing, according to Tian, the greater antioxidant captivity detected to other possible mechanisms [241].

In another study, the flavonol glycosides in sea buckthorn as well as anthocyanins in chokeberry remained unaffected by the fermentation with several several L. plantarum strains obtained from DSMZ (Braunschweig, Germany) [242].

Wei et al. [243] reported general decrease in anthocyanins, phenolic acids, flavonols and flavanols in bog bilberry juice fermented with L. plantarum B7 or L. plantarum C8-1.

Therefore, this suggests that careful selection within the L. plantarum species is crucial in order to identify the most suitable strains to be used for each specific biotechnological application aimed at improving the functional properties of the final products [244].

4. Conclusions

In the last decades, the needs of both producers and consumers in the food sector have continuously grown, thus requiring a particular attention not only to organoleptic aspects but also in terms of health and well-being. This review shows how the enzyme β-glucosidase can be considered crucial for the hydrolysis of several glycosides that give added value to the fermented food matrix. In particular, the activity of β-glucosidase during fermentation by L. plantarum can be considered an important biotechnological strategy in order to increase the nutritional, sensory and functional properties of specific fermented foods. The studies cited in this review showed that the optimal conditions for the β-glucosidase activity extensively differs among the L. plantarum strains and is significantly affected by substrate composition and culture conditions. Therefore, it is essential to optimize these conditions to improve this enzymatic activity also according to the production process adopted to obtain each specific fermented food. Considering that lactic fermentation is an important technology to increase functional properties of fermented foods, we believe that the selection of β-glucosidase-producing L. plantarum strains should remain a focal point of interest in future research, since it can be a valid tool for the design of new functional foods.