Postbiotic–Botanical Co-Formulations for Nutraceutical Powders: Interaction Mechanisms, Processing Design, Characterisation and Evidence Gaps

1. Introduction

Interest in microbiota-directed products has shifted from live probiotics towards preparations that do not depend on viability. In 2021 the International Scientific Association for Probiotics and Prebiotics defined a postbiotic as a preparation of inanimate microorganisms and/or their components that confers a health benefit on the host [1,2]. The definition requires that whole inactivated cells or cell components be present, with or without metabolites, and it excludes purified metabolites and cell-free filtrates on their own [1,2]. Postbiotics are attractive to manufacturers because they tolerate heat, low water activity, acidic matrices and ambient storage, which removes the cold chain and the viability losses that limit live probiotics [2,3,4]. Heat-killed and tyndallised bacteria have a long history of use, particularly in Japan, and several have shown health effects in humans [5].

Botanical extracts are the other major class of nutraceutical actives in tropical and Asian markets. Many of their key constituents, including curcuminoids, methoxyflavones, catechins and saponins, are poorly soluble, unstable or extensively metabolised before absorption, and their activity often depends on transformation by gut microorganisms [6,7,8]. The relationship runs in both directions: polyphenols modulate the composition of the gut microbiota, and microbial metabolism converts them into absorbable metabolites [8]; some polyphenols are among the candidate substrates discussed in the consensus definition of prebiotics [9]. Combining a botanical extract with a postbiotic in one powder is therefore an appealing design: the microbial component may modify how the botanical behaves in the gut, and the botanical may protect or complement the microbial component. Products of this kind are already marketed, and ingredient suppliers increasingly offer fermented or postbiotic-enriched botanical extracts.

The published review literature has not kept pace. Two reviews published in Foods in 2026 cover the postbiotic development pipeline from fermentation and non-thermal inactivation to food biopreservation and regulation [3], and the characterisation of postbiotic preparations in relation to gut–brain axis evidence and food matrices [10]. The first states explicitly that nutraceutical applications are outside its scope and should be addressed separately [3], and the second excludes studies of polyphenols or fermented foods without a defined non-viable microbial preparation [10]. A 2026 review of probiotic and plant bioactive combinations focuses mainly on live probiotics and antimicrobial effects and concludes that the field is promising but insufficiently standardised [11].

This review therefore addresses a specific question: what is known, and what still needs to be shown, about postbiotic–botanical co-formulations intended for oral nutraceutical powders? It covers the interaction mechanisms between the two components, the manufacturing sequences used to combine them, the characterisation needed to define the product, the human evidence available, and the regulatory constraints on claims. The perspective is that of a manufacturer of botanical ingredients and finished supplements, and particular attention is therefore given to process design and claim substantiation.

2. Methods of Literature Selection

This is a narrative review. The three source publications provided by the author [2,3,10] were read in full, together with the consensus statement on which they rely [1]. Additional publications up to mid-2026 were identified through targeted searches of public bibliographic databases and publisher websites using combinations of the terms postbiotic, paraprobiotic, heat-killed, inactivated, polyphenol, botanical extract, fermented herb, co-fermentation, biotransformation, encapsulation, spray drying and randomised trial. Human intervention and pharmacokinetic studies were prioritised. Because study designs, preparations and endpoints differ widely, no quantitative synthesis was attempted. Following the recommendation of Elmas and colleagues [10], studies were classified by preparation type, and evidence from live probiotics, isolated metabolites or fermented foods is identified as such rather than presented as postbiotic evidence.

3. Definitions and Scope

3.1. What Counts as a Postbiotic in a Co-Formulation

Three boundaries matter for co-formulated products. First, viability: the microorganisms must be inanimate, not merely dormant, so inactivation must be verified rather than assumed [1,2]. Second, composition: a preparation must contain cells or cell components; a cell-free supernatant or a purified metabolite such as a short-chain fatty acid or a bacteriocin is not a postbiotic under the consensus definition, even though such fractions dominate the food science literature [3,10]. Third, benefit: the health benefit must be demonstrated in the target host, and evidence obtained with one preparation cannot be transferred to another made from the same strain by a different process [1,10].

For co-formulations these boundaries create practical consequences. A botanical extract fermented with lactic acid bacteria and then heat-treated and dried with its biomass falls within the definition, whereas the same ferment filtered before drying does not. A blend of a postbiotic powder with a botanical extract is a postbiotic-containing product, but its health benefit cannot be inferred from the separate evidence for each component.

3.2. Categories of Postbiotic–Botanical Products

Four product categories can be distinguished (Table 1). Fermented botanicals are produced by growing microorganisms on a plant substrate and inactivating the whole ferment. Blended products combine a separately produced postbiotic with a botanical extract. Co-processed products bring the two together in liquid form, for example by adding a postbiotic liquid to an extract, before drying, so that the components interact during processing. Co-encapsulated products enclose both in a common carrier such as a liposome, a biopolymer matrix or a yeast cell wall particle. The categories differ in the degree of interaction between components, in what must be characterised, and in how a health claim could be substantiated.

4. Interaction Mechanisms Between Botanical Constituents and Postbiotic Components

4.1. Binding of Polyphenols to Microbial Cell Surfaces

Polyphenols bind to bacterial cells. Koren and colleagues showed that several microbial species, including probiotic lactic acid bacteria, irreversibly bound flavonoids and anthocyanidins, and that the coated bacteria acquired substantial oxidant-scavenging capacity measurable by several independent assays [12]. Coloured polyphenols such as catechin and tea extract remained attached after washing, and binding varied with the polyphenol and the bacterial species; the cell wall, particularly the peptidoglycan of Gram-positive bacteria, was proposed as the main binding site [12]. Because binding does not require viability, inactivated cells are expected to retain this property, and heat treatment that exposes cell wall components may even increase it, although this has not been tested systematically.

Yeast cells and yeast cell wall particles have been studied more directly as carriers. Hydrophobic compounds, including curcumin, β-carotene, vitamin D and essential oils, can be loaded into yeast cells by passive or vacuum-assisted diffusion, and pretreatments such as plasmolysis and autolysis increase loading capacity by removing soluble cell contents [13]. Spent brewer's yeast has been used to encapsulate nanostructured curcumin [14]. Inactivated microbial cells therefore have a dual role in co-formulations: they are postbiotic actives in their own right, and they are potential carriers for botanical constituents.

4.2. Effects of Polyphenols on Microbial Adhesion and Host Interaction

Plant phenolics can change how microorganisms interact with the intestinal epithelium. In Caco-2 and mucus-producing HT29-MTX cells, apple peel extract and its main phenolics (procyanidin B2 and chlorogenic acid) increased the adhesion of two lactobacilli by up to about 36 per cent, whereas apple pulp extract reduced adhesion of one strain by up to about 46 per cent [15]. These data were obtained with live bacteria, but adhesion of inactivated cells depends largely on the same surface structures, so the direction of effect may differ with the botanical and must be measured for each combination rather than assumed to be favourable.

4.3. Biotransformation During Co-Fermentation

When a botanical is used as a fermentation substrate, microbial enzymes can transform its constituents before the product is ingested. The clearest human example is ginseng. Lactic acid bacteria and other microorganisms hydrolyse the sugar chains of protopanaxadiol ginsenosides to compound K, which in humans is normally formed only by colonic bacteria with large inter-individual variation [16]. In a randomised crossover trial in 16 healthy men, 6 grams of fermented red ginseng produced a compound K peak concentration about 69-fold higher and an area under the curve about 75-fold higher than the same amount of non-fermented red ginseng [16]. Turmeric has been fermented with lactic acid bacteria; one strain of Lacticaseibacillus rhamnosus increased curcuminoid, phenolic and flavonoid content and antioxidant activity compared with unfermented turmeric [17]. Turmeric fermented with Aspergillus oryzae and dried as a whole powder reduced alanine transaminase by 26.5 per cent compared with 3.4 per cent for placebo in a 12-week randomised trial in adults with elevated liver enzymes [18]; in that trial the product was a whole fermented powder, which would contain the inactivated fungal biomass, but the contribution of the microbial fraction was not separated from that of the transformed botanical. Fermented foods as a class have their own consensus definition, which requires desired microbial growth and enzymatic conversion of food components but not that the microorganisms remain alive [19]; a fermented botanical may therefore be both a fermented food and, if inactivated with its biomass and shown to confer a benefit, a postbiotic.

For botanicals whose actives are already aglycones or lipophilic, such as the methoxyflavones of Kaempferia parviflora (black ginger) [7], the main effect of fermentation is less likely to be deglycosylation and more likely to be changes in matrix, solubility or minor constituents; this needs to be measured rather than extrapolated from glycoside-rich plants such as ginseng.

4.4. Protection, Solubilisation and Antioxidant Effects

Botanical extracts rich in phenolics can protect oxidation-sensitive postbiotic fractions, such as unsaturated membrane lipids and peptides, during drying and storage, while postbiotic exopolysaccharides and proteins can form complexes with polyphenols that alter their solubility and release [3]. Postbiotic preparations themselves contribute antioxidant and antimicrobial activity: spray-drying of a fermented Spirulina preparation with maltodextrin increased retained phenolics and antioxidant and antimicrobial activity compared with uncoated material [3]. These interactions are plausible routes to complementary activity but are largely described in vitro or in food models.

4.5. Antimicrobial and Barrier Effects

Probiotic–phytochemical combinations have shown additive or synergistic antimicrobial effects through membrane destabilisation, inhibition of adhesion and biofilm formation, and interference with quorum sensing, as well as support of epithelial barrier and immune regulation, but these effects vary with strain, phytochemical composition, dose and model, and most data come from in vitro or animal studies [11]. Bacteriocin-containing postbiotic fractions and plant phenolics also act together in food biopreservation [3]. Whether such effects occur in the human gut after ingestion of a dried co-formulation remains to be shown.

Table 2 summarises these mechanisms with the level of evidence available for each.

5. Manufacturing Sequences and Critical Control Points

The order in which botanical and microbial components are combined determines which interactions occur and what must be controlled. Four sequences are in use.

5.1. Fermentation of the Botanical Substrate

In this sequence the botanical, as a fresh material, an aqueous extract or a powder suspended in medium, is fermented and the whole ferment is inactivated and dried. It offers the strongest potential for biotransformation (Section 4.3) and yields a single integrated ingredient. Its critical control points are strain identity and purity; substrate composition and pretreatment, since plant antimicrobials can inhibit fermentation; fermentation time, pH and temperature, which govern both biomass and metabolite profiles [3]; and verification that no viable cells remain after inactivation. Batch variability is higher than for blends because both the plant material and the fermentation vary.

5.2. Blending After Inactivation

Separately produced postbiotic and botanical powders are dry-blended. This is the simplest and most reproducible approach, allows each component to be specified independently, and is compatible with existing evidence for a defined commercial postbiotic. It produces little interaction before ingestion, so any synergy must arise in the gut. Critical control points are blend uniformity, particularly when the postbiotic is dosed at a few milligrams per serving as in some clinical studies [20], and moisture and oxidation control during storage.

5.3. Co-Processing Before Drying

A postbiotic liquid or slurry is combined with a liquid botanical extract and the mixture is dried together, usually by spray drying with a carrier such as maltodextrin. Co-processing allows binding of polyphenols to cell surfaces (Section 4.1) and formation of polyphenol–protein and polyphenol–polysaccharide complexes before drying, and the drying step yields a single homogeneous particle. Spray drying is the most industrially scalable option for postbiotic stabilisation but exposes the feed to heat during atomisation, so protective wall materials and controlled outlet temperatures are needed to limit loss of heat-sensitive fractions [3]. For co-processed products the inlet and outlet temperatures, feed solids, carrier ratio and residence time are critical parameters for both components. Avoiding a separate drying step for the botanical, by processing a fresh-plant extract directly with the postbiotic liquid, can reduce cumulative heat exposure of volatile and thermolabile constituents but increases the microbial and enzymatic load of the feed, which must be controlled.

5.4. Lipid-Based and Biopolymer Co-Encapsulation

Liposomes, alginate and chitosan systems and yeast cell wall particles can enclose both components. Liposomal nanoencapsulation of bacteriocin-like substances increased their antibacterial activity fourfold, and alginate–chitosan microcapsules protected a postbiotic metabolite at gastric pH and released about 75 per cent in simulated intestinal and colonic fluids [3]. Lipid carriers also address the poor solubility of lipophilic botanical actives. The added complexity is justified only when a specific barrier, such as low solubility of the botanical or gastric lability of a postbiotic fraction, has been identified and the carrier is shown to address it. Spray-dried liposomal powders can retain intact vesicles after rehydration when phospholipid composition and drying conditions are controlled [21].

Table 3 compares the four sequences.

6. Characterisation of the Co-Formulated Product

A postbiotic is defined by its preparation, not only by its strain, and two preparations of the same strain made by different processes may differ in composition and activity [1,10]. Co-formulation adds the botanical as a second source of variability, so both components and their interaction must be characterised. Five groups of attributes are proposed.

Microbial identity and inactivation: the strain designation and deposit number; the method and conditions of inactivation; confirmation of absence of viable cells by culture with appropriate enrichment; and a quantitative measure of the inactivated biomass, such as total cell count by flow cytometry or microscopy, or a validated marker of cell wall content. No internationally agreed method yet exists for enumerating inanimate microbial cells, which is one of the main obstacles to standardisation [1,3].

Botanical identity and markers: botanical name and plant part; extraction solvent and ratio; and quantification of marker or active constituents by validated chromatography, before and after co-processing, so that degradation or transformation is detected.

Interaction metrics: for fermented and co-processed products, the conversion of precursor to product constituents (for example, glycoside to aglycone); and, where binding is claimed, the fraction of botanical constituent associated with the cell fraction after separation.

Physical and stability attributes: moisture and water activity, particle size, colour and oxidation markers over shelf life under the climatic conditions of the intended market.

Safety: absence of pathogens, mycotoxins and biogenic amines; residual solvents; and, for fermented products, absence of toxin production by the production strain.

7. Human Evidence

Table 4 summarises representative human studies. They fall into two groups.

The first group tested defined postbiotics alone. Heat-killed Lactiplantibacillus plantarum L-137 at 10 milligrams per day for 12 weeks reduced the incidence of upper respiratory tract infection in 78 adults under high psychological stress, with increased mitogen-induced lymphocyte proliferation, although duration and severity were not significantly changed [20]. Heat-inactivated Lactobacillus gasseri CP2305 for 12 weeks improved sleep quality, prevented the rise in salivary cortisol and normalised bowel habits in 69 medical students before a national examination [22]. Pasteurised Akkermansia muciniphila at ten billion (10¹⁰) cells per day for three months improved insulin sensitivity by about 29 per cent and reduced insulinaemia and total cholesterol compared with placebo in overweight insulin-resistant adults, and was well tolerated [23], consistent with earlier animal work showing that the pasteurised bacterium was at least as effective as the live one [24]; this preparation has since been assessed as safe and authorised as a novel food in the European Union [3,25]. A structured review of gut–brain axis studies found effects of defined postbiotic preparations on stress-related symptoms and sleep, while warning that mechanistic data from live probiotics, metabolites or fermented foods should not be read as direct postbiotic evidence [10].

The second group tested fermented botanicals. Fermented red ginseng increased systemic exposure to compound K about 70-fold [16], and Aspergillus-fermented turmeric reduced elevated liver enzymes [18]. These studies show that fermentation can change both pharmacokinetics and clinical outcomes, but neither compared the fermented product with the unfermented botanical plus an equivalent dose of inactivated biomass, so the contribution of the microbial fraction as a postbiotic cannot be determined.

No controlled human trial was identified that compared a defined postbiotic–botanical combination with each component alone and with placebo. Claims of synergy for such products therefore rest on mechanistic plausibility and on evidence for the separate components, not on direct demonstration.

8. Critical Appraisal and Evidence Gaps

8.1. Preparation-Level Evidence Is Not Transferable

Because postbiotic activity depends on strain, substrate, inactivation method and retained fractions, clinical evidence for one preparation does not apply to another, even of the same strain [1,10]. A co-formulation that uses a strain with published trials but produces it by a different process, at a different dose, or combines it with a botanical that binds to its cell surface, is a new preparation. Product claims should cite evidence for the preparation actually sold.

8.2. Synergy Is Asserted More Often than Tested

Synergy means that the combination produces an effect greater than expected from the components. Demonstrating it requires a factorial design in which the combination, each component alone and a placebo are compared at the same doses. In vitro, isobologram or combination-index methods can quantify interaction, but they have rarely been applied to postbiotic–botanical pairs, and probiotic–phytochemical studies use heterogeneous synergy assays [11]. Without such designs, a positive result for a combination cannot be attributed to interaction.

8.3. Dose Metrics Are Inconsistent

Postbiotic doses are reported as cell numbers, milligrams of powder or milligrams of dry biomass, and botanical doses as milligrams of extract with or without marker content. Co-formulated products need both components declared in comparable units, otherwise dose–response relationships and comparisons between products are impossible [3,10].

8.4. Cell-Free Fractions Dominate the Mechanistic Literature

Much of the mechanistic and food-application evidence for postbiotic activity comes from cell-free supernatants, which do not meet the consensus definition [3]. Co-formulations that use filtered ferments should be described accurately, for example as containing fermentation metabolites, rather than as postbiotics.

8.5. Regulatory Constraints on Claims

No major jurisdiction has yet defined postbiotic as a regulatory category, so products are assessed under existing food, supplement or novel food rules [3]. Because the consensus definition includes a health benefit, use of the word postbiotic on a label may be treated as an implied health claim in jurisdictions that require pre-approval of claims [3]. Botanical claims are separately regulated. A combination product therefore needs two independent compliance assessments, and claims for synergy require evidence for the combination itself.

9. A Minimum Reporting Standard and Research Agenda

The following minimum standard is proposed for studies and product dossiers of postbiotic–botanical co-formulations.

• Identify the microorganism to strain level with a culture collection deposit number, and describe the growth medium or botanical substrate, fermentation conditions and inactivation method.
• Confirm the absence of viable cells by a stated method, and report the quantity of inactivated biomass in a defined unit.
• Identify the botanical by scientific name and plant part, describe the extraction, and quantify marker constituents before and after combination.
• State the manufacturing sequence (fermentation, blending, co-processing or co-encapsulation) and the drying conditions, including temperatures and carrier ratios.
• Report stability of both components over the intended shelf life under the climatic conditions of the target market.
• Declare the dose of each component per serving in comparable units.
• For efficacy claims, use a randomised, placebo-controlled factorial design that includes each component alone, with pre-registration and a power calculation.
• Report adverse events and disclose funding and commercial interests.

Standardised in vitro digestion protocols [26] should be used as a common screening tier to compare release of botanical constituents and survival of postbiotic fractions across manufacturing sequences before human testing. Priority research questions follow from the gaps identified: whether polyphenol binding to inactivated cells alters the release and absorption of the polyphenol or the immune activity of the cells; whether co-processing before drying gives different outcomes from simple blending at equal doses; which fermentation and inactivation conditions maximise biotransformation without destroying cell-associated activity; and how such products perform in human trials designed to detect interaction.

10. Conclusions

Postbiotic–botanical co-formulations combine two classes of ingredient that are each supported by some human evidence and that interact in plausible ways: polyphenols bind to microbial cell walls, fermentation can transform botanical constituents into more absorbable forms, and each component can protect or complement the other during processing. The manufacturing sequence, whether fermentation of the botanical, blending, co-processing before drying or co-encapsulation, determines which of these interactions occur and what must be controlled. Current human evidence supports defined postbiotics and fermented botanicals separately but does not yet demonstrate synergy for any combination. Applying a preparation-level characterisation standard, declaring doses of both components in comparable units and testing combinations against their components in factorial trials would allow this growing product category to move from plausible to proven.