Introduction

Aspergillus flavus and Aspergillus parasiticus produce aflatoxins (AFs) which have mutagenic, carcinogenic, hepatotoxic, immunosuppressive and teratogenic effects in humans and animals, and are responsible for the loss of crops and animal feed (Hernandez-Mendoza et al., 2009a, 2009b). They are present in food products, such as corn, peanuts, beans, dried fruits and dairy products, including milk (Ambadoyiannis et al., 2004; Dalié et al., 2010). Owing to their high thermal stability, aflatoxins remain in raw food and are not destroyed by processing and cooking, and are trasferred to humans through food (Zain, 2011). Since mycotoxins have low molecular weight, they are promptly absorbed in the gastrointestinal tract (GIT) and can be discovered in blood and milk 15 min and 12 h after intake, respectively (Martins et al., 2001). More than 20 metabolites of the AF group are known, among which the most important ones are B₁, B₂, G₁ and G₂ with different levels of toxicity (Alshannaq and Yu 2017; Laciakova et al., 2008; Mahmood et al., 2018). These terms are derived through fluorescence under ultraviolet (UV) rays, blue (B) or green (G), and comparative chromatographic migration patterns through thin layer chromatography (TLC) (Lizárraga-Paulín et al., 2011; Vosough et al., 2013). AFM₁ and AFM₂ are hydroxylated metabolites of AFB₁ and AFB₂. Approximately 0.3–6.2% of AFB₁ ingested by animals is metabolized to AFM₁ (Figure 1) (Afshar et al., 2020; Battacone et al., 2003; Kumar et al., 2017; Oatley et al., 2000).

Figure 1. Sources of aflatoxin exposure and its flow in the food chain.

The order of acute and chronic toxicity of different aflatoxins is AFB₁ > AFG₁ > AFB₂ > AFG₂, where AFM₁ and AFM₂ are less toxic than their precursors (Ahmadi, 2020). Among these, AFB₁ showed the highest toxicity for animals and humans (Dhanasekaran et al., 2011; Sklan et al., 2001; Yu et al., 2008). The International Agency for Research on Cancer (IARC) classified AFB1 and AFM1 as human carcinogens belonging to Group 1 and Group 2B, respectively (Caloni et al., 2006; Darwish et al., 2014; Marchese et al., 2018). The Codex Alimentarius specifies maximum limit of aflatoxins as 15 µg/kg for almonds, hazelnuts, Brazil nuts, peanuts and pistachio nuts for further processing, and 10 µg/kg for almonds, Brazil nuts, hazelnuts and pistachio nuts for direct consumption and dried figs, with a maximum limit of AFM₁ as 0.5 µg/kg for milk. However, the maximum levels of aflatoxins in foods vary globally, depending on the type of product and also on the import/export regime (Food and Agriculture Organization (FAO), 2003; Pickova et al., 2021). There are various approaches to reduce or remove mycotoxins, including physical (thermal inactivation, radiation, magnetic carbon absorption), chemical (treatment with acid, alkali, or oxidizing agent), and biological (microorganisms) treatments (Peng et al., 2018). Among these methods, biological methods attracted a great deal of attention because of their high efficiency, low price, compatibility with the environment, and complete detoxification under favorable conditions (Adebo et al., 2017; Armando et al., 2012; Shetty and Jespersen, 2006). Among the biological methods, detoxification using postbiotics has gradually become the most appropriate biological detoxification method, because the use of microorganisms in food chains required a long process of national and international authorization (Zhou et al., 2017; Zychowski et al., 2013).

Some microorganisms, especially probiotic bacteria, have received special attention because of their various beneficial effects, such as an easy, controllable, and safe method without loss of nutritional value to remove aflatoxins (Mahmood et al., 2018). Along with the remarkable progress made in this field, the researchers got acquainted with a new concept of probiotics called postbiotics. The word “postbiotic” refers to bioactive compounds and does not match the definitions of prebiotics, synbiotics and probiotics. In mid-2021, the International Scientific Association for Probiotics and Prebiotics (ISAPP) provided a precise and well-considered definition of postbiotics, which is the “preparation of inanimate microorganisms and/or their components that confers a health benefit on the host” (Salminen et al., 2021). According to the above definition, the term postbiotic refers to a set of macro and micro molecules, such as inactive microorganisms (cells that are not alive), parts of a cell (surface layer proteins, endo- and -exo-polysaccharides, teichoic acid, muropeptides and cell-free supernatants [CFS]), or metabolites of a cell (bacteriocins, short-chain fatty acids [SCFA], organic acids and enzymes that are produced naturally by living probiotic cells during fermentation) (Figure 2) (Homayouni-Rad et al., 2021).

Figure 2. Definition of the family of probiotics, prebiotics, synbiotics and postbiotics (Vinderola et al., 2022).

Terms such as parapsychobiotics, paraprobiotics, non--viable probiotics, heat-killed probiotics, ghost probiotics, meta-biotics, cell-free supernatant, biogenic, tyndallized probiotics, bacterial lysates, and postbiotics are also used (Homayouni-Rad et al., 2021; Salminen et al., 2021), although term postbiotic is used more commonly.

Probiotics are mostly lactic acid bacteria (Walhe et al., 2022) and have many health benefits; however, because they are alive, they have limitations for use and treatment. Probiotic bacteria must maintain their viability even in inappropriate conditions, such as preparation of food, storage, processing, distribution as well as passing through the digestive system, so that they can perform their health-giving effects (Homayouni-Rad et al., 2020; Rad et al., 2020a). In spite of passing these challenges and survival, adverse reactions may occur, such as resistance gene transfer between other intestinal microbiota, different colonization patterns, presence of virulence factors in some probiotic species, preventing the colonization of other intestinal microbiota, meningitis, infectious endocarditis, urinary tract infections, and bacteremia, especially in infants, patients treated with antibiotics and have weak immunity (Abbasi et al., 2021a, 2021b; Abriouel et al., 2015; Kim et al., 2018; Liong 2008).

Postbiotics have overcome these adverse effects and appeared as a suitable alternative to probiotics. In recent years, it has been shown that the viability of a probiotic is not necessarily to benefit from its favorable effects (Piqué et al., 2019). It has been proven both in vivo and in vitro that the beneficial effects of probiotics and postbiotics include antibacterial, antiviral, antifungal, antioxidant, anti-proliferation, anti-mutation, anti--cancer, -anti-obesity, anti-diabetic, anti-blood pressure, and immune system booster (Moradi et al., 2021). In addition, postbiotics have unique features, such as safe origin, no toxic effect, low cost of preparation and transportation, and long shelf life (Homayouni-Rad et al., 2021). Many results obtained from clinical studies indicated the suitability of digestion, absorption, metabolism and distribution of postbiotics (Asif et al., 2023; Gao et al., 2019; Shenderov 2013; Singh et al., 2018), and these characteristics are suitable for the food and pharmaceutical industries (Aghebati-Maleki et al., 2021).

Many studies were carried out to reduce the amount of aflatoxins by postbiotics in food model systems (Karazhiyan et al., 2016; Nassar et al., 2018) and culture media (Taheur et al., 2019; Vosough et al., 2014), but, so far, no study is discovered on the systematic review of articles for (1) determining the amount of aflatoxin reduction by postbiotics and (2) identifying and introducing the most effective postbiotic to reduce aflatoxins.

Materials and Methods

Search Strategy

This systematic review was based on the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA), with the reports cited in this review being consistent (Moher et al., 2015). In this study, the population, intervention, comparison and outcome (PICO) approach was used to identify terms.

Our study started on 01 September 2022 and continued up to 27 October 2022. Four electronic databases, including Web of Science, Google Scholar, PubMed and Scopus, were used from the beginning till 25 September 2022. The following keywords were used to search databases: “Probiotics” AND “Postbiotics” OR “Non-viable probiotics” OR “Parapsychobiotics” OR “Paraprobiotics” OR “Heat-killed probiotics” OR “Ghost probiotics” OR “Metabiotics” OR “Cell-free supernatant” OR “Biogenic” OR “Tyndallized probiotics” OR “Probiotic lysates” AND “Detoxification” AND “Aflatoxin.” These words along with synonyms or Medical Subject Heading (MeSH) terms were added to the search terms.

Duplicate cases were excluded from the final selection of the study. As this review focused on published papers, gray literature found on Google Scholar was not included. After selecting relevant titles and abstracts, all texts of the selected articles were evaluated. A PRISMA flowchart was used to graphically represent the selection of final articles (Figure 3).

Figure 3. PRISMA flow chart of the systematic review search process.

Inclusion and exclusion criteria

Articles were considered eligible if the full-text articles met the below-mentioned conditions:

1. Full text of articles must be accessible

2. Article must be written in English

3. Articles were peer-reviewed

4. Articles written about the detoxification of aflatoxin by postbiotics

5. Articles about the inhibitory effect of probiotic metabolites or killed probiotics against aflatoxin

6. In vitro articles

7. Articles that considered probiotic bacteria

Articles that did not use postbiotics as an inhibitory agent and dealt with other methods or that document in vivo conditions were excluded from the study. Furthermore, conference abstracts, letters to editors, and non-peer-reviewed studies were also excluded. In this study, two reviewers were used to include the articles. They screened all titles and abstracts independently to find eligible articles. Sometimes, if needed, the third referee was consulted to clear doubts, increase self-confidence and reduce errors. For example, if it was not specified in articles whether postbiotics or probiotics were used, a third reviewer was consulted for clarity.

Data extraction

Two reviewers independently extracted the data using a pre-designed table. The extracted data included article title, year of publication, probiotics type, type of aflatoxin, results, and other relevant findings. The extracted data were analyzed.

Quality assessment

Risk of bias assessment for each article included evaluation in the six areas of research logic: description of methodology, rationale of research, description of results, characterization and testing, overall conclusions, and description of discussion (Khizer et al., 2021).

The current risk of bias framework was adapted from the Cochrane Handbook for Systematic Reviews of Interventions (Tarsilla, 2010). Owing to the lack of a suitable risk assessment tool in this particular field, namely food safety, the evaluation process of each included study was carried out independently by two members of the review team. The final decision was that once any disagreements were resolved in the process, a categorization and binding agreement would be reached.

At the end of the work, a systematic review checklist was used to ensure accuracy of work process (Page et al., 2021).

Results and Discussion

Content extraction

The present paper considered a new field of research that focused on biological compounds derived from probiotics, with a limited number of available studies. The information for this study was extracted from 29 articles in line with the objectives of the present study. It should be noted that the results of this systematic review have limitations, as in some papers, it was not mentioned whether the strain in question was a probiotic or not.

Various factors, such as proving that LABs are probiotics, type of inactivation of microorganisms and extraction methods, and the exact nature of the consumed postbiotic (cell-free supernatant or cells killed by different physical and chemical treatments), were also considered. Therefore, in the future studies, it would be important to determine the aforementioned factors to obtain healthy and anti-toxic effects of the considered postbiotics.

Creation and isolation of postbiotics

As mentioned, probiotics must be alive to exert their health effects, but their viability has created limitations for use and treatment. In recent years, it has been shown that the viability of a probiotic is not related to its beneficial effects. Hence, the focus on the health benefits of microorganisms has shifted from live probiotics to postbiotics. According to different definitions, postbiotics include both nonviable bacteria and substances derived from bacterial metabolism. These side products provide specific physiological effects to host by providing more biological activity (Cicenia et al., 2016), and because postbiotics do not contain live microorganisms, the risks associated with their consumption are minimized. Furthermore, postbiotics can be used in a controlled and standardized manner (Shigwedha, 2014). The problem of dose standardization, which is a significant issue in probiotic production, does not exist in the case of postbiotics. Production methods have been investigated by researchers to produce postbiotics in a controlled, standardized, and efficient manner to enable their analysis and use in food, pharmaceuticals and nutritional programs. Although there are no clear international regulatory standards for postbiotics, in some countries, postbiotics have been included in their national regulations and their products have entered public domain (Gelmetti et al., 2022; Kiran et al., 2023). The use of postbiotics derived from probiotics is a specific, effective, environment-friendly, cheap and safe strategy. Since postbiotics have unique features, such as specific chemical structure, safe and long shelf life, they can be used in a delivery system, such as medicine or beneficial foods, to achieve wellness goals, prevention and treatment of diseases (Shigwedha, 2014; Tomar et al., 2015).

In the food industry, postbiotics are produced using natural and laboratory (physical and chemical) methods. One of the most important methods to produce postbiotics in a natural form is the fermentation process. In this, prebiotic compounds are rendered to microorganisms involved in the fermentation process to produce postbiotics (Izuddin et al., 2018). By using different laboratory methods, postbiotics are produced in a pure form with higher performance (Dunand et al., 2019). To inactivate and destroy bacterial cells, physical methods, such as mechanical breaking, heat, gamma, or ultraviolet radiation, high hydrostatic pressure and ultrasound, or chemical methods, such as inactivation by acid, are used (Fan et al., 2021). These methods change cellular structures and their physiological functions. Bacteria are destroyed by applying these methods and are no longer able to grow, but they still maintain their positive effects (de Almada et al., 2016; Rad et al., 2020b). In order to destroy and inactivate bacterial cells, extraction postbiotics by solvent and sonicator are also used (Amaretti et al., 2013). After this, steps such as centrifugation, dialysis and freeze-drying are also used to increase the efficiency of production and storage of these compounds (Amaretti et al., 2013). Despite the existence of various techniques to obtain postbiotics, it is essential to optimize culture environments and bacterial growth conditions along with using new methods of extraction and identification. After achieving optimal production conditions on a laboratory scale, optimization in industry is recommended to maximize production. Finally, appropriate dose and frequency of use are determined.

It is important to note that postbiotics, unlike probiotics, do not add new species to intestinal microbiota, but like prebiotics, they develop the existing probiotics in the digestive system (Homayouni-Rad et al., 2020, 2021). Therefore, it is necessary to carry out a comprehensive investigation for safety, optimization of different extraction methods, identification of the chemical composition of postbiotics, and the physiological role of postbiotics in the biological system to facilitate and use postbiotic compounds in food formulations.

The mechanism of action of postbiotics on the detoxification of aflatoxins

In the biological strategy, detoxification is done through a microbe or non-pathogenic enzymes and/or their catabolic pathways not only to reduce/remove but also to suppress toxins (Afshar et al., 2020). One of these non-pathogenic microbes is probiotics, considered as safe (GRAS) and having health benefits (Afshar et al., 2020). Probiotics can counteract the toxic effects of aflatoxins by the following three mechanisms: (1) Possibility of probiotics competing with aflatoxigenic mold strains to occupy space or an ecological niche, use available nutrients, and reduce the biosynthesis of aflatoxins (probiotics), (2) degradation of aflatoxins by enzymes (postbiotics), and (3) bind aflatoxins to the cell wall of probiotic strains and prevent its intestinal absorption (postbiotics) (Zhao et al., 2016). In general, postbiotics can be classified into different groups based on their biological structure (lipid, protein, carbohydrate, etc.) (Thorakkattu et al., 2022). Different types of postbiotics and their action in detoxification of aflatoxins are shown in Table 2.

Some studies reported that probiotics can produce metabolites, such as organic acids, bacteriocins and even hydrogen peroxide, that inhibit production of aflatoxins and also interfere in the biosynthesis of aflatoxins (Gerbaldo et al., 2012; Gourama and Bullerman 1995).

Probiotics can even remove the produced aflatoxins using biodegradation or biosorption mechanisms (Afshar et al., 2020). For removal of aflatoxins, biodegradation is a permanent process but takes a longer period than biological absorption. Biodegradation is done by extracellular or intracellular enzymes (Zhao et al., 2016). Aflatoxins are decomposed by different mechanisms, such as epoxidation, hydroxylation, dehydrogenation, and reduction, depending on the decomposing agent (Martinez et al., 2019). Double furan ring and coumarin structure are common in the chemical structure of aflatoxins (Adebo et al., 2017; Dalié et al., 2010). During microbial degradation, microbial catabolic pathways and specific enzymes present in cell-free supernatant influence coumarin conformational changes and difuran ring cleavage in aflatoxins (Adebo et al., 2017; Dalié et al., 2010).

Degradation or enzymatic conversion is the biological breakdown of mycotoxins into less or nontoxic compounds; only certain cells of bacteria and fungi are effective in this change (Chlebicz and Śliżewska 2020). In biodegradation, aflatoxins are converted into other undesirable metabolites, such as aflatoxicol (AFL), which is possibly harmful to health (Ji et al., 2016). Therefore, it appears that biological absorption is a more suitable method than biological degradation.

Biological absorption is a short-term process, and the formed complex is unstable and depends on the number of absorption sites and the affinity of aflatoxins to these sites (Adebo et al., 2017; Afshar et al., 2020; Luo et al., 2018). In biological absorption, an unspecified physical connection to the cell wall takes place, and aflatoxins are removed by physical adhesion. Aflatoxins bind to the carbohydrate components of the cell wall. Covalent bindings or metabolic degradation is not used in this mechanism. Therefore, nonviable bacteria also can bind and remove aflatoxin (Bueno et al., 2007; Hernandez-Mendoza et al., 2010).

Probiotic bacteria, similar to other Gram-positive bacteria, have a complex arrangement of macromolecules. The wall with a special structure contains compounds such as peptidoglycan (PG) sacculus that surround cytoplasmic membrane and other glycopolymers, such as teichoic acids (TAs) or polysaccharides (PSs) and proteins that decorate PG. Each of these compounds has a different function, for example, the TA unit is divided into two groups: (1) wall teichoic acids (WTAs), which are covalently linked to the PG molecule and (2) lipoteichoic acids (LTAs), which are anchored in cytoplasmic membrane with a glycolipid moiety (Chapot-Chartier and Kulakauskas 2014). WTA composition may constitute half of the total dry weight of the cell wall in certain bacterial species and has high hydrophobicity ( Chapot-Chartier and Kulakauskas 2014; Swoboda et al., 2010).

Exopolysaccharide) EPS(, teichoic or lipoteichoic acid, and peptidoglycan are the three main forms of carbohydrates present in the cell wall. As reported by Lahtinen (2004), there was no evidence of the involvement of EPS, cell wall proteins, Ca²⁺, or Mg²⁺ in AFB₁ binding. The study confirmed that EPS is rejected as a possible component of binding AFB1, and cell wall peptidoglycan appeared as the most likely carbohydrate component effective in the binding process (Lahtinen et al., 2004). Teichoic acid also has a little effect on binding of mycotoxin (Zhao et al., 2016).

Aflatoxins bind to bacterial cell wall through weak non-covalent interactions, which occur with hydrophobic pockets on its surfaces, such as peptidoglycan and other cell wall polysaccharides, or their compounds that are strongly associated with bacterial peptidoglycan (Figure 4) (Fouad et al., 2021). These non-covalent interactions include van der Waals interaction, electrostatic interactions, and hydrogen bonds. Van der Waals interaction was weaker, compared to hydrogen bonds, but since their large numbers maintain the stability of molecular structure, they play a major role in AFB₁ binding (Fouad et al., 2021). Hydrogen bonds account for a small fraction of binding energy; however, hydrogen bonding may be important in the binding of aflatoxins ( Haskard et al., 2001; Lahtinen et al., 2004).

Figure 4. Schematic mechanism of aflatoxin detoxification in the cell wall of Gram-positive bacteria, such as probiotics.

With the two parameters mentioned below, the efficiency of a microorganism as an aflatoxin binder can be determined using the following formula:

Efficiency = (M × Keq),

where M is the number of binding sites in the cell wall of the microorganism, which is different in each microorganism, and Keq is the equilibrium constant of the reaction (Bueno et al., 2007).

According to the above-mentioned formula, differences in the structure of the cell wall and cell membrane of bacteria lead to differences between strains in the absorption of aflatoxins ( Kosztik et al., 2020; Pop et al., 2022; Serrano-Niño et al., 2013).

However, it seems that more studies are needed to identify the precise structures of the cell wall responsible for binding mechanism. Therefore, after recognizing the involved components, their functions are defined in vivo experiments and can be used as a method to detoxify food and feed (Lahtinen et al., 2004).

Necessary conditions to increase the removal of aflatoxins in postbiotics

In addition to the strain, some experimental factors also affect the capacity of aflatoxin removal by probiotics. Since the viability of probiotics at low pH conditions is reduced by passing through the stomach, the ability of nonviable probiotics to detoxify aflatoxins is an important topic to explore (Hamidi et al., 2013; Topcu et al., 2010). It has been shown that bacterial viability is not a prerequisite for aflatoxin detoxification (Sarlak et al., 2017). Using killed probiotics is a safe method, because according to the literature, nonviable bacteria can remove more aflatoxin than live bacteria without causing any problems to humans (Vosough et al., 2013). Bovo et al. (2013), Kabak and Var (2008) and Pierides et al. (2000) found that by inactivating the lactic acid bacteria by heat, their ability to remove AFM₁ significantly increases in PBS and milk, compared to live bacteria. El-Nezami et al. (1998a) reported that heat-treated dairy strains had the same removal capacity as live strains. It appears that by applying heat treatment, polysaccharides and peptidoglycans are strongly affected, resulting in denaturation of proteins, and the hydrophobic nature of the surface increases, or Maillard reaction products are formed. These changes cause aflatoxins to bind to bacterial cell wall and plasmatic membrane components, which are not available if the cell wall remains intact (Haskard et al., 2001).

Acid treatment has also been shown to significantly increase the ability of bacteria to bind to AFB₁ (El-Nezami et al., 1998a; Peltonen et al., 2001). Haskard et al. (2001) also stated that in most cases, acid treatment is more effective than heat treatment. Several studies showed that acid treatment of yeast or LAB had the highest aflatoxin absorption, compared to live and heat-treated microorganisms (Haskard et al., 2001; Hegazy et al., 2011; Rahaei et al., 2010). A study conducted in 1998 by El-Nezami and colleagues (1998a) stated that acid pretreatment increases the binding ability of nonviable LABs. Acidic pretreatment may lead to the cleavage of amine bonds in peptides and proteins, yielding peptides and amino acids (El-Nezami, et al., 1998a). Haskard et al. (2001) reported that acid treatment may affect cell wall components, such as peptidoglycan and polysaccharide, releasing monomers and further fragmentation into aldehydes after breaking glycosidic bonds. Haskard et al. (2000) pointed out that acid-treated LAB may show more hydrophobic binding sites for aflatoxins due to protein denaturation, thus expecting hydrophobic interactions. Although the peptidoglycan layer in probiotics is very thick, the acid may reduce thickness and cross-links, leading to larger and more pores being available (Moghaddam et al., 2019). Finally, aflatoxins are easily and quickly bound by the components of cytoplasmic membrane under acidic conditions (Bejaoui et al., 2004).

Pretreatments of probiotic bacteria increases the efficiency of probiotics as adsorbents of aflatoxins because they cause protein denaturation, release some components and increase pore size, and, probably, affect change in charge distribution and hydrophobic nature of bacterial surface (Ahlberg et al., 2015; Karazhiyan et al., 2016). As a result, it allows aflatoxins to bind to the cell wall or its associated components because the available aflatoxin binding sites are increased (El-Nezami et al., 2002). Haskard et al. (2000) expected salts to affect the surface charge of the bacteria, followed by electrostatic interactions, and the effect of salt addition was greater for nonviable cells, as demonstrated by Balsini et al. (2021).

In one study, treatment with sodium dodecyl sulfate caused a significant increase in the ability of the studied strain to bind AFB₁. Under sodium dodecyl sulfate, the protein was denatured and the cell wall composed of peptidoglycan was separated (Maragos, 2008).

Treatments such as divalent cations (Ca²⁺ and Mg²⁺), chelators (EDTA and EGTA) as well as ultrasound and enzyme treatments, including various proteases, were ineffective to bind aflatoxins (Lahtinen et al., 2004).

Yeasts have been shown to have a mechanism similar to probiotic bacteria in binding AFB₁ as a tool for detoxification (Shetty and Jespersen, 2006; Wu et al., 2009). The cell walls of probiotic yeasts (e.g., Saccharomyces cerevisiae) also contain polysaccharides, such as glucan and mannan, which provide different mechanisms to bind with toxins (ionic, hydrogen, and hydrophobic bonds) (Saeidi and Safari, 2010). In probiotic yeasts, heat causes the denaturation of proteins or the formation of Maillard reaction products in the cell wall. By dissolving some of the mannoproteins in the cell wall, the permeability of the wall increases and leads to increased access to the hidden places of the cell wall as observed in probiotic bacteria. In addition, acid conditions act by affecting polysaccharides, converting monomers and breaking them into aldehydes, and some of the reactions may be intracellular in these acidic conditions (Karazhiyan et al., 2016; Oliveira et al., 2013; Rahaie et al., 2010). As a result, aflatoxin are easily and rapidly bound by yeasts under acidic or thermal treatment (Zhong et al., 2022).

Thermal treatments are the main and most common methods for preparing postbiotics, and these include pasteurization, sterilization, Ohmic heating, radio frequency heating and microwave heating. The most common deactivation mechanism of these methods is the thermal effect, but due to different modes of heat generation, they show different deactivation efficiencies. Thermal treatments are low-cost, simple, and common methods for microbial inactivation, but they can cause irreparable damage to some physiologically active substances in microbial cells, such as proteins and polypeptides. Therefore, it is necessary to use more efficient and practical microbial inactivation methods, such as nonthermal treatments (Zhong et al., 2022). In general, heat treatments mainly damage the cell membrane, break the nucleic acid, inactivate essential enzymes, denature protein and polymerizes such as the ribosome. However, the choice of temperature and time for postbiotic preparation depends on the type of microorganism, culture conditions, growth stage, PH and water activity (Zhong et al., 2022). Some microorganisms inactivated by heat treatment showed various beneficial effects for aflatoxin removal (Abdelmotilib et al., 2018; Montaseri et al., 2014).

Removal and release of aflatoxin is a fast and reversible physical process without any chemical change in aflatoxins. The amount of toxin that is removed depends on various factors, such as probiotic/postbiotic strain concentration, treatment time, amount of cell wall components obtained from viable or nonviable bacteria, toxin characteristics, such as toxin concentration and toxin type, and pH of the environment (Ahlberg et al., 2015; Pfliegler et al., 2015). For example, it appears that 10⁸–10⁹ CFU/mL of viable or nonviable probiotic LAB are required to achieve a 50% aflatoxin removal (Bovo et al., 2013; El-Nezami et al., 1998a). Maximum and minimum binding capacity was observed at 10¹⁰ CFU/mL and 10⁷ CFU/mL, respectively (Ismail et al., 2017). Maximum binding occurs within the first few minutes of the reaction. The quick connection process is between 0 and 4 h. When the maximum amount of aflatoxins was removed by probiotics strains, no further reduction occurred. Lee et al. (2003) reported that more the aflatoxin is adsorbed by bacterial cells, more the time the adsorbed aflatoxin molecules would remain on the cell surface of bacteria (Lee et al., 2003). In some studies, it was shown that the mixture of probiotics, bacteria or bacteria with yeast had a better effect in reducing aflatoxins (Florina et al., 2018; Mokoena et al., 2006; Okeke et al., 2015). Owing to different food matrices, physicochemical characteristics, and treatment conditions, the rate of aflatoxin reduction is different.

Probiotic strains or postbiotics can bind with aflatoxins and form a probiotic–aflatoxin or postbiotic–aflatoxin complex. The stability of probiotic/postbiotic and aflatoxin complexes depends on strain, treatment and environmental conditions. Since the binding of aflatoxin to probiotics/postbiotics is weak and partially reversible (Peltonen et al., 2001; Pizzolitto et al., 2012; Topcu et al., 2010), in vivo experiments appear as necessary, because aflatoxins may be released from the gastrointestinal tract, resulting in negative health consequences. The possibility of reversibility is 27.8–94.4%, depending on the strain (Moghaddam et al., 2019). It is determined that LAB shows a significant adhesion to intestinal cells, which lose this characteristic by binding to aflatoxins, so the bacteria–-aflatoxin complex is quickly excreted in the digestive system (Gratz et al., 2010).

As mentioned in the detoxification of aflatoxins by probiotics/postbiotics, the process depends on the strain, irrespective of viable or nonviable probiotic (Topcu et al., 2010). The studies conducted on the anti-aflatoxin effect of postbiotics included B₁, B₂, G_1,and G₂, the most effective of which are listed in Table 3.

Summarizing the previous table, postbiotics derived from probiotics can remove aflatoxins with a mean of 80%. Based on our review, studies have been done more on AFB₁, followed by AFM₁, probably because of the greater importance of these two aflatoxins. Strain L. rhamnosus (GG) has been mostly used in the literature, and studies have obtained different results, which could be due to food matrix, type of medium, physicochemical characteristics, and different treatment conditions. Among the food items, milk and dairy products are used maximum.

As already reported, after comparing the studies, we concluded that the acid-treated strain was more effective than the heat-treated strain. We also found that cell-free supernatant was very effective in removing aflatoxins. In fact, cell-free supernatant contains active ingredients and bacteriocin (Tenea and Barrigas, 2018), and probably the presence of several compounds makes it effective. It seems that the simultaneous use of several postbiotics derived from different strains can remove aflatoxins in large amounts, resulting in food safety. The result obtained from this study supports the development of future studies for the formulation of biological additives that could be used in functional foods.

Probiotics, prebiotics, synbiotics and postbiotics are novel concepts. However, biological detoxification technology is currently far from complete, and the determination and purification of metabolites is incomplete in many cases. In the future studies, the determination of various factors, such as the proof of LAB being probiotic, types of inactivation of microorganisms and extraction methods, and the exact nature of postbiotic (cell-free supernatant or cells killed by different physical and chemical treatments), to identify health effects and toxicity are essential. More studies are necessary to accurately identify and understand complex relationships between these concepts; this would help to understand the pathway of their mechanism of action. In addition, more research is needed to reveal the dose, time of detoxification, and usage of new preparations to maximize prevention and beneficial effects to reduce toxins on humans. In addition, in vivo, systematic review studies are also needed to support the health claims of postbiotic functional foods. At the same time, a big challenge exists, such as transferring scientific knowledge to business applications, which are a bridge between science and industry. Therefore, it would take a long time to develop biological additives to realize large-scale industrialization (Saeidi and Safari, 2010).

Author contributions

S. Zamanpour and B. Shokrollahi Yancheshmeh independently searched databases and extracted data individually. Any disagreement was resolved by consulting SM Noori, M. Hashemi and A. Afshari. Finally, the full text was written by S. Zamanpour, B. Shokrollahi Yancheshmeh, M. Hashemi and A. Afshari. Text editing was done by SM Noori.