APPLICATIONS OF NON-BACTERIOCINOGENIC LAB AS PROTECTIVE CULTURE
During bacterial fermentation, organic acids are produced along with other metabolites that are also produced. The fermentation of organic acids viz., lactic, acetic, and propionic acids lower the pH of the food matrix, which plays a vital role in food preservation. Other antifungal metabolites are: H,02, reuterin, phenyllactic, and hydroxyphenyllactic acids, diacetyl, acetoin, and fatty acids. On the other hand, LAB producing biocompounds other than bacteriocins, which exhibit synergistic effect, have been explored as non- bacteriocinogenic PC . Fungal growth in foods is associated with the production of volatile compounds, bad odor, and release of mycotoxins, thus antifungal PC have gained importance. Sorbates, propionates, and benzoates have been used to overcome the problem of fungi, nevertheless, their usage is recently frowned upon by the consumers. Thus as an alternative antifungal protective agent, LAB has great potential to produce preservative-free foods without compromising shelf-life [5, 35].
Many researchers have isolated antifungal strains from different niches including fermented foods and have reported antifungal activity in yogurt, cheese, and bakeiy products. Lactobacillus species were evaluated with LAB in foods as antifungal PC and there are promising applications in dairy products . For example, strains belonging to Lb. alimentarius, Lb. sakei, Lb. fermentum, Lb. paracasei or Lb. casei species demonstrated good inhibition of Penicillium, Kluveromyces, Rhizopus, and Yersinia spp. in yogurt . Use of Lb. reuteri, Lb. plantartini and Lb. rlianmosus strains in Cheddar and cottage cheese resulted in inhibition of Penicillium spp [21, 35]. The Lb. sakei, Lb. piantarum, Lb. spicheri, Lb. brevis and Leu. Citrium have shown in situ antifungal activity in bakeiy products, out of 270 LAB strains assessed for antifungal activity in vitro against Penicillium, Cladosporium, and Wallemia spp. .
Aflatoxins are used as a food safety hazard produced by Aspergillus spp. and the major ones are Bl, B2, Gl, G2; and Ml is considered as a human carcinogen. The use of protective culture is an attractive preventive approach to manage the risk of aflatoxins in foods . Mainly Lactobacillus, Bifidobacterium, and Propionibacterium are reported to have the ability to bind Aflatoxins. The aflatoxin Bl in contaminated wheat flour during the bread-making process was found to be reduced by fermentation with yeast and Lb. rhamnosus .
The application of LAB caused 84-100% reduction of aflatoxin in bread with the extension of shelf-life up to 4 days . While the addition of Lb. piantarum resulted in a decline of aflatoxin Bl concentration from 11 to
5.9 pg/kg during the storage of olives . Similar observations have been noticed in milk S. cerevisiae and a pool of LAB strains 100% binding of aflatoxin Ml in milk for 60 minutes . Lb piantarum was the highest aflatoxin Ml binding strain in yogurt during storage when used with Str. thermophilus and Lb. bulgaricus . The aflatoxin binding ability of LAB is associated with factors, such as : bacteria strain, food matrix, storage temperature and the storage period, etc. Furthermore, the binding is a surface phenomenon that is strongly correlated with the involvement of various metabolites released by LAB.
The mechanism of antifungal action of LAB is not well-known as compared to antibacterial peptides. Antifungal principle of LAB in foods may be attributed to acidification (lowering of pH), production of antifungal metabolites (antibiosis), and competition for specific nutrients . However, the mechanism of antifungal action of a strain may not solely depend on the concentration of one type of antimicrobial. For instance, organic acids can diffuse into the cell through the cell membrane and can dissociate inside causing reduction of intracellular pH, disruption of membrane function leading to cell death. In addition, complex interactions between numerous metabolites released by LAB are believed to exert combined stress on fungal physiology with synergistic action causing greater inhibition. The interactive effect between the metabolites confirms the complexity of the antifungal mechanism .
Several methods have been proposed for optimization of antifungal activity in food products to enhance of production of antifungal metabolites and broaden antifungal spectrum :
- • Inducation of stress conditions to cultures;
- • Use of co-cultures of antifungal strains;
- • Supplementation with precursors, which trigger biosynthesis of metabolites (e.g., glycerol);
- • Combination with other molecules, such as chitosan.
Even though so many antifungal strains have been identified, yet only a few cultures are made commercially available for utilization in food products.
PROTECTIVE CULTURES (PC) IN HURDLE TECHNOLOGY
Combinations of cultures for in situ production of bacteriocins and other hurdles have also been studied mainly to control L. monocytogenes or S. aureus in foods. Bacteriophage PI00 and Lb. sakei together was successfully applied to prevent the progression of L. monocytogenes on cooked ham. This combined approach can also be a beneficial tactic to reduce the emergence of resistant pathogens .
The effectiveness of the combination is dependent on the product and target bacteria. Several authors have studied to develop PC to combine with other hurdles in food products. The successful combination should act synergistically, and the effect of the combination is more than an independent application . The protective culture-based combination has shown to increase the effectiveness of HHP, active packaging, modified atmosphere packaging (MAP), and super chilling against pathogens and spoilage bacteria . However, the selection of PC having compatibility with other hurdles need to be evaluated carefully.