study the effect oflactobacillus plantarumstrains isolated fromdifferent stages of productionlighvan cheese on rodotorulamucilaginosa as a spoilage indicator in fruit juice

introduction : nowadays, attention to foods free from chemical preservatives is on the rise. recently, consumers have concerned about foods containing these preservatives in their formulation .therefore,the use of antimicrobial peptides produced by lactic acid bacteria (lab) are strongly highlighted. ribosomally-synthesized peptides, which possess antimicrobial properties, are produced by a vast range of organisms including prokaryotes and eukaryotes. yeasts are the most important microorganisms which are responsible for fruit juice and soft drink spoilage. in case of growth and production of by-products like co2, acid and other contaminants, the spoilage will be visible. lactic acid bacteria isolated from natural, local sources such as dairy products have presented potentially antifungal features against food spoilage fungi. among these, rodotorula and penicillium are regarded as the most critical genera in fruit juice spoilage. the main objective ofthis study was the evaluation of anti-yeast activity of lactobacillus plantarum isolates from different stages of lighvan production against rodotorulamucilaginosa as fruit juice spoilage indicator. in the next step, technological parameters effects were analyzed on antimicrobial potential of cell-free supernatant of these isolates. finally, we aimed at finding the most effective isolate on aforementioned yeast. materials and methods: nineteenlb.plantarumisolates, which were identified previously, were subjected to antifungal assay. for this purpose, rodotorulamucilaginosaptcc 5257 was selected. agar spot and well diffusion assay (wda) were applied for antifungal assay in solid and liquid media, respectively.determination of yeast colony: following the cultivation of yeasts in potato dextrose broth, it was determined using spectrophotometer. preparation of lb.plantarum cell-free supernatant (cfs) was carried out. in agar spot method, clear zones of inhibition around the spotted colonies were evaluated after 24-48h incubation. in wda, cfs of lb.plantarum isolates were poured in wells and clear zones were evaluated around each well after 24-48 h incubation. technological properties: the influence of different levels of ph (2, 3, 4, 5, 6, and 7) was analyzed on cfs of lb.plantarum isolates. this assay was done according to wang et al., 2011. finally, results were reported using the measurement of clear zone diameter in mm. all experiments were performed in duplicates. the effects of various temperatures were applied on cfs and remaining the antifungal activity was evaluated according to rouse et al., 2008. finally, all cfs of lb.plantarum isolates were subjected to proteniase k and antifungal properties of cfs were assayed by wda according to ghrairi et al., 2005. results and discussion:agar spot results showed the highest and lowest clear zone of inhibition related to c28 and lf49 with 16 and 6 mm diameter, respectively. in this method, 11 out of 19 isolates (60%) presented anti-yeast activity with clear zone formation. in comparison between two incubation temperatures (25 and 30◦c), all isolates stronger clear zone in 30◦c than in 25◦c. this was due to enhancement of yeast growth in 25c rather than in 30c. also, with respect to the mesophilic nature of lb.plantarum isolates, the possibility of metabolite production are more likely in 30c. it was reported that antifungal activity of lab is mostly due to synergistic effect of lactic acid and acetic acid. in agar spot, some colony-associated antimicrobial compounds are responsible for antifungal activity. in wda, 8 out of 19 isolates (42%) were positive for their inhibitory effects. the highest anti-yeast activity was seen at ph=2. it seems that antimicrobial compounds are likely more stable at acidic conditions than at alkaline ones. among isolates, c28 presented the highest stability at alkaline conditions. with ph increase, the antifungal activity decreased so that no anti-yeast activity was seen at ph=7. regarding the different temperatures, we should mention that thermal resistance of isolates' cfswitnessed declining trend with increasing of temperature. this fact implies the presence of low- molecular weight compounds in cfs. finally, all isolates' cfs was subjected to proteinase k. all isolates have lost their anti-yeast activity after enzyme treatment showing their proteinaceous nature. conclusion: in wda, the number of positive isolates showing anti-yeast activity declined in comparison with agar spot. since some isolates retain their inhibitory activity toward food spoilage yeast at low ph, their cfs can be applied in acidic foods like fruit juice. also, some isolates showed their antifungal activity at high temperatures (80c and 100c) which are applied for fruit juice pasteurization, so they can be applied in fruit juices as a bio-preservative.