Diversity Analysis And Cleaning Methods Of Mycoflora On Raisin Surface

Raisin is dried grape obtained by drying and other processing technology.Raisin is popular snacks for its pleasant favor,antioxidant activity and various nutrients.However,raisins are easily contaminated with microbes due to environments of processing,storage and transportation.Raisin a_w is generally controlled at 0.55,under which the microorganism cannot grow.If the processing and storage conditions are not suitable,and when a _W is higher than 0.64,microbial pollution should be occur.A large number of fungal spores are always existed raisins;the spores could germinate when mixed with other food materials with high water activity.The characterization study of mold-contamination and the cleaning methods on raisins surface is great significance for raisins production and consumption.Two kinds of raisins,i.e.,Blackcurrant from Uzbekistan and Thompson Seedless from Xinjiang of China were selected as the materials.The communities of filamentous fungi of raisins were analyzed by high-throughput sequencing and cultural method.The representative fungal strains isolated from raisin surface were identified by morphological and transcriptional spacer sequences（ITS）and physiological characterization.Meanwhile,the toxigenic activity was further examined for Aspergillus strains.After that,the cleaning effect under different ultrasonic cleaning conditions and cleaning solution were tested for raisins.High-throughput sequencing showed that the Aspergillus spp.was the dominant genus of the raisin surface,more than 70%（Thompson Seedless）and 95%（blackcurrant）Aspergillus strains were found in the total flora.In addition,Starmerella spp.Issetchenkia spp.and other yeast fungi accounted for about 10-20%of the total flora in Xinjiang,and Penicillus spp.accounted for about 4%of the total flora in Uzbekistan.Three representative isolates Z1,Z2 and Z3 were selected from the cultured strains.Physiological characterization showed that the optimal temperature of Z1 was 28-30℃,the optimum temperature of Z2 and Z3 is 25-30℃,the three strains did not grow at 4℃.The optimum pH of Z1 is 5,strain Z2 did grow at pH 3,and strain Z3 showed wide adaptation under different pHs.Three strains showed water activity（a_w）ranged from 0.99 to 0.87.Combined with the morphological,molecular and physiological results,Z1 was recognized as Talaromyces sp.,Z2 as Aspergillus Niger,and Z3 as Aspergillus flavus.Toxigenic test showed Aspergillus niger Z2 did not produce ochratoxin,while Aspergillus flavus Z3produced aflatoxin AFB₁,but the activity was weak（0.64μg/kg）.The cleaning results showed that ultrasonic cleaning was better than ordinary cleaning,and the best cleaning time was about 5 min;in multriple-short time cleaning method,4 times cleaning with 90 s in each cleaning time was the best choice.The best material-liquid ratio should be 1:10（g/mL）,the optimal antibacterial ethanol concentration was 10%.However,the cleaning effect of 0.1%unacidified sodium chlorite was better than that of 10% ethanol（increased by 5%）.