Microbiological analysis and the antioxidant capacity of edible biofilms enclosing Bacillus subtilis

Abstract

The aim of this research was to design edible biofilms on a basic matrix of sodium alginate (Al) and other hydrocolloids in which different proportions of onion extract (OE), yeast extract (YE) and Bacillus subtilis (B. subtilis) was introduced. These biofilms were evaluated from microbiological and antioxidant perspective. The EC50 (mg sample/ml DPPH solution 6×10-5 mol/L) value was calculated, representing the active compound concentration which inactivates 50% of the total radicals DPPH. For the YE the EC50 value resulted 0.88 mg/ml and for the OE resulted with 30% smaller (0.60 mg/ml). The microbiological study targeted the influence of the OE and YE on cell viability of B. subtilis used as inoculum. A standardized inoculum of B. subtilis with a concentration of more than 1011 spores/g was used and a base culture of Luria Bertani Agar, Miller. After 48 hours of thermostating at 37° C the number of CFU/g resulted over 1010 in all experimental variations. A quantitative evaluation was made through indirect culture methods for a period of 8 days time in which the samples were preserved in refrigerated conditions at 40 C. After analyzing the data we observed a constant behavior of the probiotic bacteria B. subtilis through all the conservation period. The B. subtilis + OE sample presented a reduction of CFU/g with one unit after 24 hours until the value of 9.85×107 CFU/g was reached most probably due to the OE which has an antimicrobial effect. The best behavior over the preservation period was observed in case of B. subtilis + YE (4.51×108 CFU/g) sample as we consider due to the YE which offers a good source of nitrogen and vitamin content that is necessary to maintain the viability of B. subtilis cells. Similar values have been obtained for B. subtilis + OE + YE (3.57×10 8 CFU/g) with both extracts included onion and yeast that manifest a prebiotic effect for the bacterial cells. This study showed that obtaining Al based films and incorporating distinct active ingredients with unique properties provide a „clean” solution and a future reference to healthy packaging materials. An edible biofilm which contains a mixture of YE, OE and a number of more than 108 CFU/g probiotic bacteria even after 8 days in refrigerated conditions can be considered as an efficient solution of bio conservation and more than that could manifest a nutraceutical effect for consumer’s immune system stimulation.