The Study Of Lactobacillus Acidophilus NX2-6 Fermenting Food-Based Media And Its Production On Food Preservation

Nowadays, consumers pay a lot of attention to the relation between food and health. So the considerable developments of food preservatives have brought about the prevalence of studying the safe and natural antibacterial agents to control undesirable bacteria in food. Antibacterial substances producing by Lactic acid bacteria (LAB) fermenting food, directly used in foods can not only inhibit the growth of spoilage bacteria and pathogens, but also enrich nutrients of food.Based on preliminary study regarding Lactobacillus acidophilus NX2-6, it produced antibacterial substances as one food preservative may be directly used as a new food preservative. The study of a food-based medium was carried out based on the optimization of the improved tomato juice agar (modified TJA) to lead L. acidophilus NX2-6 produce antibacterial substances. Then the application was carried out by researching the preservative effect of its food-based fermentate on chilled pork patties. At the meantime, a further study was carried out based on the low concentration of skim-milk. All these studies provided scientific bases for the application of the antibacterial substances from L. acidophilus NX2-6 in the food preservation. The main results of these studies are as follows:1. Optimization of a food-based medium for L. acidophilus NX2-6 producing antibacterial substances based on the modified TJA medium. By reducing factors of the modified TJA medium, it screened out that the production of antibacterial substances of L. acidophilus NX2-6 required for lactose, tomato juice and yeast extract. Then the medium components were improved and the medium of tomato juice, skim milk and beer turned out to have highest antibacterial activity against Escherichia coli ATCC25922. Then response surface methodology (RSM) of a central composite design (CCD) was used to optimize levels of these three components in the medium. The result showed that the optimal concentrations were determined as:tomato juice 92.5 mL/L, beer 208.5 mL/L and skim-milk 300.3 g/L, and the diameter was 19.50 mm. The diameter of inhibition zones of validation tests was up to 19.78±0.23 mm. At the same time, the antibacterial spectrum of cell-free supernatant was board, which could not only inhibit food spoilage bacteria such as E. coli, Bacillus and Pseudomonas, but also inhibit foodborne pathogens like Staphylococcus aureus, Enterobacter sakazakii, Salmonella and Listeria monocytogenes. Meanwhile, the minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) against Bacillus cereus AS 1.1846 and B. pumilus CMCC(B) 63202 in Brain heart infucion (BHI) broth were both 6.25%(v/v) and 12.50%, respectively; while in Luria-Bertani (LB) medium and nutrient broth (NB) medium the MICs were 3.13% and 1.56%, and MBCs were both 6.25%.2. The preservative effect of food-based fermentate from L. acidophilus NX2-6 on chilled pork patties. Experiments were conducted to assess its application as a preservative in pork patties. The effect of freeze-dried FBF on the microbiological parameters, physicochemical changes, and sensory evaluations of chilled pork patties stored for 15 days at 4℃ was investigated. The five treatments evaluated included a control (meat only), nisin (meat plus 0.5% nisin), L.I (meat plus 2% freeze-dried FBF), L.2 (meat plus 4% freeze-dried FBF), and L.3 (meat plus 8% freezedried FBF). The results showed that freeze-dried FBF could significantly (P< 0.05) inhibit aerobic bacteria, coliforms, Pseudomonas spp., and lactic acid bacteria, with the lowest microbial counts observed in L.3. The addition of freeze-dried FBF resulted in concentration-dependent decreases in total volatile basic nitrogen values and pH values but increases in lipid oxidation and color instability. Based on the criteria regarding microbiological and physicochemical parameters, the shelf life was 9 to 12 days for L.1,12 to 15 days for L.2, and over 15 days for L.3, while the shelf-lives of the control and nisin treatments were 3 to 6 days, indicating that freeze-dried FBF could extend the shelf life by more than 3 days. Although the shelf life of L.1 was shorter than those of L.2 and L.3, the appearance of L.1 was much better than those of L.2 and L.3. Overall, treatment with 4 or 8% freeze-dried FBF could be improved if color and lipid oxidation could be improved by appropriate stabilizers, and a lower concentration (2%) of freeze-dried FBF has great potential as a natural and safe preservative in chilled pork patties.3. Optimization of a dairy medium for L. acidophilus NX2-6 producing antibacterial metabolic products and its preservative effect on pasteurized milk. The results of lots of experiments have showed that the skim-milk was a significant effective factor for L. acidophilus NX2-6 to produce antibacterial substances. Based on 50.0 g/L skim-milk, the single factor experiment, Plackett-Burman (PB) test and CCD experiment was used to optimized the medium. The components of the medium was:lactose 20.0 g/L, soybean 90.9 g/L, sodium acetate 22.4 g/L, Tween-802.5 g/L, tomato juice 10.0 mL/L, glucose 10.0 g/L, heptahydrate magnesium sulfate 1.0 g/L and skim-milk 50.0 g/L, and the diameter was 23.83 mm. At the same time, the antibacterial spectrum of cell-free supernatant was board, which could not only inhibit food spoilage bacteria such as E. coli, Bacillus and Pseudomonas, but also inhibit foodborne pathogens like Staphylococcus aureus, Enterobacter sakazakii, Salmonella and Listeria monocytogenes. The diameter of inhibition zones of validation tests was up to 22.75±0.35 mm. The antibacterial activity kept stayable under the treatment of different temperatures and the different times of UV radiation, but it influenced by changing of pH. When pH changed from 2.0 to 4.0, the inhibition changed significantly. At pH 5.0, the antibacterial activity showed only in four strains, Salmonella paratyphi-A CICC21501, S. enterica subsp. Enterica CMCC50071, B. cereus AS 1.1846 and P. fluorescens AS 1.1802. The antibacterial spectrum of cell-free supernatant was board. The inhibitory activity of antibacterial substances from L. acidophilus NX2-6 did not changed because of varied of the temperature and the exist of UV radiation.4% fermentation added in the pasteurized milk can extend the preservation period of 2 to 5 times.