Abstract
An antioxidant is a substance that inhibits the oxidation of other molecules caused by free radicals. The inbuilt antioxidant systems possessed by living organisms are generally not enough to prevent them from oxidative damages and the uses of synthetic antioxidants also have some harmful effects. This study was aimed at evaluating the antioxidant activities of exopolysaccharides produced by lactic acid bacteria isolated from yoghurt. Lactic acid bacteria (LAB) were isolated from six different brands of commercially available yoghurt using deMan Rogosa Sharpe (MRS) agar. The LAB isolates were identified based on morphological and biochemical analyses and were screened for exopolysaccharide (EPS) production. The LAB isolates screened positive were used for EPS production in a liquid medium and the EPS produced were purified and quantified using standard methods. Antioxidant activities of the EPS were evaluated by determining the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity, ferric ion reducing power, and total phenolic contents. Data obtained were analysed using Analysis of Variance. Total lactic acid bacterial count obtained from the yoghurt samples ranged from 0 - 3.9 × 104 CFU/mL with sample A (Fan Yoghurt) having the highest LAB count (3.9 × 104 CFU/mL). The isolated LAB and their incidence rate were Lactobacillus plantarum (25.49%), L. delbrueckii (19.61%), L. fermentum (15.69%), L. acidophilus (13.73%), Leuconostoc mesenteroides (11.76%), Lactococcus lactis (7.84%), and Lactobacillus casei (5.88%). Fifty-one out of the 64 LAB isolates were screened positive for EPS production and only six were able to produce substantial quantity of EPS ranging from 127.4 - 208.5 mg/L. The exopolysaccharides produced by L. fermentum had the highest DPPH radical scavenging activity (62.90%) while that of L. plantarum had the lowest (23.10%) at a concentration of 1000 μg/mL. Also, the EPS produced by L. fermentum recorded the highest ferric ion reducing power (12.89 mg AAE/mL) at 1000 μg/mL while that of L. plantarum had the lowest (5.62 mg AAE/mL). At 1000 μg/mL, the total phenolic contents of the EPS samples ranged from 1.41 - 1.58 mg GAE/mL, and the EPS produced by L. fermentum had the highest (1.58 mg GAE/mL) while those produced by L. paracasei had the lowest (1.41 mg GAE/mL). This study revealed that the exopolysaccharides produced by the LAB isolates showed high antioxidant activities with respect to their DPPH free radical scavenging activity, ferric ion reducing power and total phenolic contents.
Highlights
Yogurt is among the most common dairy products consumed around the world [1]
This result is in agreement with Ishola and Adebayo-Tayo [15] who reported that L. plantarum was predominant among the lactic acid bacteria isolated from yoghurt and other fermented dairy products such as Nono, Fura and Wara
This study revealed that some commercial yoghurts have lactic acid bacteria while some did not have
Summary
Yogurt is among the most common dairy products consumed around the world [1]. It is mainly obtained from fermentation of fresh milk or reconstituted milk with lactic acid bacteria (LAB) [2]. Yogurt is considered a probiotic because it contains live microorganisms (usually LAB) that provide significant nutritional and therapeutic values. It stabilizes gut microflora, produces antimicrobial compounds, reduces serum cholesterol and stimulates the immune system [3]. Lactic acid bacteria (LAB) are widespread in nature and are found primarily in the environments where there are high level of carbohydrates, peptides, amino acids and vitamins. They are Generally Recognized as Safe (GRAS) bacteria that have been used in the food industry for centuries [4]. Lactic acid bacteria are able to produce exopolysaccharides in the surrounding medium as a slime or on the surface of bacterial cells to form a capsule [6]
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