Production of exopolysaccharides in a Kenyan fermented milk, Mursik

Abstract

The aim of this research was to establish whether Mursik, a Kenyan traditional fermented milk product, contained Exopolysaccharides (EPS) and EPS producing Lactic acid Bacteria (LAB). It also sought to establish the optimum conditions for their production in Mursik. Samples of Mursik were collected from 39 randomly selected homesteads in Njoro District, Nakuru County, Kenya, and transported to Egerton University for analysis. Their EPS levels were determined by isolating them and quantifying by phenol-sulphuric acid method. EPS producing LAB were screened using sucrose agar and identified by phenotypic and biochemical methods. Optimum incubation period for EPS production in traditional Mursik was determined by fermenting Mursik in gourds and determining their EPS levels after every 24 hours for 5 days. The effect of incubation period and growth medium on EPS yield by Mursik culture were determined by propagating it in four different growth media, Reconstituted skim milk (RSM), RSM with 5% sucrose (RSM-S), Whey (WH) and WH with 5% sucrose (WH-S) at 20 ± 1ºC and determining their EPS levels after every 6 hrs for 30 hours. The data obtained was used to compute for analysis of variance (ANOVA) and means separated by least significance difference (LSD). The study established that the EPS levels in traditional Mursik differed with households at 5% level of significance (P< 0.05), and ranged between 17.86 - 59.77 mg/L glucose equivalent (Glu.equiv), the mean being 44.83 mg/L Glu.equiv. A total of 122 EPS producing LAB were isolated and characterized into five genera namely, Lactococcus (76%), Lactobacillus (11%), Enterococcus (7%), Leuconostoc (5%) and Weissella (1%). Their EPS yields ranged between 44.3 and 449.9 mg/L Glu.equiv. EPS production during traditional fermentation of Mursik was not growth associated and its optimum incubation period was 4 days (mean yield being 73 mg/L Gluc.Equiv). The EPS yields by the Mursik culture in the four growth media and at the five incubation periods were significantly different (P< 0.05). Based on the mean EPS level, the growth media and incubation periods ranked from the highest to the lowest as; RSM-S>WH-S>WH>RSM and 24hrs>18hrs>30hrs >12hrs >6hrs respectively. However, 18 and 24 hours were not significantly different. The Growth medium/incubation period interaction which yielded maximum EPS level (141.5 mg/L Glu.equiv) from Mursik culture was RSM-S/ 24 hours. From these results, it was therefore concluded that Mursik contains EPS and EPS producing LAB. Although their levels were low, they can be improved by supplementing the milk with sucrose and incubating for 24 hrs. Therefore, it is recommended that Mursik should be developed as a functional food product by enhancing production EPS during its fermentation. This study has a significant impact on development of Mursik as a functional food product and its commercialization.