Characterization of the dominant microbiota in nyarmie a Ghanaian fermented milk product

Abstract

The characterization of microbiota from dairy products manufactured using traditional techniques without commercial starter cultures has been considered in the search for industrially important characteristics and for the selection of potential starter cultures. One dairy product which has gained an interest in the search for starter cultures is nyarmie, a Ghanaian fermented milk product manufactured from raw or pasteurised cow milk using traditional methods. Its unique organoleptic characteristics are derived from local environmental conditions, including the race and nutrition of the cows, as well as its manufacturing practices. Different stages of nyarmie were sampled from three production sites (processors) in three separate periods for the study. The micro flora of the samples was characterised by traditional phenotypic (physiological and biochemical) tests and molecular (16S/18S PCR-denaturing gradient gel electrophoresis) approaches to examine population diversity. The dominance of lactic acid bacteria (LAB) present in nyarmie was demonstrated by culture-based and non culture-based methods. The dominant LAB from the phenotypic and DGGE analysis were Lactococcus lactis, Streptococcus thermophilus, Leuconostoc mesenteroides, Lactobacillus delbrueckii subsp. bulgaricus, Lb. delbrueckii subsp. delbrueckii, Lb. delbrueckii subsp. lactis, Lb. helveticus, Lb. fermentum, while the dominant yeast species was Saccharomyces cerevisiae. All the LAB isolates were homolactic fermenters and they grew at 37oC with variable growth at 45oC, 15oC and10oC. Pulse field gel electrophoresis was used for strain differentiation and strain presence during the time of sampling. Apart from Lc. lactis, the isolates did not show any correlation between the PFGE types and the time of sampling. Lc. lactis showed seven PFGE types with PFGE type1 occurring throughout the fermentation process. Strain clustering resulted in 59 genotypes from 72 Lc. lactis isolates, 10 genotypes from 16 Lactobacillus delbrueckii subsp. lactis and 12 genotypes from 36 Saccharomyces cerevisiae indicating subspecies variation. Heat tolerance studies showed that strains surviving to the final product and the dominant Lc. lactis strain were all capable of surviving the pasteurisation stage. Organic acids and in some cases hydrogen peroxide were produced as the antimicrobial substances by selected strains but no bacteriocin production was evident. Growth of Salmonella typhimurium was inhibited at different rates when inoculated simultaneously with selected starters. S. typhimurium in nyarmie declined within 4 h from the selected starters. Distinct volatiles were produced by the mesophilic and thermophilic starters and in nyarmie samples from the processors without any consistent pattern. The mesophilic starters were mainly associated with diacetyl, 3-metylbutanal and 2-methylpropanal whilst the thermophilic starters were associated more with acetoin and dimethylsulfon. However, all the nyarmie samples and starters were found to contain significant amounts of decanoic acid. Samples from processors A and B produced more acetoin and acetaldehyde whilst those from processor C produced more furanmethanol. It was concluded that, Lc. lactis subsp. lactis strain BO-4 as a single starter and this isolate with Leuconostoc mesenteroides strain AU-3, Streptococcus thermophilus strain B3-1 & Lactobacillus delbrueckii subsp. bulgaricus strain B4-20 as paired starters were potentially suitable for production of nyarmie. However, the overall flavour profiles produced never matched those of the processor made nyarmie and hence more work is needed to design the ideal starter