Investigation of the native microflora and isolation of protective starter culture for a traditional Vietnamese fermented meat

Nem chua

is a traditional Vietnamese uncooked fermented pork sausage, which has been one of the most important traditional foods consumed in festive celebrations and also as a part of the daily diet. However, despite its popularity,

nem chua

is still mainly produced by small family businesses using traditional methods and practices that are based on art rather than science. The fermentation process, the key hurdle to ensure the product safety, occurs naturally without the use of starter culture or only with back-slopping, a practice employing a small amount of previous successful batch as the starter for the fermentation of a new batch. The practice of natural fermentation or the repeated use of back-slop in fermented meat production implies significant risks of contamination. This project was thus objected to investigate the hygienic status and native microflora of

nem chua

and develop the protective starter culture for this product.

The outcome of the study on the hygienic status of

nem chua

revealed that 93% of the samples tested were classified as unacceptable. Using the Australian microbiological guidelines for uncooked fermented meat, 92.2% of the samples were classified as unacceptable for

Enterobacteriaceae

, 53.3% as unacceptable for

Escherichia coli

and 56.6% as unacceptable for coagulase-positive staphylococci.

E. coli

O157:H7 was unexpectedly detected in

nem chua

but confirmed to be non-toxigenic. These results indicate hygienic deficiencies in sausage processing and the need to improve poor handling practices.

The investigation of the native microflora of

nem chua

revealed that lactic acid bacteria (LAB) were predominant in

nem chua

microflora, accounting for 80 − 100% of total viable counts. It was found that 67.6% of all LAB isolates belonged to the species

Lactobacillus plantarum

, followed by

Pediococcus pentosaceus

(22.7%) and

L. brevis

(9.3%) with only one isolate (1.4%) being a

L. farciminis

. A great genetic plasticity was observed among LAB isolates within the same species, with similarities ranging from 16.9 to 100%. These results suggest the great diversity of

nem chua

sensory characteristics among different regions of Vietnam.

For the potential use as protective starter culture, two strains of LAB, namely

L. plantarum

A17 and

L. plantarum

B21 with the strongest production of acid and antimicrobial agent were isolated and identified. These two starter cultures were successfully used to control

E. coli

in the challenge studies with both strains capable of reducing

E. coli

by more than 4 logs. The strain

L. plantarum

B21 was also proven to produce an acid and heat stable bacteriocin. In view of its interesting characteristics, this bacteriocin has an application potential as a natural preservative in

nem chua

as well as other pasteurised, acid or non-acid foods. The successful isolation and identification of two new strains of LAB will have enormous benefit as potential starter cultures for commercial production of

nem chua

with export potential to the Asia Pacific region.