Inhibitory substances produced by probiotic bacteria for control of food-borne pathogenic and spoilage microorganisms in dips

The success in using a food product as a delivery vehicle for probiotics depends on its ability to maintain required level of viable cells (at least 107 cfu g-1) and to suppress the growth of spoilage and pathogenic organism. Cheese-based dips could deliver probiotic bacteria owing to its stable pH, buffering capacity of ingredients and the presence of prebiotics. The anti-microbial properties of probiotics can also be employed for controlling the spoilage organisms such as yeast and mould. The work described in this thesis focused on the survival of probiotics and their anti-microbial effects in dips. Effective selective enumeration methods were first identified for specific probiotic cultures to enumerate their numbers, the ideal conditions in which the organisms survive better were evaluated and the mechanism by which the probiotic organisms antagonise pathogenic and spoilage organisms were then elucidated. The cultures of Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, Lactobacillus casei, Lactobacillus rhamnosus, Lactobacillus acidophilus, Bifidobacterium spp. and propionibacteria were tested in selected bacteriological media to evaluate their suitability as selective media. Nineteen bacteriological media were evaluated at different incubation conditions, including Streptococcus thermophilus (ST) agar, pH-modified MRS agar, MRS - vancomycine agar (MRS-V agar), MRS-bile agar, MRS-NaCl agar, MRS-lithium chloride agar, MRS - NNLP agar, RCA agar, sugar-based (such as maltose, galactose, sorbitol, manitol, esculin) agar media, sodium lactate agar (NaLa), arabinose agar, raffinose agar, xylose agar and LC agar. Aerobic and anaerobic incubations were carried out at temperatures of 27°C, 30°C, 37°C, 43°C and 45°C for the duration of 24h, 72h and 7-9 days. ST agar and aerobic incubation at 37°C for 24h were suitable for S. thermophilus. L. delbrueckii ssp bulgaricus can be enumerated in MRS agar (pH 4.58 or pH 5.20) and anaerobic incubation at 45°C for 72h. MRS-V agar and anaerobic incubation at 43°C for 72h was suitable to enumerate L. rhamnosus. Anaerobic incubation in MRS-V agar at 37°C for 72h was selective to enumerate L. casei. It is recommended that subtraction method should be implemented when L. rhamnosus is present in the product. To do this, the count of L. rhamnosus recorded on MRS-V agar at 43°C for 72h under anaerobic incubation should be subtracted from the total count of L. casei. and L. rhamnosus recorded on MRS-V agar 37°C for 72h under anaerobic incubation