Development of a sheep's milk kefir using species isolated from kefir products : a thesis presented in partial fulfilment of the requirements for the degree of Masters in Food Technology at Massey University, Palmerston North, New Zealand

Abstract

The aim of the research described in this thesis was to develop a good tasting kefir using sheep’s milk. Kefir is a refreshing and effervescent milk beverage fermented with bacteria and yeast. Different combinations of bacteria, yeast and treatment result in different qualities of Kefir. An optimal kefir has a pleasant slightly sour flavour and is slightly effervescent. It is drunk chilled. To arrive at preferred or optimal kefir the following steps occurred: Yeast and bacteria were isolated using MRS, M17, and DRGB agar from five commercial and three homemade kefir products. 54 isolates were identified using 16s rDNA PCR for bacteria, and 26s rDNA for yeast. The commercial yeasts were: Saccharomyces cerevisiae, and Torulaspora delbrueckii, the commercial bacteria were: Lactococcus lactis subsp. lactis, Leuconostoc mesenteroides, Lactobacillus plantarum, Lactococcus lactis subsp. cremoris, and Leuconostoc pseudomesenteroides, and Leuconostoc pseudomesenteroides. The homemade yeast were: Kluyveromyces marxianus, Kazachstania unispora, Pichia membranifaciens, and Clavispora lusitaniae, and the homemade bacteria were: Lactobacillus delbrueckii, Lactobacillus kefiranofaciens, and Lactobacillus kefiri. Streptococcus thermophiles was the only isolate found in both homemade and commercial kefir. One isolate of each species identified was used to form a starter culture and grown in gamma sterilized sheep’s milk, allowing the assessment of the growth characteristics required for kefir. The characteristics assessed were; cell counts, pH, textural properties, effervescence, and the levels of lactose, glucose, galactose, ethanol, lactic acid, acetic acid, and diacetyl using HPLC. The results from these tests were used to determine the optimum mix of species for a sheep’s milk kefir. Four different mixes of 5-6 isolates were chosen based on the results of the individual isolates and grown in sheep’s milk and tested for the same characteristics as the pure isolates as well as taste tested. This optimised kefir was made by inoculating 1x106cuf/mL of each isolate to sheep’s milk, sealed in the final container and fermented at 30°C for 24 hours. After cooling to 4°C the final product has a refreshing sour taste and effervescence, with a pH of 4.6, and a cell count above 3x109cfu/mL which decreases to above 9x108cfu/mL after five weeks, which is over 106cfu/mL required for labelling purposes