The diversity and technological properties of yeasts from indigenous traditional South African fermented milks

[摘要] English: Yeasts are widely dispersed in nature and considered as natural inhabitants of the dairy environment. They are also natural contaminants in milk and milk products causing either spoilage or leading to fermentation with beneficial effects. In kefir, yeasts belong to the typical microflora and are responsible for the prickly and refreshing taste due to carbon dioxide production. In a number of other fermented dairy products, yeasts play an important role in metabolic interactions, thereby ensuring a product with typical sensoric properties. In this study, yeasts from kefir milks fermented by seven indigenous kefir grains were isolated and characterised in order to determine the diversity of the yeast flora in South African kefir. These kefir grains were inoculated into full cream milk and incubated at 25°C tor 18 h. The fermented kefir milks revealed yeast counts between 2.00x108 and 3.00x108 cfu/ml. The representative yeast strains were dominated by Kluyveromyces marxianus, Kluyveromyces laetis and Saccharomyces cerevisiae. Other species encountered were Saccharomyces unisporus, Saccharomyces rouxii, Torulaspora delbrueckii and Debaryomyces hansenii. Furthermore, a total of 50 yeasts were isolated and identified from 14 different traditional fermented milks manufactured in households by small-scale dairy farmers in rural areas. Growth patterns and the characteristics of Kluyveromyces marxianus, Debaryomyces hansenii and Torulaspora delbrueckii, the three dominant yeast species encountered in the fermented milk samples, were determined. A commercial lactic acid bacteria (LAB) culture was used in various combinations with the yeasts in the growth and fermentation studies. Growth at different pH values suggested that these yeasts (especially T. delbrueckii) play a metabolically active role during, as well as right to the end of the lactic acid fermentation of milk. T delbrueckii also showed significant growth at 15°C, but was not able to ferment lactose, as was the case with Kluyv. marxianus and D. hansenii. The growth of these two yeasts was also directly proportional to the concentration of lactose. All three yeasts were able to grow in the presence of 0.6, 0.85, 1.25 and 1.5 % lactate. Although the best aroma profiles were experienced at 22°C, no specific yeast or combination was responsible for the production of a desirable aroma. The highest alcohol and CO2 production resulted from a yeast:LAB combination with Kluyv. marxianus. These results could not highlight a specific combination of yeasts and LAB of all the variables tested to include in a fermented milk product to enhance or improve its characteristics. Some combinations produced desirable aromas while other combinations produced better alcohol or CO2 yields. However, it indicated that yeasts play a role in the improvement of the characteristic fresh pungent flavour encountered with traditional fermented milks.

[发布机构] University of the Free State