Lab-scale optimisation of Kefir beverage production from mass-cultured and freeze-dried kefir grains

[摘要] ENGLISH ABSTRACT: Kefir is a fermented dairy beverage resulting from the fermentation of milk withreusable Kefir grains. The grains consist of a complex combination of lactic acidbacteria and yeasts in a symbiotic relationship, embedded in a polysaccharidematrix called kefiran. Various problems are experienced during thecommercialisation of the ready-made Kefir beverage and, therefore, it is moreadvantageous to market the grains, enabling the consumer to produce thebeverage at home. Kefir grains could be mass-cultured and then preserved byIyohilisation for successful long-term storage and easy distribution, duringcommercialisation. The microbial balance of the Kefir grains changes during bothmass-culturing and freeze-drying, which will have an influence on the sensoryproperties of the Kefir beverage produced. The aim of this study was theoptimisation of the production of Kefir from mass-cultured grains and from freezedriedmass-cultured grains respectively. The sensory characteristics of thefermented beverages produced from these mass-cultured and preserved grainswere determined.Mass-cultured Kefir grains were activated and Kefir produced using ninemethods with different activation times and temperatures, different grain:milk ratios(36, 72 and 108 g grains.l⁻¹) and with different heat-treated milks (pasteurised,double pasteerised and UHT). The best Kefir beverage was produced byactivation of the grains at 22°C for two successive 24 h incubation periods,followed by Kefir production at 22°C for 18 h and a maturation period at 18°C for 6h. The milk was replaced before every incubation period, excluding the maturationperiod, and the fermentation vessel was swirled five times at the start offermentation and after 18 h. This method resulted in a sour beverage with a thickconsistency and the characteristic effervescence and flavour of Kefir. The optimalgrain:milk ratio was identified as 36 g grains.l⁻¹ and the best heat-treated milks forthe production of Kefir beverage were UHT and double pasteurised milk.Mass-cultured Kefir grains were freeze-dried for 1, 2, 3 and 6 d and themoisture loss determined. Freeze-dried grains were rehydrated for 1, 2, 6, 12 and18 h to determine the optimal rehydration time. A sensory analysis was performedto compare the properties of Kefir produced from mass-cultured grains (Me),freeze-dried mass-cultured grains that were rehydrated and activated (FDRA) and activated mass-cultured grains that were freeze-dried and rehydrated (AFDR).The chemical compositions of mass-cultured grains (MC), mass-cultured, freezedriedgrains (MCFD), mass-cultured, freeze-dried grains that were rehydrated andactivated (FDRA) and activated mass-cultured grains that were freeze-dried andrehydrated (AFDR), were also investigated. The optimum time to freeze-dry grainswas 2 d and to rehydrate freeze-dried gtains was 1 h. The sensory analysisindicated that Kefir beverages prepared from FDRA and AFDR grains did not differsignificantly and were less fermented than Kefir produced from MC grains.It was concluded that Kefir with excellent sensory characteristics can beproduced from mass-cultured grains. Freeze-drying is a better method to preserveKefir grains than freezing due to mass loss during freezing and easier distributionand storage of freeze-dried grains. The supplementation of freeze-dried grainswith additional lactic acid bacteria and yeast isolates should be investigated.

[发布机构] Stellenbosch University