Fractionation and characerisation of a commercial yeast extract to facilitate acceleration of yogurt fermentation

[摘要] English: In order to study the decrease of yogurt fermentation time, the effects of a wide range ofsupplements on yogurt fermentation time were evaluated. YE was identified as the onlysupplement which showed potential. Unfortunately it resulted in a product with anunacceptable flavour. It was therefore important to identify and isolate the specificcomponent responsible for the decrease in yogurt fermentation time.YE was fractionated with size exclusion chromatography and it was subsequentlydetermined that the accelerating fraction had a low molecular weight.It was important to establish whether the accelerating component was of mineral, vitamin oramino acid origin. Three cocktails containing the most common minerals, vitamins andamino acids were prepared and their respective effects on yogurt fermentation weredetermined. Results indicated that when compared to the respective controls, no decrease infermentation time was observed with the mineral and vitamin cocktails. A decrease infermentation time was observed with the amino acid cocktail, indicating that the acceleratingcomponent present in YE was of amino acid origin. It was however not clear whether it wasa single amino acid or a peptide.The accelerating fraction was further analysed by SDS-PAGE and due to no visible bands inthe respective region (<1kDa), it could not be analysed with mass spectrometry. The fractionobtained directly after size exclusion chromatography was however analysed by using massspectrometry in order to determine the total amino acid content of the accelerating fractionafter which it was evident that the fraction containing the accelerant contained an abundanceof peptides.The individual effects of the 17 identified amino acids were determined in respective yogurtfermentations. Results indicated that no single amino acid was responsible for the decreasein yogurt fermentation time. Although a combination of the 17 amino acids in onefermentation run resulted in a decrease in fermentation time in comparison tounsupplemented yogurt, the decrease was not as considerable as that of the acceleratingfraction obtained after size exclusion chromatography. Due to the fact that it was not practical to evaluate the effect of all possible combinations ofthe 17 amino acids, the combinations that were evaluated were based on literature reportson stimulation of Streptococcus thermophilus growth. Focus was placed on the growth ofStreptococcus thermophilus due to this organism being the growth limiting organismbetween the two starter organisms used for yogurt fermentation. None of the evaluatedamino acid combinations decreased yogurt fermentation time, and it was thereforeconcluded that the accelerating component was a peptide and not a free amino acid.In order to establish the mechanism of acceleration of the isolated YE fraction, it wasimportant to determine whether the addition of YE to milk increased the rate of starterbacteria growth or whether it completed the growth requirements of starter bacteria. Thelatter could either result in earlier initiation of lactic acid production or in increased levels oflactic acid production. Results indicated that YE had no effect on the total growth rate of thestarter bacteria. However, when examining the effect on the respective bacteria individually,it became clear that YE increased Streptococcus thermophilus cell numbers in comparisonto the unsupplemented control. Using resazurin, it seems YE increased the metabolic rate ofthe starter bacteria.Although supplementation of yogurt with YE did not influence combined starter bacteriagrowth, it influenced lactic acid production. The addition of YE to yogurt resulted in increasedlactic acid levels as well as an increase in lactic acid production rate. Lactic acid was alsoinitiated earlier in the fermentation process, resulting in higher lactic acid levels incomparison to unsupplemented yogurt.It was therefore evident from this study that YE did not provide nutrients that are not alreadypresent in milk, but rather provided nutrients in a readily available form at the beginning ofthe yogurt fermentation process resulting in the reduction of the lag phase of lactic acidproduction.

[发布机构] University of the Free State