Quintas C, Leyva J S, Sotoca R, Loureiro-Dias M C, Peinado J M
Universidade do Algarve, Escola Superior de Tecnologia, Campus da Penha 8005-139, Faro, Portugal.
Int J Food Microbiol. 2005 Apr 15;100(1-3):125-30. doi: 10.1016/j.ijfoodmicro.2004.10.009. Epub 2004 Dec 8.
Zygosaccharomyces bailii, a spoilage yeast, capable of metabolic activity in food environments with low pH, low a(w) and in the presence of weak acid preservatives was chosen for a study on the effect of benzoic acid on growth parameters. In batch cultures, under controlled pH, this food preservative inhibited growth, decreasing the specific growth rate (mu) and the yield coefficient (Y(S)) on glucose. Data obtained at pH 3.5, 4.0 and 4.5 showed that this inhibition was exclusively promoted by the undissociated form of the acid since the effect was independent of pH when the concentration of the acid was expressed in this form. Moreover, the relationship between the values for mu and Y(S), provided evidence that the specific consumption rate of glucose (q(S)) was not affected by benzoic acid, indicating that the inhibition of growth should be completely explained by a decrease of Y(S). The outcome of parallel experiments performed in continuous culture was that the decrease of Y(S) was due to an increase of the maintenance coefficient (m), defined as the fraction of q(S) diverted from growth to cope with stress, represented in this case by the presence of the preservative. Based on these results a model was built, assuming that m increased hyperbolically with the concentration of benzoic acid, from zero in the absence of the acid up to q(S) when growth was completely inhibited. The concentration of the acid, for which m=q(S)/2, is a constant (K(W)), and represents a measure of the tolerance for a preservative, in this case benzoic acid. The simple equation mu/mu(0)=1+W/K(W) predicts the value of mu for a concentration (W) of the preservative, requiring the knowledge of two parameters: the specific growth rate in the absence of the preservative (mu(0)) and K(W). The equation fitted very well the data of the effect of benzoic acid on the specific growth rate of Z. bailii, having K(W)=0.96 mM benzoic acid. The model was also validated with other spoilage yeasts grown in the presence of benzoic acid in microtiter plates in an automated spectrophotometer. The values obtained for K(W) under these conditions confirm Z. bailii as the most tolerant (K(W)=2.1 mM) followed by Pichia sp. (K(W)=0.78 mM), Saccharomyces cerevisiae (K(W)=0.53 mM) and Debaryomyces hansenii (K(W)=0.11 mM).
拜耳接合酵母是一种腐败酵母，能够在低pH值、低水分活度以及存在弱酸防腐剂的食品环境中进行代谢活动。本研究选用该酵母来研究苯甲酸对生长参数的影响。在分批培养中，在控制pH值的条件下，这种食品防腐剂抑制了酵母的生长，降低了葡萄糖的比生长速率（μ）和产率系数（Y(S)）。在pH值为3.5、4.0和4.5时获得的数据表明，这种抑制作用完全是由酸的未解离形式引起的，因为当以这种形式表示酸的浓度时，其效果与pH值无关。此外，μ和Y(S)值之间的关系表明，葡萄糖的比消耗速率（q(S)）不受苯甲酸的影响，这表明生长抑制应完全由Y(S)的降低来解释。在连续培养中进行的平行实验结果表明，Y(S)的降低是由于维持系数（m）的增加，维持系数定义为从生长中转移出来以应对压力的q(S)的比例，在这种情况下，压力表现为防腐剂的存在。基于这些结果建立了一个模型，假设m随着苯甲酸浓度呈双曲线增加，从无酸时的零增加到生长完全被抑制时的q(S)。当m = q(S)/2时的酸浓度是一个常数（K(W)），它代表了对一种防腐剂（在这种情况下是苯甲酸）的耐受性的度量。简单方程μ/μ(0)=1 + W/K(W)可以预测防腐剂浓度（W）下的μ值，这需要知道两个参数：无防腐剂时的比生长速率（μ(0)）和K(W)。该方程很好地拟合了苯甲酸对拜耳接合酵母比生长速率影响的数据，K(W)=0.96 mM苯甲酸。该模型也在自动分光光度计中用在微量滴定板中在苯甲酸存在下生长的其他腐败酵母进行了验证。在这些条件下获得的K(W)值证实拜耳接合酵母是耐受性最强的（K(W)=2.1 mM），其次是毕赤酵母属（K(W)=0.78 mM）、酿酒酵母（K(W)=0.53 mM）和汉逊德巴利酵母（K(W)=0.11 mM）。
