Chemical and Biochemical Process Technology and Control Section (BioTeC), Department of Chemical Engineering, Katholieke Universiteit Leuven, Belgium.
Food Microbiol. 2010 Apr;27(2):220-8. doi: 10.1016/j.fm.2009.10.005. Epub 2009 Oct 9.
Several studies have shown that food structure causes slower growth rates and narrower growth boundaries of bacteria compared to laboratory media. In predictive microbiology, both a(w) or corresponding solute concentration (mainly NaCl) have been used as a growth influencing factor for kinetic models or growth/no growth interface models. The majority of these models have been based on data generated in liquid broth media with NaCl as the predominant a(w) influencing solute. However, in complex food systems, other a(w) influencing components might be present, next to NaCl. In this study, the growth rate of Salmonella typhimurium was studied in the growth region and the growth/no growth response was tested in Tryptic Soy Broth at 20 degrees C at varying gelatin concentration (0, 10, 50 g L(-1) gelatin), pH (3.25-5.5) and water activity (a(w)) (0.929-0.996). From the viewpoint of water activity, the results suggest that NaCl is the main a(w) affecting compound. However, gelatin seemed to have an effect on medium a(w) too. Moreover, there is also an interaction effect between NaCl and gelatin. From the microbial viewpoint, the results confirmed that the a(w) decreasing effect of gelatin is less harmful to cells than the effect of Na(+) ions. The unexpected shift of the growth/no growth interface to more severe conditions when going from a liquid medium to a medium with 10 g L(-1) gelatin is more pronounced when formulating the models in terms of a(w) than in terms of NaCl concentrations. At 50 g L(-1) gelatin, the model factored with NaCl concentration shifts to milder conditions (concordant to literature results) while the model with a(w) indicates a further shift to more severe conditions, which is due to the water activity lowering effect of gelatin and the interaction between gelatin and NaCl. The results suggest that solute concentration should be used instead of a(w), both for kinetic models in the growth region and for growth/no growth interface models, if the transferability of models to solid foods is to be increased.
已有多项研究表明，相较于实验室培养基，食物结构会使细菌的生长速度减缓、生长范围变窄。在预测微生物学中，水活度（a(w)）或相应的溶质浓度（主要是 NaCl）均被用作动力学模型或生长/不生长界面模型的生长影响因素。这些模型中的大多数都是基于在含有 NaCl 的液体肉汤培养基中生成的数据建立的，而 NaCl 是主要影响 a(w)的溶质。然而，在复杂的食物体系中，除了 NaCl 之外，可能还存在其他影响 a(w)的成分。在本研究中，在 20°C 下，研究了肠炎沙门氏菌在含有不同浓度明胶（0、10、50g/L 明胶）的胰蛋白胨大豆肉汤中的生长率，以及在不同 pH（3.25-5.5）和水活度（a(w)）（0.929-0.996）条件下的生长/不生长响应。从水活度的角度来看，结果表明 NaCl 是主要的影响 a(w)的化合物。然而，明胶似乎也对培养基的 a(w)有影响。此外，NaCl 和明胶之间还存在相互作用。从微生物的角度来看，结果证实明胶对细胞的 a(w)降低效应不如钠离子的影响严重。从液体培养基到含有 10g/L 明胶的培养基时，生长/不生长界面的意外偏移到更严格的条件，这在以 a(w)为参数构建模型时比以 NaCl 浓度为参数构建模型时更为明显。在 50g/L 明胶的情况下，以 NaCl 浓度为参数的模型向较温和的条件转移（与文献结果一致），而以 a(w)为参数的模型则进一步向更严格的条件转移，这是由于明胶的降低水活度效应以及明胶和 NaCl 之间的相互作用。研究结果表明，如果要提高模型向固体食品的可转移性，则应该使用溶质浓度来替代 a(w)，无论是在生长区的动力学模型还是在生长/不生长界面模型中。
