Chirife J, Buera M D
Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina.
Crit Rev Food Sci Nutr. 1996 May;36(5):465-513. doi: 10.1080/10408399609527736.
Water is probably the single most important factor governing microbial spoilage in foods, and the concept of water activity (a(w)) has been very valuable because measured values generally correlate well with the potential for growth and metabolic activity. Despite some drawbacks (e.g., solute effect), the concept of a(w) has assisted food scientists in their effort to predict the onset of food spoilage as well as to control food-borne disease hazards in food products. In the last decade the concept of a(w) has been challenged. It has been suggested that reduced-moisture food products (e.g., low and intermediate) may be nonequilibrium systems and that most of them are in the amorphous metastable state, which is very sensitive to changes in moisture content and temperature. It has been proposed that the glass transition temperature Tg (temperature at which the glass-rubber transition occurs), is a parameter that can determine many product properties, the safety of foods among them. The concept of water dynamics, originating in a food polymer science approach, has been suggested instead of a(w) to better predict the microbial stability of intermediate-moisture foods. The usage of a(w) to predict microbial safety of foods has been discouraged on the basis that (1) in intermediate-moisture foods the measured water vapor pressure is not an equilibrium one, and because a(w) is a thermodynamic concept, it refers only to equilibrium; and (2) the microbial response may differ at a particular a(w) when the latter is obtained with different solutes. This review analyzes these suggestions on the basis of abundant experimental evidence found in the literature. It is concluded that nonequilibrium effects (e.g., inability of water to diffuse in a semimoist food) appear to be in many cases slow within the time frame (food's shelf life) of the experiments and/or so small that they do not affect seriously the application of the a(w) concept as a predictor of microbial stability in foods. The claims that a food science polymer approach to understanding the behavior of aqueous sugar glasses and concentrated solutions may be used to predict the microbial stability of food systems is not substantiated by experimental evidence. This approach does not offer, at the present time, a better alternative to the concept of a(w) as a predictor of microbial growth in foods. It is also recognized that a(w) has several limitations and should be always used carefully, and this must include precautions regarding the possible influences of nonequilibrium situations. This aspect may be summarized by simply saying that anyone who is going to employ the term water activity must be aware of the implications of its definition.
水可能是决定食品微生物腐败的唯一最重要因素，水分活度（a(w)）的概念非常有价值，因为测量值通常与微生物生长和代谢活性的可能性密切相关。尽管存在一些缺点（例如溶质效应），但a(w)的概念在帮助食品科学家预测食品腐败的开始以及控制食品中食源性疾病危害方面发挥了作用。在过去十年中，a(w)的概念受到了挑战。有人提出，水分含量降低的食品（例如低水分和中等水分食品）可能是非平衡体系，并且其中大多数处于无定形亚稳态，这种状态对水分含量和温度的变化非常敏感。有人提出玻璃化转变温度Tg（发生玻璃-橡胶转变的温度）是一个可以决定许多产品特性的参数，其中包括食品的安全性。源自食品聚合物科学方法的水动力学概念已被提出，以取代a(w)，以便更好地预测中等水分食品的微生物稳定性。基于以下原因，不鼓励使用a(w)来预测食品的微生物安全性：（1）在中等水分食品中，测得的水蒸气压力不是平衡压力，并且由于a(w)是一个热力学概念，它仅指平衡状态；（2）当通过不同溶质获得特定的a(w)时，微生物的反应可能会有所不同。本综述基于文献中发现的大量实验证据对这些建议进行了分析。得出的结论是，在实验的时间框架（食品保质期）内，非平衡效应（例如水在半湿食品中无法扩散）在许多情况下似乎很缓慢，并且/或者非常小，以至于它们不会严重影响a(w)概念作为食品微生物稳定性预测指标的应用。关于食品科学聚合物方法用于理解水性糖玻璃和浓缩溶液行为可用于预测食品体系微生物稳定性的说法，没有得到实验证据的证实。目前，这种方法并不能提供比a(w)概念更好的替代方案来预测食品中的微生物生长。还认识到a(w)有几个局限性，应始终谨慎使用，这必须包括对非平衡情况可能影响的预防措施。这方面可以简单地概括为，任何打算使用水分活度这个术语的人都必须意识到其定义的含义。
