Manzocco Lara, Nicoli Maria Cristina
Dipartimento di Scienze degli Alimenti, University of Udine, Via Marangoni 97, 33100 Udine, Italy.
J Agric Food Chem. 2007 Aug 8;55(16):6521-6. doi: 10.1021/jf070166k. Epub 2007 Jul 20.
This work was addressed to study the chemical stability of coffee brew derivatives as a function of water activity (aw) and storage temperature. To this purpose, coffee brew was freeze-dried, equilibrated at increasing aw values, and stored for up to 10 months at different temperatures from -30 to 60 degrees C. The chemical stability of the samples was assessed by measuring H3O+ formation during storage. Independently of storage temperature, the rate of H3O+ formation was considerably low only when aw was reduced below 0.5 (94% w/w). Beyond this critical boundary, the rate increased, reaching a maximum value at ca. 0.8 aw (78% w/w). Further hydration up to the aw of the freshly prepared beverage significantly increased chemical stability. It was suggested that mechanisms other than lactones' hydrolysis, probably related to nonenzymatic browning pathways, could contribute to the observed increase in acidity during coffee staling. The temperature dependence of H3O+ formation was well-described by the Arrhenius equation in the entire aw range considered. However, aw affected the apparent activation energy and frequency factor. These effects were described by simple equations that were used to set up a modified Arrhenius equation. This model was validated by comparing experimental values, not used to generate the model, with those estimated by the model itself. The model allowed efficient prediction of the chemical stability of coffee derivatives on the basis of only the aw value and storage temperature.
这项工作旨在研究咖啡冲泡衍生物的化学稳定性与水分活度（aw）和储存温度之间的关系。为此，将咖啡冲泡液冷冻干燥，在不断增加的aw值下进行平衡，并在-30至60摄氏度的不同温度下储存长达10个月。通过测量储存过程中H3O+的形成来评估样品的化学稳定性。与储存温度无关，只有当aw降低到0.5（94% w/w）以下时，H3O+的形成速率才相当低。超过这个临界界限，速率增加，在约0.8 aw（78% w/w）时达到最大值。进一步水合至新鲜制备饮料的aw显著提高了化学稳定性。有人认为，除内酯水解之外的机制，可能与非酶褐变途径有关，可能是导致咖啡陈化过程中观察到的酸度增加的原因。在所考虑的整个aw范围内，H3O+形成的温度依赖性可以用Arrhenius方程很好地描述。然而，aw影响了表观活化能和频率因子。这些影响通过简单的方程来描述，这些方程被用于建立一个修正的Arrhenius方程。通过将未用于生成模型的实验值与模型本身估计的值进行比较，对该模型进行了验证。该模型仅基于aw值和储存温度就能有效地预测咖啡衍生物的化学稳定性。
