Foam‐mat Freeze‐drying of Apple Juice Part 2: Stability of Dry Products During Storage

Sorption isotherms at 5, 20 and 35C and glass transition temperature as a function of water content were evaluated for foamed (3% egg white and 1% methylcellulose) and nonfoamed freeze‐dried apple juice by using the static gravimetric method and differential scanning calorimetry, respectively. Equilibrium isotherms were fitted to the Guggenheim–Anderson–de Boer equation, whereas glass transition temperature to the Gordon–Taylor model. After freeze‐drying at 20C during 48 h, the dry products were stored at different temperatures (5 and 20C) under ambient conditions or vacuum. The nutritional, physical and structural properties were assessed before and at the end of the freeze‐drying process and after storage by determining vitamin C content, solubility, color and microstructure. Freeze‐dried nonfoamed juice retained more vitamin C and was more soluble than foamed products after freeze‐drying. However, foam‐mat juice powders presented higher stability during storage at 20C, which agreed with their higher values of glass transition temperature. Freeze‐dried juice stored at this temperature collapsed showing a decrease in solubility and a marked color change. PRACTICAL APPLICATIONS Quality of dry products depends largely on how they are stored (relative humidity, temperature, presence of light and vacuum). Degradation of flavor, color and texture occur under adverse storage conditions. Nutritional compounds such as vitamins or unsaturated fats could also be negatively affected because of oxidation reactions, which could be a function of the dehydration method or the pretreatment used to manufacture the dry product. The determination stability and quality degradation of foamed and nonfoamed apple juice products during freeze‐drying and storage of the dry products will complete the evaluation of foaming as a potential pretreatment for freeze‐drying.