MOISTURE TRANSFER AND NUTRIENT RETENTION IN PROCESSING AND STORAGE OF FRESH FRUITS

Abstract

The growing of fruits and vegetables is limited in many countries to certain seasons and localities. This makes food processing and storage important for food processors. Also, interest in quantitative approaches to the analysis of food quality deterioration has increased due to growing consumer awareness and mandatory governmental requirements such as nutritional labelling. Kiwifruit, a delicious, unique flavored fruit, has attracted great attention in the last few years. Its popularity is rapidly increasing. It contains a large amount of nutrients, especially vitamin C. The present study includes some of the important aspects in processing and storage of kiwifruits. It consists of three parts: sorption isotherm, drying kinetics and nutrient retention during drying and storage. In the sorption isotherm study, the desorption and adsorption characteristics of kiwifruits were investigated at 30, 45 and 50°C, using the static method of saturated salt solutions. The GAB (Guggenheim-Anderson-de Boer) and BET (Brunauer- Emmett-Teller) equations fitted the data well in their respective range of applicability. The net isosteric heat of sorption as a function of moisture content was determined using the Clausius-Clapeyron equation. A heat pump drier was used to control drying conditions accurately to study drying kinetics. Shrinkage of kiwifruits was taken into account and the diffusion coefficients for the two falling rate periods were calculated. An empirical correlation has been developed for diffusivity under different drying conditions. For nutrient retention, ascorbic acid (vitamin C) was selected as an index because it is known as the least stable one among all the nutrients present in kiwifruits. A static test was adopted to investigate the effect of temperature and water activity on vitamin C degradation during storage. As for drying, due to the high moisture content of fresh kiwifruits, a dynamic test during which temperature and moisture content changed continuously was used to develop a kinetics model for ascorbic acid deterioration. Predictions from this model and experimental vitamin C retention agreed well, indicating the potential of this method for estimating deterioration of ascorbic acid during air drying.