Abstract
Freeze-drying is an important processing unit operation in food powder production. It offers dehydrated products with extended shelf life and high quality. Unfortunately, food quality attributes and grinding characteristics are affected significantly during the drying process due to the glass transition temperature (during drying operation) and stress generated (during grinding operation) in the food structure. However, it has been successfully applied to several biological materials ranging from animal products to plants products owning to its specific advantages. Recently, the market demands for freeze-dried and ground food products such as spices, vegetables, and fruits are on the increase. In this study, the effect of the freeze-drying process on quality attributes, such as structural changes, the influence of glass transition during grinding, together with the effect on grinding efficiency in terms of energy requirement, grinding yield, and morphological changes in the powder as a result of temperature, drying time were discussed. An overview of models for drying kinetics for freeze-dried food sample, and grinding characteristics developed to optimize the drying processes, and a prediction of the grinding characteristics are also provided. Some limitations of the drying process during grinding are also discussed together with innovative methods to improve the drying and grinding processes.
Highlights
Drying is a unit operation by which free water in the food is significantly reduced, promoting the concentration of dry matter without damaging the tissue, wholesomeness, and physical appearance of the food
It has been practiced as sun-drying method, which is considered as a traditional method practiced in most of the developing countries, or it can be conducted with improved technology such as spray drying, hot air drying, vacuum drying and freeze-drying [2]
When deciding on the type of processing methods to be used for a particular product, it is required that we look beyond the surface, as freeze-drying is reported to be accompanied by several complex interactions which may prove to be a problem at varying processing stages [23,24,25]
Summary
Drying is a unit operation by which free water in the food is significantly reduced, promoting the concentration of dry matter without damaging the tissue, wholesomeness, and physical appearance of the food. Though the operational methods of conventional drying differ from modern technology, drying from these methods is all achieved either through radiation, convection, conduction, or a combination of heat transfer mechanisms All of these drying methods may affect partially or totally the quality of the product [2]. Other limiting factors include reductions in quality as a result of friction during grinding, reduction in flavor, aromatic components, and the removal of essential oils All these aforementioned properties affect the quality of the resulting powder product from grinding unit operation. Drying and grinding unit operations are coherent towards the production of food powder, and as such, the parameters involved should be critically considered These lead to a number of researches being conducted with the aim of improving food powder quality by freeze-drying operations [17,18,19]. Further discussions will be made on future trends and solution to grinding challenges
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