Thermal and Non-thermal Processing on the Physical and Chemical Properties of Tree Nuts: A Review

Tree nuts are an essential component of the modern diet and are consumed worldwide for their health benefits and great taste. The quality and nutritional value of nuts can be altered by processing. Therefore, it is important to understand nut processing methods and resulting property changes. In this review, thermal processing (drying, roasting, blanching) and non-thermal processing (HPP, irradiation, cold plasma, PEF, radio frequency heating) effects on the physicochemical properties of tree nuts are summarized. Different thermal and non-thermal methods employed in the last ten years are compared and perspectives on their current challenges and future prospects are given. The review reveals that, although, thermal processing can enhance the dark brown color, crispy texture, and flavor of nuts, its impact on nutritional quality (e.g. vitamins, lipids, and phytochemicals) can be undesirable due to the effect of heat. In contrast, non-thermal processing has little effect on losses in the nutritional quality of nuts; yet, it has other limitations, e.g. incomplete inactivation of pathogens, unexpected chemical reactions, and non-robust process control. Considering these challenges, future perspectives are described and discussed, including the application of newly developed thermal techniques (e.g. superheated steam roasting, cooling with dehumidification) in nut processing, integration of thermal and non-thermal processing methods with enhanced efficiency, and the use of artificial intelligence and mathematical models for optimization of processing parameters, amongst others. This review will benefit nut researchers and help the industry develop and optimize nut processing strategies. Graphical abstract