# Non-Thermal Technologies in Food Processing: Implications for Food Quality and Rheology
###### Abstract
Furthermore, during non-thermal processing techniques, the rheological properties of the food determine the flowability, deformation behaviour, and texture under physical forces. This can determine the choice of processing method because of its influence on the processability and stability of food products, mouthfeel attributes, digestion, and absorption of food [33]. Overall, the changes in rheological properties during non-thermal processing are crucial in determining the quality characteristics of the food products [34]. Thus, there is a correlation between non-thermal processing and changes in the rheological properties and quality of different foods including fruit/vegetable products (juice, nectar, and puree) and starchy food products. Studies on food rheology are important because of the complex nature and interaction of food components. There is a broad spectrum of rheological behaviours and textural features, from low-viscosity fluids like milk or honey to hard solids. (e.g., saline crackers or raw carrots) [35]. The ability of the food products to deform and flow under shear force defines the rheology, which can be characterised via viscosity, plasticity, and elasticity. The rheological properties can be influenced by changes in the processing temperature, pressure, and external forces. As a result, the flow behaviour of liquid food products such as juice, nectar, puree, honey, mayonnaise, dough, chocolate, etc., can be modified to either Newtonian or non-Newtonian. Overall, in most cases, the complex microstructure of the food products confer mostly non-Newtonian behaviour, in which their apparent viscosity might increase or decrease with a shear rate or both [36]. Other important rheological properties include the yield stress, shear response, and recovery time. For certain foods, the rheological behaviours during processing can be modelled as Newtonian and non-Newtonian (e.g., pseudoplastic, dilatant, Bingham, Herschel-Bulkley, and thixotropic), with the viscoelasticity properties of these food products (which describe the viscosity and elasticity state) signifying the exhibition of both liquid-like (viscous) and solid-like (elastic) behaviours. In food processing systems, these rheological models can be used to predict the texture of food products and shelf life as a function of processing conditions [37]. Hence, an in-depth understanding of food rheology in various non-thermal food processing technologies can be of benefit to the process design, optimisation of processing conditions, product development, and quality control, as the rheological properties of food products can influence their mass transport during non-thermal processing such as drying, thus providing food scientists with quantitative and qualitative data on foods' mechanical properties and approximate texture [38]. Similarly, for food ingredient analysis and processing,rheological measurements of ingredients and mathematical modelling can be undertaken to ensure appropriate mixing ratios. This reduces production and testing costs and allows for more fine input control using sensitive methods [36]. A variety of factors affect food rheology. For example, temperature fluctuations during the processing and storage of milk may affect yoghurt's rheology. The numerous iterations of the Arrhenius equation may show how temperature influences the food fluid viscosity [38; 39]. Therefore, food companies test the rheological properties of the fluid, thus reducing production loss and improving the product quality, as exemplified in mashed fruits or ground meat, which are characterised as semisolid foods and would exhibit Non-Newtonian behaviour [36; 40; 41]. The challenge associated with the processing of foods with a complex rheology is the utilisation of an adequate processing technology that does not destroy the food's properties or is insufficient to ensure food safety [42; 43].