Application of new drying technologies to aromatic herbs and vegetableseffects on physico-chemical, sensory and functional properties

Abstract

Product quality of dehydrated fruits, vegetables and aromatic herbs is a key feature in innovation of future drying technology, which is closely related to development of novel drying techniques and process optimization. This was the general aim of the present Ph.D. Thesis. The main objective of this Ph.D. dissertation was to evaluate the application of (i) convective drying, (ii) freeze drying, (iii) microwave drying, (iv) vacuum-microwave drying and a (v) combination of convective pre-drying followed by vacuum-microwave finishing drying in the processing of dehydrated (i) rosemary, (ii) basil, (iii) thyme, (iv) garlic and (v) pomegranate. The parameters under study were related to kinetics and energy consumption aspects together with quality parameters, such as chemical composition, functional properties and sensory quality. The microwave drying kinetics of rosemary consisted of two periods: (i) linear until a critical point and (ii) exponential beyond that point. The time needed to dry rosemary was shorter for high values of microwave power and vacuum intensity. The drying kinetics of sweet basil using convective drying was described by a two term exponential model, while vacuum-microwave drying kinetics consisted of two periods: (i) linear until a critical point and (ii) exponential beyond that point. The convective drying kinetics of thyme was described by a two term exponential model, while vacuum-microwave drying kinetics consisted of two periods: (i) a constant rate period until a critical moisture ratio and (ii) falling rate period beyond that point. The drying kinetics of convective and vacuum-microwave drying of pomegranate arils and rind were described using seven basic drying models; however, vacuum-microwave finishing drying was modeled only by one term exponential equation. Drying kinetics of garlic slices dried by the convective method was described by a two-term exponential model, while drying kinetics of vacuum microwave finishing drying garlic was described by a one-term exponential function. Energy consumption of both pomegranate arils and rind was reduced when the air temperature and microwave wattage were increased. Regarding garlic slices, vacuum-microwave drying required less energy than convective drying. Convective drying energy consumption can be reduced by its combination with vacuum-microwave drying (convective pre-drying followed by vacuum-microwave finishing drying) and this reduction was more important when the vacuum-microwave finishing drying was applied earlier. The quality parameters under study included, volatile compounds, sugars and organic acids compositions, contents of punicalagins, ellagic acid, and total polyphenols, total antioxidant activity and sensory quality. In general, drying led to a reduction in all studied parameters; however, the behavior of each plant material was different. The total quantity of volatiles of fresh rosemary decreased during drying, independently of treatments. Soft conditions of vacuum-microwave drying (low vacuum level and microwave power) are recommended to get the highest concentrations of volatile compounds and the best sensory quality. The total quantity of volatiles of fresh sweet basil decreased considerably during both convective and vacuum-microwave drying. Combined methods were the best option for drying sweet basil; the time required was relatively short, and aroma quality was good according to instrumental data and sensory data. The total quantity of volatiles of fresh thyme was reduced by most of the drying treatments, except for the treatments with low wattage of vacuum-microwave drying. The combined method was the best option for drying thyme; the time required was relatively short, and aroma quality was good according to instrumental data and sensory evaluation results. Phenolic content of garlic slices was not significantly improved by the used of the combined drying method compared with the convective one. However, a decrease in the moisture produced an increment in the antioxidant capacity of garlic slices. Dried garlic showed even higher antioxidant capacity than fresh samples. Results suggested that phenolic compounds and other chemical compounds, such as allicin, were responsible for the antioxidant capacity of dried garlic slices. Vacuum-microwave drying of pomegranate was the best drying treatment for arils, while rind required freeze drying or soft conditions of convective drying. Further research is needed to obtain proper results with combined drying techniques. With proper selection of the drying protocol, high-quality dried arils will be available for consumers; these arils will be characterized by high contents of fructose, punicalagins, total polyphenols, high antioxidant capacity and high intensities of garnet color, sweetness, sourness and fresh pomegranate aroma. Besides, dried rind with very high contents of active compounds and high antioxidant capacity will be also available as a functional ingredient. On the whole, the optimal drying technology is different for each of the materials studied and specific conditions must be recommended after proper evaluation of the drying protocols.