# The Impact of Freeze Drying on Bioactivity and Physical Properties of Food Products
## 5 Impact on Physical Properties and Flavor

Table 1: _Cont._color, smell, and taste, can change after drying. For instance, according to the research of [PERSON]. [PERSON] et al., FD increases the flavor retention of tomatoes via affecting their volatile compounds [152]. FD provided better results in flavor protection, with spray drying results being close to FD. However, the hot air drying technique had the lowest flavor retention capacity for tomatoes. In another study on _Gastrodia elata_, FD was the most effective technique in terms of appearance and color maintenance compared with hot air drying, vacuum drying, microwave drying, and microwave vacuum drying, despite having the longest drying time [75]. However, microwave vacuum drying was the most practical technique for _Gastrodia elata_, as it preserved the original shape, color, and active components while reducing the drying time. In another study on _Dendrobium officinal_ flower tea, natural air drying and oven drying, except FD, were also applied, and the study determined that vacuum FD was the most effective technique for the preservation of the shape, color, taste, and antioxidant activity of the sample compared with other techniques [153]. However, vacuum FD showed the least protective effect on the aromatic compounds of the sample. When the results obtained were evaluated, natural air drying was determined to be the best technique because of its preservation ability on the bioactive and physical properties of the fresh flower scent. In another study, microwave drying, vacuum microwave drying, sun drying, vacuum drying, hot air drying, and vacuum FD were used to improve the preservation and long-term storage of the kiwifruit pomace, and these drying methods were compared [154]. As a result of the study, while vacuum FD caused the longest drying time and the highest energy consumption with better retention capacity, vacuum microwave drying showed a shorter drying time and lower energy consumption. Vacuum microwave drying was considered the most suitable drying technique for the pomace because of its retention ability on the bioactive contents and vitamin C. Also, vacuum FD was good at the protection of the original structure of the pomace through preserving its physical properties, nutrient values, antioxidant activity, and microstructure. In another article on vacuum FD, air impingement drying, hot air drying, pulsed vacuum drying, and medium- and short-wave infrared radiation drying were applied to garlic slices to determine the effects of these drying techniques on the slices [155]. As a result of the study, while vacuum FD caused the longest drying time and the highest energy consumption with better retention capacity, vacuum microwave drying showed a shorter drying time and lower energy consumption. Vacuum microwave drying qualified as the most suitable drying technique for the pomace because of its retention ability on the bioactive contents and vitamin C. Also, vacuum FD was good at the protection of the original structure of the pomace through preserving its physical properties, nutrient values, antioxidant activity, and microstructure. In another article about vacuum FD, air impingement drying, hot air drying, pulsed vacuum drying, and medium- and short-wave infrared radiation drying were applied to garlic slices to determine the effects of these drying techniques on the slices [146]. A high amount of chlorophyll and flavonoid contents were retained by this drying technique. In some cases, FD may not be the best drying technique to preserve the bioactive and physical properties of the samples compared with other drying techniques. For example, coffee pulp tea was dried via applying the FD to preserve the original color, total phenolics, trigonelline, total soluble sugars, chlorogenic acid, and protocatechuic acid of the tea [156]. As a result of the study, a long drying time and the lowest taste were obtained from applying the FD compared with sun drying, hot air drying, microwave drying, and vacuum drying.