# The Impact of Freeze Drying on Bioactivity and Physical Properties of Food Products
## 4 Impact on Bioactive Properties
FD that used a lower temperature than convective hot air drying effectively preserved ascorbic acid, total phenolics, total flavonoids, antioxidant activity, and total anthocyanins of the fruits, more effectively than convective hot air drying. In an experiment with orange puree dried only using the FD technique to assess the impact of FD on the bioactive and physical properties of the orange puree [80], a clear, yellowish, darker, and more saturated color was obtained and high porosity was maintained after FD of the orange puree, while a slight decrease in the amounts of vitamin C, carotenoids, and phenols was obtained in the freeze-dried orange puree. In the study by [PERSON] et al., FD was used to obtain a high-quality final product, and it caused a slight degradation in vitamin C and polyphenic content as bioactive properties and in the flavor and texture values as physical properties of the fruits [116]. Not only was any color alternative not obtained at the end of the study, but also the high porosity of strawberries was observed. Moreover, FD was used with different drying techniques including hot air drying, vacuum drying, ultrasound-assisted vacuum drying, and ultrasound-pretreated FD on goji berries in the study to separately determine their effects on the berries in terms of their bioactive properties [117]. For each drying technique, the determination of the total phenolic content, phenolic profile, antioxidant capacity, carotenoid profile, and color change in the sample was conducted to contrast the drying techniques for maximum product quality. The study showed that the best technique for obtaining a maximum-quality product in regard to the bioactive properties was hot air drying including the carotenoid profile and antioxidant activity of the sample. Although better product quality was also provided by FD and ultrasound-assisted vacuum drying, the high cost and energy consumption of FD generally take second place in this technique. Some cases show that FD is not the most effective technique to preserve the product quality among other drying techniques. For example, [PERSON] et al. determined that walnut kernels as a stone fruit provided a useful basis to compare FD with other drying techniques, including gradient hot drying and constant hot drying, and to determine the degradation of bioactive properties of the walnut kernels after FD [118]. As a result, gradient hot drying was chosen as the most effective technique to preserve the total phenols and antioxidant activity of walnut kernels. Similar to fruits, drying techniques are also applied to vegetables to extend their shelf life and preserve their sensory and rheological properties [119; 120; 121]. As an example, in the research of [PERSON] et al., both FD and air drying techniques were used to dry purple carrots to obtain a high-quality product [74]. At the end of the study, while FD did not cause any change in the carotenoid and anthocyanin content, air drying reduced the number of carotenoids, anthocyanins, and polyphenols in purple carrots. However, the total phenolic compounds of purple carrots were reduced by FD. Also, in the research of [PERSON] et al., African eggplant was dried with FD, oven drying, vacuum drying,and solar drying to determine the best technique to obtain maximum product quality [122]. The results demonstrated that African eggplant was most effectively preserved while retaining its bioactive properties using FD. Several results were also obtained aside from FD. These results explained that oven drying at 70 \({}^{\circ}\)C significantly degraded the beta-carotene of the sample and indicated high temperatures that caused the degradation of the beta-carotene, while vacuum drying at 70 \({}^{\circ}\)C preserved more beta-carotene of the sample compared with oven drying at the same temperature, and solar drying caused the highest degradation in total phenolic compounds of the sample due to the activation of polyphenol oxidase enzymes.