*5.1. Fruits*

Table 2 presents the wide range of freeze-dried fruits that have been reported in literature, including strawberry, blackberry, guava, pineapple, etc. [10,12,14,44–48]. Fresh fruits containing high moisture levels are di fficult to dehydrate by classical drying techniques due to significant damage to their physical attributes, mainly collapse and severe bleeding due to the rupture of skin. Shishehgarha et al. [14] studied the various quality parameters (drying kinetics, color, and volume variation) of sliced and whole strawberries under di fferent FD shelf temperatures (30, 40, 50, 60, and 70 ◦C). The level of shrinkage was found to be independent of FD shelf temperature, where whole and sliced strawberries had an average volume reduction of 8% and 2%, respectively. On the other hand, the risk of collapse was found to increase exponentially once the FD shelf temperature surpassed the glass transition temperature of dried strawberries [14]. Shrinkage and collapse of fruits are often encountered during drying, mainly by air-drying techniques. Hawlader et al. [12] compared quality of guava obtained by heat pump dryers (RH = 10%, v = 0.7 m/s, T = 45 ◦C, 8 h), vacuum (vacuum pressure = 15,000 Pa, T = 45 ◦C, 8 h), and freeze-dryers (freezing at −20 ◦C for 24 h, and less than 613.2 Pa vacuum pressure, 10 ◦C shelf temperature, 24 h for freeze-drying). Porosity, color, rehydration, and vitamin C retention of guava produced by FD were better, resulting in FD guava being the most desirable powder compared to that produced by vacuum and heat pump dryers. FD was able to retain 63% vitamin C, whereas heat pump dryer retained only 25%.

It is often challenging to dehydrate fruits with waxy impermeable skin, as described previously, such as seabuckthorn berries (*Hippophae rhamnoides* L.), which are delicate fruits containing natural antioxidants, ascorbic acids, carotenoids, and flavonoids. Araya-Farias et al. [44] investigated the effect of hot air drying (HAD) at 1 m/s and 50 ◦C or 60 ◦C, and FD (4 Pa of vacuum, 20 ◦C or 50 ◦C shelf temperature) to obtain dried seabuckthorn pulp (shown in Table 2). They found that FD retained 93% more carotenoids, 34% more vitamin C, and 11% more phenolics than HAD. Increased drying temperature resulted in increase of drying kinetics. However, no significant impact of drying temperature on overall nutritional retention was noticed for FD or HAD samples, although a slight decrease was found for particular compounds such as vitamin E for both drying methods while increasing the operation temperature. Between two drying processes, it was reported that drying kinetics were surprisingly faster in FD.
