**1. Introduction**

Plant-based foods, including fruits, vegetables, seeds, beans, spices, etc., are important components of a healthy diet, and their sufficient regular consumption could help to prevent certain major diseases such as cancer and cardiovascular diseases, etc. According to the combined report of World Health Organization and Food and Agriculture Organization, it was recommended that a daily minimum consumption of 400 g of fruits and vegetables may help to minimize the occurrence of chronic diseases along with the mitigation of micronutrient deficiencies [1]. Fresh plant-based foods may not available all year round for consumption and the long-term storage of fresh foods could be challenging due to high water content, unavailability of cold-storage facilities (particularly in underdeveloped and

developing countries), and possibility of nutritional deterioration. Consequently, drying of such foods may allow their long-term consumption and eases handling, transportation, and storage.

Freeze drying (FD), also known as lyophilization, is a well-known technique for the production of high quality food powders and solids [2,3]. It is a preferred method for drying foods containing compounds that are thermally sensitive and prone to oxidation since it operates at low temperatures and under high vacuum. Application of FD to various plant-based foods, such as apple, guava, strawberry, blackberry, pumpkin, tomato, asparagus, co ffee, tea, garlic, ginger, maple syrup, etc., has already been reported in literature [4–15].

Drying of food results in food quality changes [3]. The quality of foods can be divided into three sections: physical, chemical, and nutritional. Major qualities of foods that can be a ffected during drying processes are color, odor, texture, rehydration property, bulk properties, flow property, water activity, and retention of nutrients and volatile compounds [16]. Regarding to nutritional qualities, oxygen, high temperature, and cell damage are usual enemies of bioactive compound retention during processing. The stability of the valuable compounds of plant-based foods can therefore be a ffected during dehydration. Phenolic compounds could be susceptible to enzymatic degradation due to the polyphenol oxidase activity [17]. In addition, carotenoids have a highly unsaturated nature, making them susceptible to degradation by oxidation and thermal processes. Oxidation is the major cause of carotenoids degradation and can be generally considered autocatalytic, beginning only after an induction period in which radicals are built up and antioxidants are depleted [18]. Also, the loss of vitamin C and carotenoids is a ffected particularly by the temperature and the moisture content during drying processes [19]. Vitamin C is usually selected as a reference index of the nutrient quality due to its labile nature compared to other nutrients in foods [20], and thus, if ascorbic acid is well retained, other nutrients would be as well. A few interesting reviews on the impact of drying methods and operating conditions on functional quality retention can be found in the literature [16,21].

Freeze-drying method by lack of liquid water, oxygen-free environment (if operated under vacuum), and low operating temperatures is thus the best choice to dehydrate fruits and vegetables in order to keep an optimized biocompound content in the final products. Despite the long drying time and expensive process, freeze-drying is widely used to produce high-value food products due to maximal retention of food quality when compared to other drying techniques. FD is considered as the standard or reference drying method in most research studies. Lately, various process intensification approaches have been implemented in order to overcome the challenges facing by FD methods, such as either pretreatment of the sample or the use of innovative technologies including infrared, microwave, and ultrasound energy with freeze-drying.

The aim of current review was to highlight the application of FD of plant-based foods, to point out some particular technological challenges, and describe process intensification in FD of plant-based foods to improve the quality of freeze-dried foods or to accelerate the process.
