Freeze-dried materials are especially recommended for the production of spices, coffee, dried snacks from fruits and vegetables and food for military or space shuttles, as well as for the preparation of food powders and microencapsulation of food ingredients.
Drying is a commonly used process to extend the shelf life of food. The most popular method of food dehydration is hot air drying, in which the food material is exposed to a stream of hot air. This method is simple and relatively cheap, but the quality of products obtained after drying is often significantly lower [1]. Freeze-drying (FD), also called lyophilization, is among the best methods of food preservation. Moreover, the FD technology is the most widely used for the preservation of bacteria for producing starters and probiotics [2]. At 0 °C and pressure of 611.73 Pa, the three states of water, namely vapour, liquid and ice, occur in aggregation [3]. This state of equilibrium is called the triple point. Below this point, the removal of water from the material can only occur as a result of sublimation [4]. Such a phenomenon is possible under adequate temperature and pressure (below the triple point to enable the conversion of ice into vapour), when water molecules have enough energy to break free from the frozen material, but the conditions cannot support the formation of a liquid [5,6]. Vacuum FD process is commonly carried out at a low temperature (shelf temperature below 50 °C) [7] and low pressure (below the vapour pressure at the ice surface). Typically, the vacuum levels applied in the FD range between 7 and 70 Pa [8,9,10,11]. Vacuum FD is especially recommended for delicate, thermal-sensitive and high-value food, the physical and nutritional properties of which should be maintained [12]. The absence of liquid water, low oxygen access in the drying chamber and application of low temperatures result in dried products of excellent quality [13]. In general, FD involves three stages: freezing, primary drying and secondary drying. The steps of the FD process were described in a recent study [14]. FD can also be performed under atmospheric pressure at a low-temperature range (−30 to −60 °C) with low-humidity air [15,16]. However, such a process is usually very slow and takes up to three times longer duration than vacuum FD [6].
Lyophilization allows almost complete removal of water from food [17]. In industrial conditions, freezing is mostly performed in a lyophilizer, whereas in the laboratory scale, food is often frozen in a refrigerator [5,18,19]. The rate of freezing significantly influences ice formation and determines the drying rate. A faster freezing rate results in the formation of small ice crystals. The size of ice crystals has a considerable impact on lyophilization. Sublimation of fast-frozen food, with small-sized ice crystals, occurs rapidly in the first drying period but is slower in the second period of lyophilization [17].
FD is also widely used in the pharmaceutical and cosmetic industries [20,21]. Because of its high costs (up to five times higher than hot air drying [22]), this process is mainly recommended for the preservation of heat-sensitive materials [6]. It is also widely used for the microencapsulation of bioactive compounds of food [23,24]. The reduction in FD costs with high-quality products is still considered a challenge. However, adequate food pretreatment can significantly decrease the energy consumption associated with FD [25,26,27] and improve the quality of dried food [28,29]. FD allows obtaining products of very good quality, with a low final moisture content of 1–4% [5]. The obtained materials are brittle and easy to grind, and therefore, FD can be used to produce powders from various biological substances [30].
Pretreatment of food before drying serves two purposes: reduces the drying time and improves the quality of the dried material. This review aims to point out the recent trends in the pretreatment of food before FD and show how the different methods of pretreatment influence the properties of the dried materials and drying rate.
This entry is adapted from the peer-reviewed paper 10.3390/pr8121661