*5.4. Nontraditional Source*

More recently, there is has been an increasing trend of consumption of nontraditional foods or food from an alternative source. One such nontraditional plant source food is maple syrup. Maple syrup is composed of a mixture of sugars (66% sucrose, 0.4% glucose, and 0.5% fructose), minerals and water, and traces of organic acids, proteins, and polyphenols [66]. Bhatta et al. [67] studied the drying of maple syrup to produce a maple sugar powder. FD of sugar-rich foods is challenging due to high hygroscopicity of simple sugars, the increase in solubility with temperature, a low glass transition temperature of sugars (fructose, glucose, and sucrose; *Tg* = 5, 31, and 62 ◦C, respectively) [68], and the stickiness problem in the drying equipment [69]. The dilution of maple syrup from 66 to 20 ◦Brix was needed to produce a dried maple syrup powder. Such freeze-dried maple sugar powder exhibited an instant-like property as it dissolved within 14s, but showed fair to poor flow characteristics due to cohesiveness nature of sugars. Authors have also suggested the use of glass transition temperatures (related to collapse temperature) for the determination of FD temperatures and online temperature recording with thermocouples for the identification of drying periods [67].

#### *5.5. Generalities about Impact of Freeze-Drying on Biocompounds*

Several studies investigated/reviewed the important impact of di fferent drying techniques, including freeze-drying, on the active ingredients and phytochemical contents of fruits, vegetables, and herbs and medicinal plants [70,71]. In particular due to freeze-drying, Marques et al. [57] showed significant losses in vitamin C in tropical fruits (3% to 70% depending on the fruit). Araya-Farias et al. [44] reported 20% loss in vitamin C and in total carotenoids, 35% loss in vitamin E, but only 4% loss in total phenolics when freeze-drying seabuckthorn berries at 20 ◦C shelf temperature and 30 mTorr vacuum pressure. So, although an excellent choice to preserve plant-based foods, freeze-drying cause some decrease in phytochemical content.

Nevertheless, when compared to other drying techniques, usually freeze-drying is a superior technology. Asami et al. [72] reported that freeze drying preserved total phenolics in marionberries, strawberries, and corn better than air drying. Sablani et al. [70] showed that compared to air drying, freeze drying improved retention of anthocyanins, phenolics, and antioxidant activity during processing of regular versus organic blueberries and raspberries, and in some cases it even increased the concentration of phytochemicals. Reyes et al. [73] also indicated that ascorbic acid content in blueberries was significantly reduced by freeze-drying in any operating condition, while the total polyphenol content was apparently augmented when a vacuum was used (compared to atmospheric pressure), an increase attributed to an improvement in the extractability of polyphenols. To conclude, for vitamin C and phenolic content retention, vacuum freeze drying most of the time gives the best results.

In terms of preserving β-carotene, lycopene, vitamin E, unsaturated oils, and other lipid-based oxidizable bio-compounds in fruits and vegetables, freeze-drying and storage of freeze-dried products should be taken with high consideration since autocatalytic oxidative reactions are accelerated at very low water activities achieved during freeze drying. As an example, Gutierrez et al. [45] indicated that oils from freeze-dried pulps of seabuckthorn berries had a much lower peroxide value than those obtained from air-dried berries, showing that low water activities attained during freeze-drying could damage the quality of lipid-based biocompounds.

#### **6. Pretreatments and Process Intensification**

Process intensification with innovative technologies (external to the product) or additional pretreatments (internal) prior to or during freeze-drying are key approaches aimed at efficiently overcoming processing challenges to increase mass transfer or improve product quality.

Chemical, mechanical, and thermal pretreatments have been used to reduce the effect of plant skin hydrophobicity and promote water transport during drying of whole berries. Chemical pretreatment involves immersion of the product in alkaline or acid solutions of oleate esters prior to drying. Alkaline dipping facilitates drying by forming cracks on the fruit surface [74]. However, the high temperature (100 ◦C) of the chemical solution and the long periods of soaking causes texture degradation and a low level of taste acceptability due to the incorporation of chemical residues in the fruit flesh [75]. Mechanical pretreatments might replace or complement chemical pretreatments, mainly because of the higher acceptability levels [76]. It consists of peeling, abrasion of the surface, puncturing the skin, or cutting the fruit in various shapes [77]. Araya-Farias et al. [44] halved sea-buckthorn berries before freeze-drying in order to produce high quality powders out of this oily, impermeable skinned fruit. Some other pretreatments include exposure to sulphur dioxide and thermal pretreatments such as blanching (immersion in hot water) or steaming [76]. However, blanching may cause the loss of soluble substances like proteins and mineral elements while high temperatures may induce the loss of heat labile substances such as nutrients and vitamins [78].

On the other hand, there has been little research done on freezing pretreatments prior to dehydration methods. Slow freezing helps the formation of large extracellular ice crystals damaging vegetable tissues while rapid freezing promotes intensive nucleation and formation of intracellular small ice crystals [79] and freeze-fractures and cracking in food tissues [80–82]. Water permeability of plant tissues depends on their composition [83], microstructure [84], crystalline or amorphous state of the matrix, and the lipid and glass transitions occurred during cooling or heating the tissues [85,86]. All these reported effects of freezing on vegetable and fruit tissues can certainly be used to induce positive changes in the food microstructure so as to increase drying rates or to improve dried product quality. Individual quick freezing (IQF), a rapid individual freezing of berries in a thin layer at −40 ◦C for a specified time [87], has been used in cycles with slow thawing in the refrigerator at 4 ◦C. This mild heat shock (−40 ◦C to +4 ◦C), together with the repetition in cycles, led to slight changes in the permeability of the waxy cuticle, sufficient to increase the drying rate [88].

Liquid nitrogen cyclic immersions of blueberries, seabuckthorn berries, and grapes markedly increased the drying kinetics during hot-air, vacuum, and freeze-drying [89]. The initial fruit epidermis thickness decreased between 20% to 50% (depending on the fruit) after 3–5 immersions in liquid N2. Also, dewaxing of the plant surface was observed after immersions in liquid nitrogen for lowbush (200.33 ± 3.05 to 152.70 ± 0.7 μg/cm2) and highbush (227.5 ± 2.12 to 112.17 ± 1.66 μg/cm2) blueberry cuticles [32], which explains the significant impact of this pretreatment on mass transfer acceleration during drying.

Dilution of a concentrated product is an easy solution to overcome problems indicated previously when dealing with liquid foods in high sugar/lipid concentrations. However, adding water and afterwards having to taking it out by an expensive method such as freeze-drying is not always an affordable solution unless necessary. Bhatta et al. [67] diluted maple syrup to 20% prior to freeze-drying as a first step of producing ultimate quality maple syrup powders. A pretreatment option for solving quality problems when freeze-drying high concentrated liquids, or solid plant foods having a moisture barrier (i.e., berries), is to provide the freeze-drier with frozen particulate systems instead of a tray of frozen liquid in a block. To achieve this, one simple way is to grind the frozen liquid at ultralow low temperatures. Then, the smaller size frozen particles are freeze-dried. This method was first reported for obtaining freeze-dried powders from plant tissues for botanical analytical use, by grinding the samples under liquid nitrogen, followed by freeze-drying [90]. One of the drawbacks, though, is a wide particle size distribution of the freeze-dried powder.

The more recent freeze granulation technology involves spraying droplets of a liquid slurry or suspension into liquid nitrogen followed by freeze-drying of the frozen droplets [91]. The above-mentioned process is illustrated in Figure 5 (adapted from [92]). The significance of this technology is that the structure and homogeneity of the particles in the slurry or suspension are retained in the granules.

**Figure 5.** Freeze-granulation process.

Another way of pretreating hard-to-freeze-dry solutions/suspensions/emulsions is by foaming, in the so-called 'foam-mat freeze-drying' process. In general, drying of foamed materials is faster than that of nonfoamed ones. Drying experts have repeatedly pointed to the increased interfacial area of foamed materials as the factor responsible for reduced drying time. However, because density of foamed materials is lower than that of nonfoamed ones and extends from 0.3 to 0.6 g/cm3, the mass load of the foam-mat dryer is also lower. Thus, shorter drying times should not only offset the reduced dryer load but also increase the dryer throughput. Raharitsifa & Ratti [93] revealed that freeze-drying of foamed apple juice was limited by heat transfer, while for nonfoamed one, by mass transfer. In this study, it was shown that the insulation property characteristic of foams was more significant in slowing down the freeze-drying process than the increased surface area available for mass transfer due to foaming. Although freeze-drying rates were increased by foaming, no practical minimal sample thickness could be found in order to increase freeze-dryer throughput as well. In further experiments, Raharitsifa & Ratti [94] reported that, confirming the glass transition temperature results, at 20 ◦C storage temperature and in presence of air, nonfoamed freeze-dried apple juice powders collapsed with marked change of color, while foamed freeze-dried products were stable for up to 70 days.

Some technical challenges facing freeze-drying include long residence times, batch operation mode, high operating cost, and energy consumption. It has been already pointed out that any new development to the classical vacuum freeze-drying should aim to improve heat transfer in order to help sublimation and reduce drying times, or to reduce/avoid the use of vacuum so as to decrease costs [3]. In recent years, studies have focused on development of process intensification technologies to resolve some of these issues.

Infrared energy impinges into the exposed material surfaces and propagates through the material to increase thermal energy through molecular vibration, which has relatively lower losses compared to other types of heat sources [95]. Application of infrared radiation in freeze-drying of plant-based foods significantly diminished drying time for sweet potato [96] and apple [97], and enhanced quality characteristics of the final product due to uniform surface heating [98], such as in the case of aloe vera [95], strawberry [99], and banana [100].

Lately, the application of microwave energy to intensify freeze-drying regained attention. Microwave heating has been studied since the 1970s in relation to the acceleration of freeze-drying [3]. The attractive aspect of this heating source is that it is an energy input that not only is essentially una ffected by the dry layers of the material undergoing freeze-drying, but also that is absorbed mainly in the frozen region [101]. Since the frozen region has a high thermal conductivity, microwave energy helps sublimation to decrease freeze-drying times up to 60–75% [102,103]. In addition, when compared to conventional freeze-drying, microwave assisted freeze-drying (MFD) may lead to products of similar/better quality [102,104,105]. Although microwave freeze-drying can o ffer unique advantages, the inherent problem preventing its commercialization is the di fficulty in controlling the final product quality and assuring its uniformity, resulting from corona discharge and nonuniform heating, which cause ice melting and overheating [15]. In an interesting review of microwave assisted freeze-drying of foods, Duan et al. [104] pointed out that to assure a successful implementation of this type of technology in industry, the following challenges should be tackled: operation scale-up, accurate temperature monitoring, appropriate simulation of the MW field distribution, and increase in the knowledge on dielectric properties of foods. Thus, most of the numerous articles published lately considered the impact of microwave freeze-drying on di fferent aspects of the final product quality and uniformity, such as the case for banana [106–109], barley grass [110], potato [108,111,112], mushrooms [113–115], apple [116–118], lettuce stem [65,119,120], okra [121], etc. In a recent review, [122] described an overview of the current developments in microwave assisted freeze-drying of fruits and vegetables, where they concluded the need for other novel nonthermal technologies, such as ultrasounds, high pressure processing, or pulsed electric fields, to improve quality of freeze-dried heat-sensitive fruits and vegetables.

Atmospheric freeze-drying (AFD) also has been getting increased attention recently. In this technique, freeze-drying is done at atmospheric pressure under inert dry gases. Although discovered at the end of the 1950s [123], this freeze-drying process started to pick up scientific interest mainly in the mid 1980s. Due to the lack of vacuum use, the cut o ff was approximately 34% as compared to vacuum freeze-drying [124]. However, drying times increased 1 to 3 times since the use of atmospheric pressure turns the control of the process from heat to mass transfer, which makes the kinetic extremely slow [3]. Other studies showed that in addition, quality of products was not excellent when atmospheric pressure is used instead of vacuum, since the risk of product collapse increased [125]. In order to accelerate drying kinetics, as well as improve quality issues, AFD has been combined with other techniques such as fluid-bed and spray freeze dryers [123]. As pointed out in this AFD review done by Claussen et al. [123], newer investigations of atmospheric freeze-drying in a fluid bed have looked into a process where a heat pump system is included with the drying system.

More recently, power ultrasound proved to be an e ffective, nontoxic, and environmentally friendly way to speed the AFD process [126]. Repeated compression–expansion cycles generated by the ultrasound helps to create of micropathways in the solid to ease the vapor flow and microstirring at the-solid fluid interface, reducing the external mass transfer resistance [127]. Moreover, the additional exertion of a mild heating e ffect increased the interest in the ultrasound-assisted atmospheric freeze-drying of thermally sensitive products.

A compilation of the last 15 years of scientific publications in the area of atmospheric freeze-drying of plant-based foods can be found in Table 3, where their main objectives and conclusions are detailed. Most of these articles deal with improvements of the AFD process by the use of heat pump application to enhance economy aspects or final product quality, or by spout and fluidized beds with and without immersion adsorbents, pulverization by spray, set-up temperature programs, and ultrasound applications as drying strategies to accelerate AFD. Theoretical mathematical modeling [128–130] is an interesting strategy to understand the AFD process and simulate it under diverse conditions, to tackle AFD concrete problems on a solid basis. To end, it was surprising to see that little attention has been paid in some published works to verify that actual freeze-drying instead of air-drying of a frozen product was happening through all the process under atmospheric pressure. Compulsory continuous process humidity determinations and control should be included in future atmospheric freeze-drying studies.




