**5. Advanced and original drying methods**

### **5.1 Instant controlled pressure drop method (détente instantanée controlee, DIC)**

Major technical innovations in fruit and vegetable dehydration include pre or posttreatment under high or low pressure. At present, the instant controlled pressure drop method known by its French abbreviation DIC (détente instantanée contrôlée) was innovated as a solution to overcome the issues of drying shrinkage/collapse, to obtain a better quality of the dried plants. The DIC is characterized by its ability to handle a broad range of food products and is considered one of the newest innovative drying methods, regardless of its sensitivity to heat. The DIC processing method was developed, defined, and studied by [162]. DIC as a drying method is focused on exposing the product to high-pressure saturated steam for a few seconds and then followed by an abrupt pressure drop toward a vacuum. Then hot air-drying processing took place after DIC treatment as a conventional drying method. As well, [163] mentioned that

DIC processing is based on fundamental studies concerning the thermodynamics of instantaneity. Where, DIC system consists of six main parts processing vessel, vacuum tank, pneumatic valve, steam generator, air compressor, and vacuum pump as shown in **Figure 33**. The DIC work idea depends on exposing a partially dried product (usually the humidity is close to 30% db) to a high temperature/short time (HTST) such as a vapor pressure (P < 1.0 MPa) at high temperature (below 180°C) for a short time (less than a minute). Then this HTST stage was treated with a sharp pressure drop to a vacuum (within the pressure range of 3–5 kPa, and Δt from 10 to 60 ms). This leads to a mechanical influence represented in a severe pressure drop in a very short time. As a result, automatic evaporation of a part of water inside the product and at the same time a cooling operation were stimulated, which stopped their thermal degradation and gives a controlled expansion of the product [164–168].

Hence, **Figure 34** accurately describes the different stages of the work idea of the DIC processing unit as follows:

A. An elementary vacuum is implemented to minimize air resistance and simplify the steam diffusion within the product, then rapid heating is happen.

**Figure 33.** *Schematic of the DIC technology.*

**Figure 34.** *Describes the work steps of the DIC system.*


According to, [169], over the past 33 years, this technology has continued to expand its food applications and improve its characteristics on an industrial scale. DIC technology has continued over the past 33 years, to expand its food applications and improve its characteristics on an industrial scale. Since the DIC implementation has shown a quantum leap in quality enhancement and reducing the cost in the food industry generally, this is achieved by reducing the drying time of fruits and vegetables and enhancing the essential oils extraction, vegetable oils, and antioxidant elements. Additionally, it eliminates plant microorganisms and germs, which cause contamination, and reduces non-food ingredients and allergens. As well as it provides strong decontamination eliminating vegetative microorganisms and spores and reducing non-nutritional and allergenic components. It is known that convective air drying is one of the most industrial food drying operations applied. However, it has some disadvantages such as shrinkage and collapse of the product structure, low kinetics, nutritional losses, long drying periods, and microbial contamination. To overcome these disadvantages, the convection drying method has been enhanced by combining it with the instant controlled pressure drop technology (DIC), which has obtained excellent results, to improve the drying process and the total quality of dried products [170]. This integration has been termed "Swell-drying," which involves a hot airflow for the pre-drying stage coupled with a DIC texturing stage. A swell-drying operation generally consists of a first hot airflow pre-drying stage until a specific water content (between 0.20 g and 0.50 g H2O/g of solid material), then followed by a DIC texturing stage (100–900 kPa during a few seconds, followed by an abrupt controlled pressure drop), and a final air-drying stage until the weight is stable. In this regard, [171] applied the DIC technology to the quality characteristics of cell wall polysaccharides of apple slices and studied their relationship to the texture. The results showed that it is possible to get apple chips with a crisp texture and excellent honeycomb-like structure by coupling convective air drying with the DIC technology (swell-drying). Also, the samples showed an excellent rehydration ratio to a homogeneous porous structure and a large specific surface area. As well, [172] applied the swell-drying technology on fresh banana pieces and studied the effect of dehydration kinetics, water and oil holding capacity, and nutritional characteristics. Drying kinetics showed that DIC technology increased the effective water diffusivity by 23%. Moreover, the water holding capacity increased by 290% under high-pressure conditions under DIC treatments, reaching 7.8 against 2.0 g H2O/g db, while the oil holding capacity was 0.60 against 1.30 g oil/g db for non-textured samples. Finally, it is noted that the DIC treatment inhibited the transformation of banana starch to reduced sugar. Additionally, concerning the strawberry fruits, the swell-drying technology is an excellent processing method to produce strawberry snacks with a high crispness behavior and a high rehydration capacity in less time than convect air-drying

conventional method [173]. As pointed out by [174, 175], the DIC treatment has a significant impact on strawberries' drying and rehydration kinetics, increasing the effective diffusivity, as well obtain the highest levels of phenols, flavonoids, and anthocyanins and antioxidant activities were achieved at 350 kPa for 10 s.
