and when utilized for drying, the probability of giving case hardening is less during microwave heating [4]. It could operate both in the continuous and batch process because of the integration of the microwave energy with the convection process. Microwave aids in the general energy conservation which is created from the entire plant volume. Microwave heating is also the predominant pertinent to food processing since it has the aptitude to evade the charring of temperature-sensitive resources. A key shortcoming can be the equipment’s capital rate and consequently most expected to be utilized for top-quality products [47].
Table 1.2 List of some techniques combined with microwave technology.
| Mode | Applications | Benefits | Drawbacks | Reference |
|---|---|---|---|---|
| Ultrasound | Enzyme activity Drying Extraction | Precise electronic control.Competent energy savings. | - | [38] |
| Cold Plasma | Sterilization of microorganisms | Maintained product quality. | Inappropriate for impenetrable peels.Probability of getting slenderize. | [35] |
| Infrared Heating | Tempering Baking Drying | Refining rehydration Properties and quality.Reduced processing time by 95%. | Probability of escalating compactness and slenderness standards.Manufacturing constrained by the equipment size and operating cost. | [55] |
| Freeze Drying | Dehydration | Quick energy dissipation.Energy savings up to 40%.Advanced volatiles retention level. | Tough to control quality at high MRP. Lengthier drying time at low power. | [30] |
| Convective Drying | Dehydration | Reduce overheating. | Induces slight discoloration reactions. | [68] |
1.3.2.3 Application of Microwave in Food Processing Technologies
The food processing industry utilizes microwaving immensely for different purposes like cooking, preservation, drying, sterilization, and heating of foods [26]. These particular applications have several advantages, such as microwave drying offers lower bulk density and lower shrinkage along with overhead rehydration ratio and saves power when compared to customary drying [27]. Similarly, the antioxidant activity and bioactive compounds, as well as the striking colors of different fruits and vegetables cooked with or without water, could also be maintained through microwave cooking or heating. It can also minimize antinutritional aspects, temporarily upsurge in digestibility of in-vitro protein. And when it comes to microwave sterilization, it ensures not only food safety, but also reduces the potential risk of any microbes’ attack on the food, inactivating enzymes to preserve the nourishment of food. This section reviews various reports on different applications of the microwave, their advantages, and effects on the quality parameter of food materials.
Microwave Drying Drying is a complex volumetric heating process that involves heat and mass transfer [13]. The strong microwave radiation when penetrates inside the food item generates vapor and a pressure gradient that heats the food from the inside and outside at the same time with a simultaneous increase of temperature.
Microwave drying improves the quality of some food products with minimum drying time. A microwave uses high-frequency electromagnetic energy and converts it into heat. Wet products manage the energy absorption strength which carefully heats the interior parts of food samples selectively. The moisture present in the food vigorously evaporates and travels towards the surface without affecting the exterior parts of the sample [41]. The microwave drying process goes through two successive stages, i.e., liquid evaporation [26], and three phases of drying include heating, constant rate, and falling rate [5]. Limiting diffusion rate during the falling rate drying period results in shrinkage of the structure of the food. Nevertheless, drying in the microwave generates vapor inside and develops a core pressure gradient outside the product, prevents the shrinkage to food material, and therefore, the drying in the falling rate period is appraised to be very beneficial in microwave drying. Microwave drying when combined with various other methods for example microwave-convection, hot air microwave, vacuum-microwave, and microwave-freeze, microwave-infrared gives more efficient results in terms of quality of the food products which is not achieved by only microwave drying and other conventional methods [8].
Microwave-assisted Freeze Drying Heat sensitive foods like tomatoes or berries undergo the freeze-drying (FD) method for moisture removal which promotes easy rehydration and prevents chemical decomposition. However, FD takes longer drying time as well as being expensive, which ultimately leads to excessive energy cost and lower productivity [18, 29]. Therefore, combining FD with radiation significantly eases the limitations of FD with shorter processing time, higher energy saving plus efficient drying in the falling rate period as compared to the convention freezing process [18]. Dehydration of Fuji apple was stated by [41] using FD merged with Microwave-Vacuum, the study reported that time for drying is reduced by 40% with nil nutritional change using this double-step technique.
Microwave-assisted Vacuum Drying In recent years, with the rapid accepted growth and popularity, this method comes with the combination of volumetric heating and vacuum drying. The advantages this combination provides are express moisture evaporation and minimum structural and chemical changes of the final dried products [7]. The final results revealed that a combination of both the techniques at 90°C restored the anthocyanins and augmented antioxidant activity when related to supplementary approaches enlisted [84].
Microwave-assisted Infrared Drying IR drying has been utilized in the past years for a varied range of agricultural products due to its acceptance as an alternative technique. However, because of its low penetrating power into the food material, it is combined with microwave energy, and their synergistic effect was observed and correlated by [57] on the drying attributes of kiwifruit and banana. For both the samples, they reported a good amount of moisture loss with reduced drying time up to 98% when related to traditional drying. Similarly, an alternative study [66] also reported the standard quality of raspberries with the drying kinetics which showed the superior class final product at varied vacuum pressures and power levels yielding 17.55% better anthocyanin retention, 2.4 times exclusive crispness value, 21.21% advanced radical-scavenging action, and 25.63% higher rehydration properties than infrared drying (IRD) at finest settings.
Microwave Heating Microwave heating relies on volumetric heating of the food material instantaneously and can also be combined with the convective and radiant heating process [33]. The electric field induces the dipole rotation, generating friction between molecules inside the microwave which assists in heating the food materials [2]. The penetration depth of the microwave is dependent on the food composition and its accompanying changes related to the chemical composition of the food, i.e., cook loss, bioactive components, antioxidant activity, and anti-nutritional factors, comprising phytic acid, trypsin inhibitor, tannins, and saponins [40]. The chicken streak rigidity was lowered after microwave heating which was not significant when cooked with grilling or boiling reported by [9]. However, it was observed differently in the case