Use of dehumidification in dehydration is among the oldest and most prevalent processes used for food preservation. Foods are typically dried to achieve shelf stability and therefore, moisture is removed to the point where the water activity of the product is sufficiently low to ensure that the product is microbiologically and enzymatically stable. In order to ensure stability while in storage, the water activity is usually required to be lower than 0.7. In this state, the product undergoes limited deterioration during storage which allows the product to be made available outside of the normal harvest times without necessitating expensive continuous refrigerated or frozen storage as shown in figure 1. There are other benefits to dehydrated goods, where as dried products may be incorporated more easily into manufactured food products. Common examples of this include their incorporation into baked products, breakfast cereals, or ready to eat snack mixes. Moisture removal is also an effective method to lower transportation by reducing the weight of the product.
There are however, some drawbacks to drying food products. Drying alters the characteristics of food products, where in general, dried fruit suffers from several faults including loss of juiciness and lignified tissue, both of which result in a harder or chewier product. Other typical quality loss attributes include case hardening, wherein the outer layers of the product are overdried in the process of removing moisture from the less accessible inner core of the product along with product shrinkage are among the issues associated with convective air drying. Additionally, changes can be noted in terms of loss of flavours and aroma volatiles, deterioration of colour and texture, and an overall decrease in nutritional value, effects that are largely attributed to products’ exposure to high temperatures for long periods of time in the presence of air. These effects are, therefore, exacerbated by the oldest and the most widely used dehydration method in the food industry, convective air drying. While air drying has proliferated because of its simple operation, relatively low construction costs, ability to burn biomaterials to provide the necessary thermal energy, and well characterised operating principles, convective air drying has also been shown to be hugely energy inefficient and destructive to product quality.
There is an ongoing push from increasingly health aware consumers for higher quality dried products that maintain more characteristics of the fresh product. With that in mind, the key to improving the quality of dried products is to limit changes to the aforementioned quality characteristics during processing. In addition to quality concerns, the main interest from the industrial perspective should be from the perspective of reducing energy consumption and associated long term savings.
Different techniques of dehumidification
Microwave heating in foods
Microwaves are members of the electromagnetic spectrum in the frequency range of 300MHz to 300GHz. Frequencies reserved for microwave heating applications include the 915MHz, 2450MHz, and 5800MHz bands, where 2450MHz is the most commonly used for food production, and exclusively so in consumer-based microwave ovens. Microwaves heat foods differently than conventional heating methods, where in conventional thermal processes, energy is transferred to the material through convection, conduction, and radiative heat transfers. Conversely, microwaves cause the material to heat internally according to the dielectric properties of the target material, causing an effect known as volumetric heating.
More specifically, heat production within the food product is the result of the following two mechanisms: first by molecular friction due to the rapid movement of molecules with permanent dipole moments in response to the changing direction of microwaves, which takes place 2.45 billion times per second in the case of the 2450MHz band. Secondly, the charge drift of ionic species under the action of the microwaves leads to collisions between ions and increasingly disordered kinetic energy throughout the sample and subsequent heat generation. Non-polar molecules that are asymmetrically charged may behave as dipoles in an electric field, but their responses to microwave energy are typically an order of magnitude less than that of water.
Microwave vacuum drying
The application of vacuum during microwave drying has been considered for many years to be a good solution for alleviating physical damage caused during microwave drying such as scorching, off-color production and uneven heat distribution. Because of the presence of a vacuum, during microwave-vacuum (MWV) dehydration the continuous application of microwave energy can cause massive fluxes of vapour towards the surface early in the process, and cause damaging temperature spikes later in the process. Moreover, it has been shown, at least on a bench top scale, that continuous application of microwave energy does not accelerate the rate of water removal once a critical moisture content has been reached and it been shown that there is no energy or quality advantage of continuous over pulsed application of microwave energy. As a result, the magnetron duty cycle is typically altered during these experiments in order to limit power application to the samples.
Open sun drying is a traditional method used widely after harvesting. Although its operating cost is quite low comparing with those of other drying methods, sun drying requires long drying time depending on the climate conditions. For example, a practice for cocoa production after fermentation in Brazil requires sun drying on wooden floor platforms with movable roofs for 1-12 days. Sun drying crops on concrete floor is preferred as this floor type provides better hygiene and shorter drying time; since concrete floor is better heated by the sun, it helps dry the crops faster. Natural ventilation drying, heated ventilation drying and stirred ventilation drying are also common methods that can be used to reduce the grain moisture content after harvesting.
Super heated steam drying
Superheated steam drying (SSD) involves the use of superheated steam in a direct (convective) dryer in place of hot air, combustion, or flue gases as the drying medium to supply heat for drying and to carry off the evaporated moisture. Apart from its advantages in terms of the ability to produce a dried product with higher porosity, better colour and more nutrition, SSD has also proved to be more effective than hot air drying in terms of microbial inactivation.
Heat pump drying
Heat pump drying (HPD) is a rapidly emerging technology, which can be used to dry spices within a controllable drying environment, specifically, the temperature and humidity. Heat pumps transfer heat from a source of heat to a destination called a heat sink and one of the important factors to be noted here is that it uses comparatively a small amount of external power to accomplish this task. Heat pumps have been designed in such a way that the thermal energy moves in the opposite direction of spontaneous heat flow by absorbing heat from a cold space and releasing it to a warmer one. Maintaining and controlling the correct levels of moisture content throughout the processing is the key factor in achieving the expected quality and it can be achieved using HPD system as it uses dehumidified air as shown in figure 2 for drying at relatively low temperature while preserving the volatile compounds. In addition, environmental concerns are minimal and economic viability is in an acceptable range. It is, therefore, HPD can be considered as an alternative method for drying chilli owing to its specific characters. However, designing and evaluating of HPD system in the chili processing industry is yet not well developed and it is, therefore, the study was conducted with the main objective of evaluating the compatibility of HPD system for mass drying of chili to be used in industry by analysing the specific moisture extraction rate (SMER).
Desiccant based dehumidifier drying
Desiccant dehumidifier dryer consists of a desiccant dehumidifier and drying chamber as shown in figure 3. The drying chamber is connected to dehumidifier by flexible pipes. The drying chamber made up of plywood and consists of five trays arranged one over the other with sliding rollers, plenum chamber, air inlet and chimney. Low humidity is maintained during drying ranges from 17-20 per cent and the drying temperature of 45C were selected for drying, keeping in view the product quality as well as time required for drying. For comparison of drying time and quality of dried product, samples were also dried in tray dryer at same drying air temperature. Desiccant food drying system can be used for drying various agricultural products, efficiently and economically without compromising with environmental conditions. The regeneration temperature and air flow rate of the dehumidifier was controlled to maintain constant optimal temperature (45C) and uniform drying rates within the drying chamber.
With an awareness of the dehumidification capabilities available for the food and beverage industry, many of the lessons learned in food processing facilities can be applied in the greenhouse industry. The most important aspect concerning optimisation of the greenhouse climate is with high humidity that can damage plant growth. In extreme conditions, excessive greenhouse RH can give rise to water droplets forming on the leaves, which can incubate and spread fungi, while the excessive heat will shrivel many types of fruits and hinder growth. In other industrial greenhouse projects, the conditions outside a greenhouse can impact the natural ventilation process negatively. To assure perfect conditions for plant growth, refrigeration-based dehumidification systems targeted for high temperatures and high moisture levels help sustain the best growing conditions. As with beef and poultry applications, a chilled water-pre-cooling coil is mounted directly at the dehumidifier inlet. This design allows for removal of much of the initial heat and moisture prior to entering the dehumidifier where the moisture is reduced even further.