The global population is increasing at an accelerating rate and the global climate is rapidly changing1,2. This creates a massive load on energy and food systems that must meet the requirements of an inflating population2. Energy and food systems also happen to be a major strain on the environment, positioning themselves as major hurdles that are offsetting efforts made to curb climate change3. Moreover, 30% of the crops harvested in agricultural regions are lost over their supply cycle4. The difference in changing market-supply of food items due to harvest cycles and weather conditions, against the relatively consistent market-demand of certain food items also results in significant losses of food products when the supply exceeds demand. Conversely, prices soar when the supply struggles to meet demand5. This creates an urgent need for eco-friendly food preservation processes that are reliable, safe, and fit to make global food systems more secure. Suitable food preservations processes are characterized by their easy adaptability for producers operating in agricultural regions, low energy consumption, short yield times, and competitive product quality.
Post-harvest food losses. It is known that one third of the food that is produced globally is lost over its supply chain6. In developing countries, most of this food loss occurs during post-harvest handling7. There are multiple factors that contribute to post harvest losses. One of the biggest factors happens to be the high perishability of food items8. Producers attempt to preserve their food items and increase their shelf life, or add value to them, by processing them into dried snacks. However, most affordable food processing methods fail to preserve most of the crops to an acceptable degree of quality9. While there are industrial-scale food processing systems available in developing countries, they are energy intensive and cannot be procured and operated by small and medium scale producers due to their high capital and operational costs10. Energy inefficiency of drying methods that deliver product with competitive quality makes them unsustainable and therefore unfit to meet sustainability goals in the future. Post-harvest losses are also linked to nutrient deficiencies in developing regions11. For these reasons, reducing post-harvest food loss and increasing shelf life of food items is vital to make the world’s food systems more secure and immune to climate change and the increasing demands that come with population expansion.
Climate Change. The global population is expected to reach 10 Billion by 205012. This will be followed by higher overall consumption of food and energy. Global food production will have to increase by 60% to meet the demands of future population13. The strain incurred on food systems is therefore expected to increase the overall Carbon emissions of food production and processing3,14. Food systems are already responsible for 34% of total GHG emissions, releasing 18 equivalent Giga-Tons of CO2 annually15. Given that most technologies that preserve food without deteriorating quality are energy expensive, food preservation and storage is expected to strain energy harvesting and storage systems as well17. Reduction in post-harvest losses, therefore, is not only related to economic safety of farmers and countries reliant on agriculture, but also to the environmental safety of a warming planet18.
Food dehydration. Going as far back as 1490 BC, drying is one of the oldest methods of preserving food items. In modern times, it is still one of the most popular methods of food preservation19. While most of the world dries its food by simply leaving it out in the sun with some sort of porous or transparent covering to keep the pests away20, the dried food products that meet consumers’ nutritional and cosmetic criteria to pass as ‘snacks’ are produced with energy inefficient processes like Freeze Drying, Osmotic Dehydration or Vacuum Drying21. 25% of the total energy spent on food processing goes solely into drying22. The data indicates that while drying of food items is one of the most important and scalable methods to preserve food, it also has a massive impact on the climate22. Thus, it is imperative that the academia and industry working in food preservation focus on drying methods that are low-cost, eco-friendly and produce quality product that incentivizes small and medium scale producers with high profit margins and low operational cost.
Methods of dehydration. As stated, the most popular method of food dehydration is Open Sun Drying. The primary attraction with this method is that it requires little to no capital and operational cost and can be used to dry large quantities of food. However, its primary drawback is that it can often take several days to produce a feed of dried food. During this time, the food is left vulnerable to changing weathers, fluctuating humidity levels and pests9. The long drying times also result in almost complete deterioration of nutrient content present in food and damage the color and shape of food items due to high levels of shrinkage and oxidation23,24.
Another common method of dehydrating food items is Convective Drying. Like Open Sun Drying, Convective Drying works by heating up the surface of a food item. Since the surface of the food item is saturated with moisture, the layer of air surrounding the surface becomes both hot and humid and thereby high in vapor pressure. This causes the water present on the surface to evaporate into surrounding air25. The surface also conducts heat energy into inner layers of food item, causing moisture to migrate from inner layers towards the surface where it is evaporated26. Convective Drying is often conducted with gas fired ovens to dehydrate snacks. But in many parts of the world, natural and forced convection with solar flat plate collectors is becoming more and more common to dehydrate food products27. This is called Green House Drying (GHD). The reason GHD is becoming popular is that the cost of a small green house or solar thermal collector is quite low and the system that is produced can dehydrate food items within a single day, significantly cutting down the drying time incurred with Open Sun Drying28. Compared to Open Sun Drying, Convective Drying has been found to drop the drying time of figs by 38%29, cherry tomatoes by 74%30, green chilli by 53%31 and cassumunar ginger by 67%28. Due to most of the energy used in this process coming from solar thermal sources, Solar Convective Drying is a very low emissions drying method. The problem with it, however, is that product quality is not up to mark for international market standards. Exposure to high temperatures over prolonged durations deteriorates the color of food items. While the end quality of food obtained with Convective Drying is still better than Open Sun Drying, it is not on par with high energy and eco-hazardous methods like Freeze Drying32.
Also called Lyophilization, Freeze Drying is the leading drying method used to produce dried berries and snacks retailed on international shelves33. This method of food dehydration works very differently from techniques that evaporate moisture via heating. Instead of heating the moisture in food from liquid to vapor (gas), Freeze Drying freezes it into solid ice crystals and then quickly turns the solid ice into vapor directly when temperature is elevated, via sublimation34. The water vapor is extracted by a vacuum pump. The mechanism of moisture removal in Freeze Drying is sublimation, as opposed to evaporation35. Since food is mainly damaged by exposure to high temperatures in other methods and Freeze Drying operates at low temperatures, it is characterized by high nutrient, color, and shape retention33. This makes Freeze Drying fit to dehydrate foods that are vulnerable to thermal damage 36. Since the process involves limited air contact and flow, Freeze Drying is also fit for drying foods like apples and bananas that are highly vulnerable to oxidation damage37. For these reasons, Freeze Drying is often used as a metric to test the quality of food items dried with other methods32. But the problems with Freeze Drying are its long drying times and high energy consumption.
Freeze Drying consumes 58% higher energy compared to electric resistance Convective Drying and 500% higher energy compared to Microwave – Convection Drying10. It is also found to consume 180% or 1.8 times more time to dry fruit compared to Convective Drying and 1728% or 17.3 times more time compared to Microwave assisted Convective Drying 10. Owing to these reasons, Freeze Drying is a highly eco-hazardous drying method and due to the capital cost of equipment and operational cost of energy, it is not fit for small and medium scale producers in developing countries to use Freeze Drying.
Radiation Drying is conducted using Ultrasonic, Infra-Red or Microwave radiations. Radiative methods work by exciting the moisture present within all of the food item rather than just the surface17. This results in very short drying times, and fast drying rates38. Volumetric heating of food, as opposed to surface heating, also reduces shrinkage and nutrient deterioration since the dry mass of the food item does not undergo extreme thermal stress39. Out of the three common types of penetrative radiations, Microwave radiation results in the most uniform heating and nutrient retention40. Compared to Convective Drying, Microwave Drying has been found to drop the drying time of apple by 55%41 and button mushroom by 90%25. It has also been found that MWD at lower temperatures preserves cosmetic value and prevents food from undergoing high levels of oxidation and scorching42.
While MWD is a reasonable balance between product quality, drying time and energy efficiency, the quality of product is still not comparable with Freeze Drying10. Therefore, Microwave Drying is preferably combined with other methods43. Combined methods tend to overcome the limitations of individual methods and enhance their benefits. One of the most common hybrid or combined food drying methods in literature is Microwave – Convection Drying. Microwaving a food item while blowing hot air over it results in heating of the bulk food compound without allowing the surface temperature to rise to undesirable levels44. This is because the surface temperature is maintained close to the temperature of hot air which is facilitating a higher rate of evaporation as well. This control over surface temperature reduces scorching of food surface, results in better nutrient and color retention and reduces the total drying time compared to each of the individual methods45. It has been reported that compared to other hybrid methods like MWD+CD+US, CD+USD and CD alone, CD+MWD consumes lower specific energy i.e., energy consumed to remove 1 kg of water from a food item46. It has also been reported in literature that while MWD and CD on their own result in high levels of shrinkage of food item, combining them significantly reduces volumetric shrinkage47. Microwave – Convection Drying is characterized with 50% reduction in both drying time and specific energy consumption compared to CD on its own41. For foods with high moisture content and very porous, pulpy structures like lemons and oranges, MWD+CD tends to drop the drying time by as much as 97% compared to CD48.
However, Microwave – Convection Drying still does not retain cosmetic quality to the same degree that Freeze Drying does. The reason being that Microwave – Convection Drying still causes thermal damage to the food surface49. Scorching is a consequence of high surface temperature which is needed for high rates of moisture evaporation. High temperatures are needed for drying because rate of moisture evaporation depends on vapor pressure difference between the layer of air sticking to the food surface, and the many layers of air surrounding the food surface as shown in Figure 3. However, increasing the temperature of air surrounding the surface is not the only way of increasing this vapor pressure difference. The vapor pressure of the surrounding hot air can be reduced by dehumidifying it50. This results in the same or higher vapor pressure difference than that obtained with only Convection or Microwave heating, and unlike heating methods. Achieving a higher vapor pressure difference by using dehumidification does impart any thermal damage to the food surface51,52. Therefore, dropping the humidity of the air used for convection in Microwave – Convection Drying process can result in an equivalent drying rate and drying time without having to increase the surface temperature53. And since the temperature would not be elevated to high levels, dehumidification is supposed to deliver higher nutrient, color and shape retention compared to Microwave – Convection Drying process52. Some research studies in literature have demonstrated that conducting CD in the presence of a desiccant material can increase the drying rate or conversely, reduce temperature needed against a fixed drying rate 50,52,53.
In this research work, desiccant dehumidification was combined with Microwave – Convection drying. It was theorized that Microwave radiation would create a high rate of moisture migration from inner layers towards the surface; and the dehumidified air surrounding the surface would be able to help evaporate this moisture at lower temperatures, while cooling the surface due to evaporation. This is supposed to be especially helpful towards the end of the drying process when the surface is relatively more moisture depleted and therefore more vulnerable against high temperatures. Moreover, since desiccant dehumidification can be primarily powered by solar heat, the resultant process was low-emission, eco-sustainable and cheaper to operate. This is particularly helpful as reducing food loss, improving product quality and cutting down energy consumption are primary criteria to make drying of food a more sustainable method of attaining food security for the changing planet and population54. Color retention of the final product was considered to be the primary criteria for assessment of quality. This is because this research work was targeted on improving the cosmetic quality of food and converting agricultural produce into profitable fast-moving snacks. Cosmetic quality is a major criterion when assessing snacks55.
Methodology. Solar evacuated tube collectors were used as the heat source for both convection and desiccant regeneration. Convective food drying is a low-grade energy process that can operate at temperatures below 100 oC, and the desiccant material used, CaCl2, can also be regenerated at temperatures below 100 oC. The evacuated tube thermal collectors provide sufficient heat input and temperature to regenerate the desiccant material to high concentrations, and to heat the air used for Convective Drying to the temperatures required for this study. To maximize energy efficiency, the desiccant material was selected on the basis of regeneration temperature and the degree of dehumidification required. The desiccant material selected was Calcium Chloride (CaCl2) due to its high levels of dehumidification and lower regeneration temperatures compared to solid desiccants like Silica or Zeolite.
As shown in Figure 5, A concentrated or ‘strong’ CaCl2 aqueous solution was sprayed in the dehumidifier where it removed moisture from air and dropped its RH to low values (~15 %) The CaCl2 solution was diluted and slightly heated in the dehumidification process. The air was heated in this process as well due to the heat of vaporization being released as moisture converted from vapor to liquid. The heat is added to both the CaCl2 solution and the dehumidified air. The diluted solution was heated indirectly with an evacuated tube heater and the heated solution was sprayed to remove moisture from it, regenerating and concentrating the CaCl2 solution. The concentrated solution was brought back to ambient temperature to be used again for dehumidification. This was done because at lower temperatures, CaCl2 solution is able to provide higher levels of dehumidification.
The dehumidified air was pumped by a centrifugal fan the speed of which was controlled with PWM signal. Downstream of the fan, a fin-tube type Aluminum heat exchanger was attached. The hot distilled water from evacuated tubes passed through the heat exchanger tubes and heated up the dehumidified air before it entered a Microwave oven that was modified to allow air inlet and outlet. The Relative Humidity of the air dropped even further as it was sensibly heated with the Aluminum Heat exchanger. Inside the MW oven, a Stainless-Steel grill was placed on which 3 mm thick potato slices were scattered. The potato slices were blanched in hot water at 70 oC for 30 minutes and were cooled for 5 minutes prior to the start of drying experiments. Inlet and outlet air’s temperature and RH, and food items’ mass and surface temperatures were measured at 30 second intervals during the drying process. Each sample that was tested was kept at a starting mass of 35.5 g. Diameters of potato slices were varied to keep the mass of each sample for different experiments the same. The experiments were continued until the mass of the food item dropped from 80% water activity, down to 3% water activity, which corresponds to 7 g of mass containing 6 g of dry potato mass and 1 g of water.
The complete configuration of the drying process is illustrated in Figure 5 and the process is shown on a psychrometric chart in Figure 4. Process 1-2 represents desiccant dehumidification. Due to the heat generated in the conversion of vapor to liquid, the air temperature increases. The process is isenthalpic because the loss of latent heat (dehumidification) is equal to the gain in sensible heat (heating). Following the dehumidification process, process 2-3 represents sensible heating with Aluminum heat exchanger containing hot distilled water from solar thermal collector. This process also causes the RH to drop even lower. Finally, process 3-4 demonstrates food dehydration. As the moisture from the food changes from liquid to vapor and enters into air, it not only humidifies air, but it also cools it. Microwave radiation reduces the rate of cooling. The process 3-4 is inverse of process 1-2 as apparent from the parallel lines. This is because process 1-2 is isenthalpic dehumidification and process 3-4 is isenthalpic humidification or evaporation. Finally, air is exhausted after process 3-4.
Equipment. The weighing machine used to weigh the potato slices was calibrated against lab-weights. The machine had a least count of ±0.1 g. The air speed of the fan was measured via a hot wire anemometer that was calibrated by the manufacturer. The anemometer had a least count of ±1 m/s. Its calibration was also tested and validated against other hot-wire anemometers in the lab. The Infra-red thermometer used to measure surface temperatures was calibrated against phase changing water at 0 oC and 100 oC and could measure temperatures at a lease count of ±0.1 oC but had an error to the grade of ±1 oC. The temperature and humidity at inlet and outlet of the oven were measured via a combined thermometer-hygrometer module that was calibrated against WBT, DPT and DBT of air, as well as a lab-grade hygrometer. The thermometer module had a least count of ±1 oC for temperature measurement and ±1% for RH measurement. However, the error and reading fluctuation for the air temperature and humidity module was found to be ±2 oC and ±3% RH.
A 200 Liter, 20 tube evacuated tube solar thermal collector was used as the primary heat source. Based on a power rating of 100 Watts per tube, the power rating of the thermal collector was considered to be 2000 Watts. The piping from the thermal collector was made of PPRC material and was not further insulated against the environment. Two 50 W diaphragm pumps were used to pump the hot distilled water flowing through two heating loops attached with the tank of the hot water heater. The heating loops were used to heat diluted CaCl2 solution and the dehumidified air, as explained earlier.
The CaCl2 salt was commercial grade and had 6% impurities as provided by the supplier. The CaCl2 solution was pumped by 20 W submersible centrifugal pumps for dehumidification and regeneration. In the desiccant regeneration chamber, an axial fan was used for air flow and the desiccant material was pumped into honeycomb structured wetting material made of Cellulose paper.
Table 1 | Key parameters obtained in the experiments that were conducted in this study
Experiment
|
Process
|
Drying Time (min)
|
Energy Consumption (kJ/g)
|
Color Retention (%)
|
1
|
Constant temperature Convective Drying at 60 oC
|
35.5
|
10.3
|
41.2
|
2
|
Microwave Drying
|
9
|
19
|
52.2
|
3
|
Microwave-Convection Drying at 60 oC
|
7.5
|
18
|
71.6
|
4
|
Desiccant assisted Microwave-Convection Drying at 60 oC
|
6.5
|
17.1
|
87.8
|