# Use of Phase Change Materials for Food Applications--State of the Art in 2022
## 2 Food Applications of PCM
### Food Processing
#### 2.1.8 Solar Food Drying  
Food drying is a way to preserve food, e.g., fruits, vegetables, herbs, grains, fish, and meat. Dried fruits and vegetables have shelf lives of 1 to 2 years. Especially in developing countries with lack of other means of food preservation, like cold storage, food drying can significantly reduce postharvest losses. The farmers can sell more, and usually at higher price since they can choose when selling their produce. And not to forget, also consumers can take advantage of carrying dried fruits without needing any other means of preservation, for example when traveling or working. Dried fruits and vegetables may be eaten plain as a wholesome snack, incorporated into baked goods or sweets, or seasoned to make pickles. Additionally, they can be reconstituted by soaking in water. Drying of food means the removal of water from the food via its surface, and connected the transport of water from within the food to the surface. The driving forces can be an increase of the vapor pressure at the food surface to a level above that of the ambient air by heating the food, or the reduction of the vapor pressure in the ambient air to a levelbelow that at the food surface. Heating of food can be by radiation, by conduction, or by convection. The reduction of the vapor pressure in the ambient air is usually by heating the air, which increases its ability to absorb water; another option is applying vacuum. Generally, temperature affects the removal of water but also the resulting food quality (e.g., proteins start to denaturate at 50-60 \({}^{\circ}\)C). Thus, the optimum temperature range for thermal drying depends strongly on the type of food to be dried; it is typically in a range between 40 \({}^{\circ}\)C and 80 \({}^{\circ}\)C. Compared to common thermal methods of food drying using heat from burning fossil fuels, drying using solar heat has the big advantage that the large amount of heat needed for drying is from a free, renewable source. Introductions to solar dryers for food drying can be found in [21; 22]. Desirable properties of the dried food are low and uniform moisture content, minimal portion of damage to the produce, high nutritive value, visual appearance and taste... The drying process itself does not only lead to the intended removal of water, but might have other effects on the food quality, e.g., negative effects due to improper drying conditions or equipment. For example, too high temperature or excessive direct exposure to sunlight might reduce food quality. Thus, classification of solar dryers is by various criteria: their temperature as high or low temperature solar dryers, their heat source as fossil fuel dryers (conventional dryers) or solar dryers, by their construction principles, or by their operation principles. For the background discussion it is not necessary to discuss any of them specifically, or in any detail. Solar heating means heating is only possible in daytime, and in addition only if the weather is suitable. This leads to two issues of economic importance. First, the dryer dries only in a fraction of the time, often less than 50%, and is thus operated well below its potential output. Second, the time needed until the desired low moisture content is finally reached is unnecessarily long and this significantly increases the risk of spoilage. Thus, besides having a lower output than possible, part of the output produced might even be spoiled and thus useless. For both reasons a backup heat source can be installed to bridge times of no or little solar heat input. Thus, at least for large-scale solar dryers, a backup source is recommended to bridge bad weather. Besides thermal issues, some further issues related to food quality are relevant. Commonly used open sun drying is low cost and easy, however it is unhygienic, and besides inefficient temperature control, direct exposure to the sun results in a poor quality of the product, e.g., loss in nutritional value of the food, loss in taste, as well as negative changes of the visual appearance.