**2. Solar drying methods**

In the past, the first method used to dry food was sun. Solar energy directly hit the food, left exposed to the air or placed on the ground. It should be considered that this type of drying is influenced by external contamination, the presence of insects and other small animals, the available area, the poor standardization of the process, and the potential development of bad smells [5]. Therefore, this methodology is a most economical solution. Fruits and vegetables that grow and are cultivated in remote areas lend themselves well to conservation by solar drying, while losing some of their peculiarities and nutritional value. But in the past, this method required large space and time and offered poor process control. This natural method takes place when materials are dried with unheated forced air, taking advantage of its natural drying potential. A natural evolution of sun drying has been realized by planning the use of solar dryers, as an efficient system for solar energy use [6]. Countries that enjoy large amount of natural sun exposure have, as the obvious option for drying, the solar dryers based on natural convection [7]. In this way, is possible to save costs and energy. However, uncontrollable climate changes are the main negatively influenced factors [8]. On the basis of climate and environment conditions or if a better quality is needed, the use of mechanical air drying can be the best solution. In any case, sun drying is the cheapest method to dry food. Nowadays, both sun drying and mechanical air driers are available on the market.

### *Drying Technology Evolution and Global Concerns Related to Food Security… DOI: http://dx.doi.org/10.5772/intechopen.109196*

More than 250,000,000 tons of horticultural products and cereals are dried every year using natural drying [8]. Especially in developing countries, the products are placed on the ground and turned periodically until completely dry. The newly developed solar drying allows the use of renewable energy sources, minimizing the defects of traditional techniques.. The drying process influences the amount and the organization of water molecules in the food cells producing changes in the product matrix [9]. Modern solar drying techniques try to combine aspects of the natural process with industrial needs, applying the concept of drying for maintenance of the characteristics of the raw materials and the environment in which they are grown (developing countries). The devices designed to effectively carry out this type of drying use mainly the energy of the sun for their functioning, shielding their deleterious effects and creating a hygienically suitable treatment environment, in tune with the microclimatic conditions of the environment. In African countries, for example, during the last 10 years, a great effort has been made to improve the horticultural drying process, with the introduction of more efficient solar drying systems [10–14].

The management of the production cycle is influenced both by intrinsic factors linked to the nature of the material to be transformed and by external factors, therefore determined by the context in which the production takes place. The final quality of the product strongly depends on the choices of technological parameters regarding processing and packaging. These choices can favor the conservation of compounds with high nutritional value present in raw materials that can still be found in final products, improving their nutritional quality. Important, especially in this pandemic era [15–17]. Solar drying has been proven to be adequate to be applied to several crops [18]. It allows the production of products with desirable quality together with minimal environmental impact. Since the process is slow and weather-dependent, so a wise strategy "fan off-fan on" must be devised considering the following conditions: air temperature, relative humidity, moisture content, and temperature of the material being dried. Solar dryers are systems capable of exploiting solar radiant energy to heat a flow of air used to dry products. There are types with natural ventilation (which exploit the "chimney" effect) or with forced ventilation, which can be powered by a photovoltaic system or other electrical source [6]. Recently, a very comprehensive review of several types of natural convective and direct-type (NCDT) solar dryers has been brought [19] with the aim of collecting the most interesting practices of solar drying technology for the benefit of one and all. Authors have classified in a precise way the different types of dryers. They explain the main characteristics of the "direct types" (i.e., (i) direct solar cabinet dryer, (ii) modified solar cabinet dryer with natural convection, (iii) direct solar drying using the chimney, (iv) foldable solar crop dryer; and the "natural convection type," then particular attention has been made at modeling the process and the evaluation of the performances. Furthermore, regarding indirect solar drying technologies, the most exhaustive review has been reported in Ref. [20] and has been written some years ago. Efforts have been made all over the world around to increase efficiency and drying time; thus, modern dryers are equipped with fans, efficient collectors, different thermal storage materials, reflectors, auxiliary heat sources, and sun-tracking systems. A recent paper [21] has reported an interesting development of closed, indirect heating, forced convection solar dehydrator with desiccant, which is economical, hygienic, and works off the grid. The whole experimental design has been discussed, paying attention to (i) energy balance, (ii) quantity of air required, (iii) flow of air required, (iv) area of the solar heater/collector, and (v) rates of dehydration.
