Acerola (Malpighia emarginata D.C.) is a fruit native to tropical America that has attracted interest from the fruit processing industry for consumption in the form of juice, jelly and compote due to its pleasant flavor and the high levels vitamin C and other bioactive compounds, such as tocopherols, carotenoids, flavonoids and phenolic compounds [1]. However, it is estimated that 40% of the processed volume of this fruit turn into waste, for example, seeds, kernels and bagasse, which is generally discarded or underutilized, causing losses of raw material and negative environmental impact [1, 2].
Some studies have shown that the residues from the processing of acerola can contain high amounts of bioactive compounds with antioxidant activity generally associated with the prevention of degenerative diseases related to aging, such as cancer and cardiovascular disease [1, 3, 4]. These aspects, along with the increasing global interest in environmentally-friendly technologies, justify the interest in a better utilization of these wastes [5–7]. However, these materials have a high moisture content level (> 80%), which limits and complicates their transport, storage, and shelf life by non-enzymatic, enzymatic, or microbial reactions that alter the product quality, including deterioration, safety, and functional properties [1, 8, 9]. Thus, it is necessary to submit these residues to an efficient dehydration process. In this way, combined drying techniques appears as an alternative to conserve the properties and the bioactive compounds present in this and other residues [10, 11]. It is known that the dried product has great potential to generate food additives, which in turn will minimize malnutrition and hunger in last developed countries where it is produced [12], thus providing a better use of such residues, in addition to representing a great opportunity to improve food security.
The drying of granular materials has been extensively studied using various types of dryers [5, 12–22]. However, acerola residues present high values of static and dynamic angles of repose, which makes drying in moving bed dryers more difficult. In contrast, some studies have shown that packed bed dryers are unsuccessful in maintaining the homogeneous properties of the product along the bed [23–25]. A solution to this limitation in terms of energy efficiency, process economy and product quality may come from hybrid drying systems and the use of pretreatments [4, 11, 26]. Some studies have proposed the combination of different drying techniques, such as hot air-microwave, microwave-vacuum, and hot air-infrared [4, 24–26]. Among these, microwave drying combined with vacuum has shown to be promising, with lower processing time, more uniform heating, safe handling, easy operation, and energy efficiency [26–29]. The most important characteristic of microwave heating is volumetric heating, where the materials can absorb microwave energy directly and generate an internal heat flux, which is quite different from conventional heating [30]. This volumetric heating combined with vacuum drying could produce a more open-porous structure that facilitates the diffusion of gases through the pores [31]. The benefits of microwave vacuum drying include increased thermal efficiency, reduction in drying time without significant damage to the final product quality, possibility of preservation of the bioactive compounds [32–34], and the obtaining of adequate moisture for a safe storage, consequently preventing or reducing the growth of microorganisms [8, 35, 36].
The hybrid drying system can also be used in conjunction with some pretreatment to enhance the drying efficiency and product quality [37–39]. The use of ethanol in pre-treatments has been investigated aiming to reduce the initial moisture content or to modify the material tissue structure in a way that the drying time can become lower [40, 41]. In addition, ethanol is a well-accepted organic compound in the food industry, being considered a safe substance [42].
The present work aimed to investigate the use of a microwave vacuum dryer associated with a pretreatment with ethanol in the drying process of acerola residues. The effects of operating variables on the drying performance and the product quality in terms of bioactive compounds content (ascorbic acid, phenolics and flavonoids) were analyzed, and the appropriate range of moisture for a safe storage that prevents the growth of microorganisms was identified.