# Valorization of Spent Osmotic Solutions by Production of Powders by Spray Drying
## 1 Introduction  
Osmotic dehydration (OD) is a method for the partial removal of water from different solid food materials, including fruits and vegetables [1, 2]. It is commonly applied before the main drying process, although it can also extend shelf life by reducing water activity, slowing down deteriorating physicochemical and biological processes [3]. The principle of OD is the immersion of plant tissue in a hypertonic solution (osmotic solution--OS)composed of sugars, salts, or other solutes, which causes counter-current mass transfer: water flows from the food material to the OS, and simultaneously, the molecules of the osmotic agent diffuse from the solution into the food [4]. Water removal from lower solute concentrations to higher concentrations through semi-permeable membranes results in equilibrium conditions on both sides of the membranes [5]. OD operates at ambient temperature; thus, it consumes less energy than drying processes and has a lower impact on the thermal degradation of sensitive bioactive compounds, including color- and flavor-related compounds [3; 5; 6]. Several OSs can be used in OD as single agents or blends. An OS should be effective, convenient, nontoxic, cheap, and taste good [5; 7]. Among the OSs applied for fruit OD, the most popular has been sucrose solution, while nowadays there is a trend to replace it with other types. This trend is caused mainly by increasing awareness of the impact of sugar/sucrose consumption on health. Applying other types of OSs can create products with a lower glycemic index and improved nutritional value. Various types of fruit juice concentrates were tested for OD. Pomegranate seeds were osmo-dehydrated using date juice by [PERSON] et al. [8]. [PERSON] et al. [9] used strawberry or cherry concentrates for strawberry OD and reported that this OS had a positive effect on the color of dehydrated strawberries, while a specific strong sensory profile of chokeberry juice concentrate passed to osmo-dehydrated strawberries. In addition, honey solution can potentially be applied as an OS. [PERSON] et al. [10] prepared carrot candy using honey by the process of OD. [PERSON] et al. [11] presented that honey was a more suitable osmotic agent than sucrose solution for yacon dehydration. Polyols are another group of substances that can be used as OS. They are characterized by their strong dehydrating capabilities, sweet taste, and low caloric content [4]. Another critical issue in the OD process is the management of the spent OS. During osmotic treatment, OS is diluted and its dewatering potential is reduced [12]. Moreover, some aromas, pigments, acids, and proteins are leached into the solution, leading to chemical, physical, and sensory changes [13]. For industrial applications, recycling the solution is necessary, and it can be achieved by concentrating, adding solute, or mixing with a more concentrated solution. It is also crucial to continuously clean it by a complex process involving coarse filtration, pasteurization, and restoring the initial concentration (i.e., by adding dry sugar). Recycling should consider microbiological safety aspects [13]. According to [PERSON] and [PERSON] [14], some authors have pointed out that the limited large-scale industrial development of OD is due to the problems of concentrated solution management. On the other hand, OD promotes OS enrichment in certain water-soluble natural components extracted from fruits, such as vitamins and minerals. For this reason, recycling it for new food formulation seems to be an attractive alternative [15]. In a recent study by [PERSON] et al. [16], tomato juice-based spent OS was valorized by spray drying to produce powders, which can be further applied as a food ingredient. However, other types of sugar-rich OSs were not examined. Spray drying (SD) is the most convenient method for transforming liquid foods into powders. It is widely applied for milk and dairy powder production, but can be also used for production of fruit, honey, or molasses powders [17].