Apples, Pyrus malus L. (Rosaceae family) are one of the most cultivated fruits around the world, its production worldwide is about 4.10×107 tons , being China the main producer. Argentina is one of the principal producers in Latin America, with 5.63×105 tons and a productive area of 5.00×104 ha . The apple-producing areas are mainly in the provinces of Río Negro, Neuquén, Mendoza, and San Juan, being the major cultivars of Red Delicious, Gala, and Granny Smith.
Apples are generally eaten fresh, although they can also be consumed like juice, or dehydrated as a snack in breakfast preparations, salads, and other culinary recipes . Apples are a vital source of many essential nutritional compounds: vitamins (A, B1, B2, B3, B5, B6, C, and E in minor amounts), minerals, fibers, and also are rich in antioxidants [3 – 6]. Granny Smith variety, widely grown in Argentina, is the most representative variety of the group of green apples. It is characterized by an intense and uniform green color, medium size compared with other varieties, also it is juicy and slightly acidic. Moreover, this variety contains more fibers and antioxidants than others .
According to Global Industry Analysis , dehydrated fruit is becoming a potential product. The study indicates that, during 2020, the consumption of dehydrated fruit was close to 4×106 tons. China is the largest exporter of dried fruits, followed by Germany, the United Kingdom, the United States of America, and Russia, exporting 150, 46, 41, 36, and 35 thousand tons respectively . The increasing consumption of dehydrated fruit is related to the global trend of consuming nutritious and healthy foods as well as avoiding wasting fruit .
To obtain dried products, conventional air drying is the most widely used drying operation due it is a simple process . This unit operation comprises the water content reduction through simultaneous mass and heat transfers . The water remotion is carried out through evaporation, consuming important energy quantities, for that, drying is denominated as an energy-intensive process . This unit operation reduces the cost of packaging, transportation, storage, and preservation.
The drying kinetics of foods are greatly affected by air temperature, moisture content, and material structure, observing contraction and changes in physical properties . According to Shrestha et al. , the drying temperature of apples must be between 40 and 80°C to avoid the decomposition of heat-sensitive biological compounds. Several authors recently investigated the drying of apples and their peel: Kidon and Grabowska  studied the effect on the bioactive compounds, antioxidant activity, color, and sensory attributes of red apple cubes by three different drying methods (convective, vacuum-microwave pretreatment with convective, and freeze-drying). Raponi et al.  real-time monitored the hot-air drying of apple cylinders using computer vision. Ma et al.  studied the effects of different methods (hot-air, heat-pump, and vacuum freeze drying) on the drying kinetics, color, phenolic stability, and antioxidant capacity of apple peel, and Chen et al.  analyzed the high-power microwave drying of apple slices to better understand the moisture kinetics and microstructure evolution.
It is important to remark that food drying is a very broad area of study, there are many experimental and theoretical reports to determine and estimate moisture transfer parameters for food drying [3, 19–21]. Heat and mass transfer models are applied to simulate drying curves under different conditions, thereby improving operational control of the process, being the most researched theoretical drying model of Fick's second law of diffusion. [22–24]. This law can be used for various forms of regular shape, such as rectangular, cylindrical, and spherical products, and commonly postulates that one-dimensional moisture movement occurs with constant diffusivity, uniform initial moisture distribution, negligible external resistance, and no change in volume .
Dincer et al.  developed and verified analytical techniques to characterize mass transfer in geometric and irregularly shaped objects (using a form factor) during drying. New drying parameters were defined, such as the drying coefficients and delay factor, based on an analogy between the cooling and drying profiles, which exhibit an exponential function in time . Few researchers studied the Dincer and Dost model to characterize the mass transfer in food geometric objects during drying . Beigi et al.  investigated the influence of drying air parameters (i.e., temperature, rate, and relative humidity of the air) on the effective Deff and the hm of apple slices. Model validation showed that the prediction of the experimental drying curves of the samples had a good precision. Bezerra et al.  evaluated the mass transfer characteristics of the passion fruit peel using the analytical model proposed by Dincer and Dost. According to the literature consulted so far, there is no known work on the effect of apple peel on drying behavior.
Considering the end-user habits of some consumers, dehydrated products must be rehydrated in solutions (e.g. water, sweetened water, or saline), before being consumed. Rehydration is the process of recovering water for dry products , in which the food mass increases according to the absorption of water during this process. The rehydration rate decreases because the value of moisture content of the product approaches the value of the equilibrium moisture content, while the water absorption rate is initially high . This process depends on structural changes in the vegetal tissues and the cells of the material during drying. It is important to remark that during the drying, contraction, and collapse are carried out, reducing the water absorption capacity and avoiding the complete rehydration of the dried product . The food rehydration process is considered as a measure of the damage degree to the raw material. However, rehydration cannot be treated simply as the opposite of dehydration. Different factors affect the rehydration process such as composition variables, drying method, physical structure, and medium characteristics. The study of the rehydration kinetics of dry vegetal tissues is composed of three simultaneous processes: water adsorption, swelling, and leaching of soluble compounds . To model the rehydration kinetics of fruits and vegetables, the equation of Fick’s second law and semiempirical equations based on it are generally used , in addition to Peleg and Weibull model, which have been used by several researchers [29, 33].
Until now, no reports have been found related to the drying of Granny Smith apple slices peeled and unpeeled varying the drying temperature and subsequently rehydration at Ta and Tb. Moreover, the effect of drying and rehydration on apple quality parameters has not been described.
1.1. Objectives of this work
In this article, the main objective was to model the drying and rehydration kinetics of green apple slices (Granny Smith variety) peeled and unpeeled to compare in what way these processes affect the quality parameters of fresh apple slices. Different drying temperatures were taken into account: 50, 60, and 70 ºC, and then samples were rehydrated in water at ambient and boiling temperatures (Ta and Tb, respectively). The drying kinetics were adjusted with the Dincer and Dost model, and the Deff and hm were calculated. Then, for the rehydration modeling, the Peleg and Weibull models were adjusted to the experimental data at both rehydration conditions, and the Deff was calculated. Diameter, pH, acidity, moisture, and solid soluble content were considered to compare between fresh, dehydrated, and rehydrated apple slices. Figure 1 shows a roadmap of this work.