applied sciences Article Effect of Hot-Air Convective Drying on Activity Retention of Amylase and Invertase in Dried Mango of Varieties Sindri, SB Chaunsa, and Tommy Atkins Adnan Mukhtar 1,2,* , Sajid Latif 1 and Joachim Müller 1 1 Tropics and Subtropics Group, Institute of Agricultural Engineering (440e), University of Hohenheim, 70599 Stuttgart, Germany; [email protected] (S.L.); [email protected] (J.M.) 2 Sub-Campus Depalpur Okara, Institute of Horticulture Sciences, University of Agriculture Faisalabad, Renala Khurd 56300, Pakistan * Correspondence: [email protected] or [email protected]; Tel.: +49-0711-459-24708 Abstract: Recently, fruit-drying industries are showing great interest in producing dry fruits that preserve a high enzyme content. Therefore, this study aimed to investigate the effect of hot-air convective drying on activity retention of amylase and invertase in dried mango of varieties Sindri, Samar Bahisht (SB) Chaunsa, and Tommy Atkins. Convection drying was conducted under over-flow mode at five temperatures (40, 50, 60, 70, and 80 ◦C), two air velocities (1.0 and 1.4 m s−1), and constant specific humidity of 10 g kg−1 dry air. The enzymatic degradation data were fitted to the first-order reaction kinetics model, in which the temperature dependence of the rate constant Citation: Mukhtar, A.; Latif, S.; is modelled by the Arrhenius-type relationship. Results showed that the maximum amylase and Müller, J. Effect of Hot-Air invertase activity for dried mango of all three varieties was best preserved in samples dried at a Convective Drying on Activity temperature of 80 ◦C and an air velocity of 1.4 m s−1. In contrast, a lower drying temperature Retention of Amylase and Invertase and an air velocity of 1.0 m s−1 contributed to a significant decrease (p < 0.05). Exploration of in Dried Mango of Varieties Sindri, SB different temperatures and air velocities to save amylase and invertase in dried mango is useful Chaunsa, and Tommy Atkins. Appl. from an industrial point of view, as mango can be a natural dietary source of digestive enzymes to Sci. 2021, 11, 6964. https://doi.org/ improve digestion. 10.3390/app11156964 Keywords: enzyme degradation; digestive enzyme; convective drying (over-flow); Sindri; Samar Academic Editors: Elisabete Maria de Castro Lima and Jorge Manuel Bahisht Chaunsa; Tommy Atkins Rosa de Medeiros Received: 7 July 2021 Accepted: 26 July 2021 1. Introduction Published: 28 July 2021 The market of industrially produced dried mango is steadily growing due to its good taste, unique flavor, attractive color, and nutritional value [1]. To produce high-quality dried Publisher’s Note: MDPI stays neutral mangoes, fruit-drying industries are paying special attention to the activity preservation with regard to jurisdictional claims in of enzymes in the dried product. The ultimate challenge for the modern food industry published maps and institutional affil- is not only to minimize the degradation of beneficial enzymes, but also to maximize the iations. preservation of these enzymes in the final dried product. As people are becoming more conscious of their diet, the use of enzyme supplements has also increased considerably in recent years. In this regard, raising consumer awareness and specifying a product by its nutrients (enzyme units) could have a substantial effect on increasing its demand. The Copyright: © 2021 by the authors. advantage of producing dried mango, in which a high proportion of the enzymes amylase Licensee MDPI, Basel, Switzerland. and invertase is retained, is that it can be used in a wide range of applications such as This article is an open access article functional foods, bread making, brewing, food supplements, and animal feed. distributed under the terms and As enzymes are proteins in nature, they must be characterized with regard to their conditions of the Creative Commons temperature-dependent behavior. In this study, amylase and invertase were selected as Attribution (CC BY) license (https:// the model enzymes. The selection was based on the enzymes that are most commonly creativecommons.org/licenses/by/ studied in fresh mango of different varieties and are present in a concentration that is easy 4.0/). Appl. Sci. 2021, 11, 6964. https://doi.org/10.3390/app11156964 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 6964 2 of 11 to measure [2,3]. In addition, amylase and invertase are the most important carbohydrate digestive enzymes, as they break down complex carbohydrates into simple sugars. Hence, mango flesh can contribute as a source of natural digestive enzymes. Fresh mango is extremely perishable and susceptible to rapid decay due to ripening and biochemical degradation, resulting in a poor shelf life. Thus, in order to consume this fruit all year long, drying is widely used technology in food processing and preservation. To date, several drying methods, such as hot-air convective drying, infrared drying, vacuum drying, microwave drying, and freeze drying have been applied to fruits, vegetables, and herbs to prepare dehydrated products [4–8]. However, convective-air drying is most commonly used due to its easy operation, as well as low investment and operating costs [9]. It is also well known that hot-air drying is primarily influenced by the drying temperature and air velocity [10–12], whereas specific humidity has no major influence on the drying rate, except that its higher value decreases the drying rate [13]. Mango and other tropical fruits are commonly dried at temperatures ranging 40 ◦C to 80 ◦C, and air velocities of 0.2 to 2.0 m s−1, to a target moisture content of 10–15 g 100 g−1 wet basis [14–17]. Several researchers have extensively documented the effects of the selected drying conditions on the physicochemical and nutritional parameters of dried mango [6,15,18–20]. Only a few studies, however, focus on the enzymatic activity in particular. In most of these studies, considerable efforts have been undertaken to understand the inactivation of the enzymes during fruit drying [21–23]. However, to the best of our knowledge, no study is available dealing with the effect of drying temperature and air velocity on the activity retention of amylase and invertase in dried mango. As enzymes are very diverse in nature, drying at different temperatures and air velocities could contribute to different degradation rates of enzymes due to differences in drying time, exposure to heat, light, and oxidation. Consequently, the activity preservation of the enzymes in the final dried product may be significantly affected under different drying conditions. Therefore, it is highly important to obtain information about the drying operating parameters and its relationship with activity retention of enzymes in dried mango, due to the health benefits of the enzymes. The aim of the present study is specifically focused on investigating and comparing the effect of the most influential hot-air drying parameters, such as temperature and air velocity, on the activity retention of amylase and invertase in dried mango slabs. The results from this research work will provide information to select the appropriate drying temperature and velocity to preserve a maximum of the digestive enzymes, amylase, and invertase in dried mango. 2. Materials and Methods 2.1. Raw Materials Mangoes (Mangifera indica L.) from three varieties, Sindri, Samar Bahisht (SB) Chaunsa, and Tommy Atkins, were used for this study. Sindri and Samar Bahisht (SB) Chaunsa were harvested from Khanewal, Pakistan, and transported by air to Frankfurt, Germany, where they were obtained through local shipment. However, Tommy Atkins was purchased from a local market in Stuttgart, Germany. During the experiments, the fruits were refrigerated at a temperature of 11 ± 1 ◦C for no more than six weeks. 2.2. Drying Experiments Convective drying was carried out using an over-flow chamber of a high precision hot-air laboratory dryer designed at the Institute of Agricultural Engineering, University of Hohenheim, Stuttgart, Germany [24], which ensured a highly accurate control of the temperature, humidity, and velocity of the drying air. Before drying, the average initial moisture contents (Sindri 86.96 ± 2.05, SB Chaunsa 82.12 ± 2.62, and Tommy Atkins 89.08 ± 2.22% w.b.) were measured when drying 24 h in an oven at 105 ◦C[25], as well as the total soluble solids (Sindri 17.33 ± 1.71, SB Chaunsa 20.69 ± 1.46, and Tommy Atkins 13.95 ± 2.81 degree Brix) by digital refractometer (ATAGO PR-201 palette, ATAGO Co. Ltd., Tokyo, Japan) and the water activity (Sindri 0.922 ± 0.012, SB Chaunsa 0.920 ± 0.010, and Appl. Sci. 2021, 11, 6964 3 of 11 Tommy Atkins 0.937 ± 0.012) using a ventilated hygrometer system (Rotronic A2, Rotronic AG, Basserdorf, Switzerland) after 30 min in a thermostatic cell at 23 ◦C. For drying, fruits were cut into slabs of 4 cm × 2 cm × 0.8 cm using a stainless steel knife and a food dicer (MultiSchneider Serano 7, Ritter, Groebenzell, Germany). A fixed dimension of mango slabs was used to achieve more homogeneity of the dried samples, as fruit size and shape affect drying time and fruit quality loss by interfering with the convective drying process [26]. Drying experiments were performed in duplication by employing different temperatures (40, 50, 60, 70, and 80 ◦C) and air velocities (1.0 and 1.4 m s−1), while maintaining a constant specific humidity 10 g kg−1. As specific humidity might vary over the experimental days, a constant specific humidity was set, which is a typical value at temperate latitudes. During the dehydration process, mass reduction was automatically recorded at a regular interval of 15 min until the target moisture content of approximately 11% wet basis was achieved [2]. This moisture content is usually considered to be under the hygienically safe microbial load with a water activity of ≤0.6 [27,28].