Quality improvement of non-sulphited mango slices by drying at high temperatures
Introduction
The mango fruit (Mangifera indica L.) is one of the most important seasonal fruits of tropical and subtropical countries. Since industrial capacities for the processing of highly perishable mangoes into storable products are limited due to seasonal over-production of the fruits, drying of excess and partly defected mangoes is a promising preservation technique, meeting the processing requirements of small- and medium-size producers (de la Cruz Medina & Garcia, 2003, Fellizar, 1994). Beside traditional sun drying by direct solar radiation, solar dryers and conventional overflow dryers are presently used by small-scale enterprises to reduce the water activity. According to common practice for preservative-free as well as for sulphited mangoes, drying air temperature ranges between 50 and 60 °C. During sun and solar drying, even lower maximum temperatures are reached. Consequently, drying usually needs at least 20 h (Candelaria & Raymundo, 1994, Chavasit et al., 2002, Kameni et al., 2003), resulting in low drying capacities or high investments, respectively. Observed quality deficiencies of the dried fruits caused by these long-term processes were mainly discolouration, such as browning or bleaching, and cracked or scorched products (Kameni et al., 2003, Walker & Wilhelm, 1995), while insufficient drying limits the shelf life of the product due to microbial spoilage. Browning of non-sulphited fruits during drying is typically caused by both enzymatic and non-enzymatic transformations, whereas the non-enzymatic Maillard reaction predominantly occurs during storage of dried products (Krokida & Maroulis, 2000). Therefore, approaches to improve the quality of dried fruits generally involve thermal, chemical and osmotic pre-treatments such as blanching, sulphitation and osmotic dehydration (Jayaraman & Das Gupta, 1992, Madamba & Lopez, 2002, Nijhuis et al., 1998, Sagar & Khurdiya, 1999, Welti et al., 1995). Beside the suppression of microbial growth, all pre-treatments aim at thermal or chemical inactivation of detrimental enzymes, chiefly of polyphenol oxidase (PPO), and at inhibition of the Maillard reaction (Wedzicha, Bellion, & German, 1994) by controlling pH, water activity, and reactive carbonyl compounds, respectively. The high-caloric, sugary dried fruit products, obtained by osmotic dehydration, often exceed maximum sulphur dioxide contents, and therefore, international quality standards are not met. Additionally, the labour-intensive application of pre-treatments contributes to prolonged processing procedures, causing significant vitamin losses. Consequently, masking of colour deficiencies, which result from carotenoid degradation, by applying synthetic food colourings is a common practice.
Therefore, the aim of this study was to investigate systematically suitable drying procedures without pre-treatments, abandoning the use of sulphur dioxide, in order to meet quality requirements and consumers' acceptance. To distinguish between unsuitable and suitable drying conditions, product colour was considered the primary quality criterion. A wide range of drying air temperatures, 30 to 90 °C, was applied, considering dew point, velocity of drying air, slice thickness and drying time as further drying parameters. Instead of the classical “one-variable-at-a-time” approach based on plotting the weight loss against drying time with stepwise variation of the drying parameters, or a highly resource-consuming traditional matrix approach (complete factorial plan), a statistical problem solving approach, that is, a fractional factorial plan, was chosen to explore the experimental space while studying many variables by use of a small number of observations (Dey & Mukerjee, 1999, Haaland, 1989). Hence, possible interactions among the variables could also be studied. Such factorial experiments have wide applications in many diverse scientific areas. To identify the most suitable drying procedures and to evaluate the various existing ones, the objective of the present experimental design was to display the important variables and to narrow their best settings down for further optimisation. Therefore, the drying experiments were performed according to a D-optimal design for 5 factors, covering the range of usual drying procedures. Because of the putative benefit of high-temperature drying, which should allow fast initial reduction of water activity and early enzyme inactivation, drying at elevated air temperatures was included in this study. Furthermore, extension of drying capacities was a further objective of the present investigation.
Section snippets
Raw material
Fresh mangoes, cv. ‘Kent’, from Ivory Coast, were purchased at the wholesale market in Stuttgart, Germany. Using another mango variety than those applied by Mahayothee, Neidhart, Mühlbauer, and Carle (2004), optimum ripeness for drying was reached after 5 instead of reported 2 days of postharvest ripening at 25 ± 2 °C and 50–70% relative humidity. In the raw material used for processing, contents of soluble solids and titratable acids were 17.5 ± 1°Brix and 0.9 ± 0.15 g/100 g, respectively, resulting
Effects on water activity
The relation between moisture content and water activity of the dried products at 20 °C is shown in Fig. 2. The J-shaped isotherm obtained is commonly observed in food products with high sugar content (Maroulis, Tsami, & Marinos, 1988). In accordance with Talla, Jannot, Kapseu, and Nganhou (2001), Henderson's sorption isotherm model (Eq. (3)) was found to fit best with the experimental data (R2 = 0.98) for the dried products derived from the given mango fruits with T = 20 °C, dry-basis moisture
Conclusions
Using the statistical approach on the one hand and considering the knowledge on the various reactions which affect the quality of dried non-sulphited mangoes on the other hand, novel strategies could be suggested concerning dryer performance and operational capacities as well as improved product quality. The experimental design and the non-linear regression models in Table 1, Table 2 were suitable to describe the complex effects of main drying parameters on process characteristics such as water
Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support of this work (MU 582/3-1) as a supplement to SFB 564 (“Research for Sustainable Land Use and Rural Development in Mountainous Regions of Southeast Asia”), and Ms. I. Amberg and Ms. D. Hirschbach-Müller for their technical assistance. Dr. E. Schumacher, Institute of Applied Mathematics and Statistics, and Dr. Th. Waldenmaier, Institute for Agricultural Engineering, Hohenheim University, are acknowledged for
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