Potravinarstvo Slovak Journal of Food Sciences Volume 13 462 No. 1/2019 Potravinarstvo Slovak Journal of Food Sciences vol. 13, 2019, no. 1, p. 462-469 https://doi.org/10.5219/1134 Received: 20 May 2019. Accepted: 30 May 2019. Available online: 28 June 2019 at www.potravinarstvo.com © 2019 Potravinarstvo Slovak Journal of Food Sciences, License: CC BY 3.0 ISSN 1337-0960 (online) OPTIMIZATION OF INFRARED DRYING CONDITION FOR WHOLE DUKU FRUIT USING RESPONSE SURFACE METHODOLOGY Laila Rahmawati, Daniel Saputra, Kaprawi Sahim, Gatot Priyanto ABSTRACT Duku (Lansium domesticum), tropical exotic fruit, was successfully preserved by drying using exposure to infrared radiation emitters. Response surface methodology (RSM) is used to optimize independent variables (IRE distance of 6 cm and 10 cm, IRE temperature of 200 °C, 300 °C, 400 °C, and IRE exposure time of 50 s, 60 s, 70 s, and to produce response variables (weight loss, fruit firmness, titratable acidity, total soluble solid, and browning index). It could be concluded from the optimization performed that drying duku skin in a whole fruit by exposing the fruit to the infrared emitter resulted in a duku fruit with a relatively good physical and chemical conditions and still be consumable. The IRE distance of 6 cm gave a desirability value of 0.80 while the IRE distance of 10 cm gave a desirability value of 0.92 however the IRE distance of 6 cm gave a better storage time. The IRE distance of 6 cm has an optimum value of weight loss 2.2%; optimum value of fruit firmness of 40.92 N; optimum value of total soluble solid of 17.48 brix; optimum value of titratable acidity of 0.33%; and optimum value of browning index of 0.9. The fitting model base on RSM resulted from this research indicated that this study could be used as the basis for alternative process in food processing of duku but still need further research to increase the shelf life and a better result in the chemical and physical characteristics of duku. Keywords: Duku; infrared; optimization; response surface methodology INTRODUCTION Food processing such as thermal and non-thermal processes could affect changes in structure and composition of the food (Mercier et al., 2011). The process of food processing with a thermal method, could cause a chemical and organoleptic properties damage and reduce nutrition or nutritional bioavailability. An example of food processing technology with thermal is drying. Drying is one of the food processing method to prolong shelf life or preserve grains, fruits, vegetables and food in all varieties. The quality of dried fruits depends on the conditions of drying process. One type of drying processes is by using infrared radiation. Infrared drying has been widely implemented in the food process because of its several advantages including to reduce water content in food, low energy consumption, short time in processing and also maintain and ensure product quality conditions (Pan et al., 2009). The advantages of infrared radiation could inhibit the pathogens in products which include mold, yeast, bacteria and spore by controlling some parameters such as power on the heater (Hamanaka et al., 2000), temperature of sample (Sawai et al., 2003), wavelength and the target wave in a wide range (Krishnamurthy et al., 2008), sample thickness (Sawai et al., 2000) and sample water content (Hamanaka et al., 2006). As with other electro magnetics wavelengths such as microwaves and radio frequencies, infrared radiation has a unique characteristic in the design of its spectral distribution and energy intensity which could be be controlled by using optical filters. Furthermore, the unique characteristic of infrared radiation to the product is the heat energy from the emitter only affected the surface of food in a short time without raising the inside temperature of material (Li and Pan, 2014a). Infrared radiation is divided into three different categories, namely near-IR (NIR) with a spectrum scale in the range of 0.75 – 1.4 m, mid-IR (MIR) with a spectrum scale in the range of 1.4 – 3 m, and far-IR with a spectrum scale in the range of 3 – 1000 m (FIR) (Sakai and Hanzawa, 1994). Infrared radiation has a longer wavelength than visible light, but is shorter than terahertz radiation and a microwave. The infrared radiation spectrum has a range of 750 nm up to 100 m and widely used in food processing in several ways including food processes involving heating processes, spectroscopic measurements of chemical composition (food analytical applications), and measurement of non-contact food temperature. The use of infrared has been carried out in previous research for drying the skin of fresh duku. The design of this research was a multi-variate process involving many factors