Effects Of Different Drying Methods On Pineapple Quality Characteristics

Pineapple is essential fruit in human nutrition for body growth and development. However, pineapple is a seasonal fruit with short shelf life due to its perishability. Our goal was to find better method for dehydration to extend pineapple shelf life for future that will make available of pineapple products throughout the year. In addition, the dried products quality is a challenge to drying technologies due to heat effects and prolonged drying time. Normal hot air drying and infrared radiation drying were combined with explosion puffing drying for the purposes of reducing drying time while maintaining product quality. Moreover, the osmotic dehydration was introduced as pretreatment prior to drying to minimize the heat effects. The objective of this study was to investigate the effects of osmotic dehydration pretreatments, hot air and infrared and its combination with explosion puffing on the physiochemical and quality indices of pineapple slices.The indices tested were drying kinetics, rehydration ratio, hardness, microstructure observation, reducing sugar, vitamin C contents, total phenolic contents, and color. The preliminary results on osmotic treatment identified the osmotic temperature of 45 ?C, concentration of 20-30 %, osmotic time of 2-3 hrs and slices thickness of 6 mm as the best parameters for further drying of pineapple at 80 ?C. The best drying temperature conditions of 80 ?C for infrared and hot air respectively also optimized the pre drying conditions of explosion puffing(puffing temperature 90 ?C, dwelling time 5 min, vacuum temperature 65 ?C, and vacuum time of 1.5 hr for hot air and 45 min for infrared).Comparing the effects of all drying methods, infrared drying produced relatively higher drying rate than hot air. Combination of infrared and explosion puffing methods achieved 50.98 % reduction of drying time than hot air. Osmotic dehydration and combination of hot air-drying or infrared radiation drying with explosion puffing enhanced rehydration ratio, maintained hardness, produced porous and puffed products. Osmotic pretreatment decreased the vitamin C contents and preserved reducing sugar especially when accompanied by infrared drying but also preserved the color characteristics. Combination of hot air-drying or infrared drying with explosion puffing preserved the Vitamin C while the total phenolic contents were the highest in AD and INF+Exp methods.Considering single method and its combination, we found that combination of infrared radiation and explosion-puffing drying reduced drying time by 75.5 % and 58.3 % of osmotic and non-osmotic treated pineapple slices respectively, compared to infrared without explosion puffing, also increased rehydration ratio. Moreover, hardness maintained and porous and puffed structure observed in all infrared radiation treatments except for infrared without osmotic dehydration in which presented undesirable products. The reducing sugar was higher in infrared with osmotic dehydration sample, while the total phenolic contents increased in non- osmotic treated samples. Vitamin C contents were high in combined drying treatments. The osmotic dehydrated followed by infrared drying produced the product with desirable color.Combination of hot air and explosion puffing reduced drying time by 57.5 % and 52.8 % of osmotic and non-osmotic treated samples respectively, compared to hot air dried samples without explosion puffing, also increased rehydration ratio. The presence of porous and puffed structure indicated that hardness maintained in all hot air treatment conditions except hot air dried sample that showed shrinkage. Combination of hot air and explosion puffing preserved vitamin C especially in hot air dried samples without osmotic dehydration(AD+Exp). Osmotic dehydration preserved reducing sugars and product color while total phenolic contents were preserved in samples dried using only hot air(AD).Therefore, combining osmosis pretreatment, infrared radiation and explosion puffing drying methods demonstrated effective way of shortening the drying time and improved quality of dried pineapple slices hence most promising methods for pineapple drying.