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Effects of sulphiting and osmotic pre-treatments on the effective moisture diffusion coefficients D<sub>EFF</sub> of air drying of pineapple slices
Abstract
Air dehydration of fruits has been reported to be limiting in some factors especially on the drying kinetics and quality of the dried fruits. Removal of moisture during drying is attributed to these. This study was designed to evaluate the effects of sulphiting and osmotic pretreatments on effective diffusion coefficient (Deff) of air drying pineapple slices at 50o and 70 oC temperatures. Smooth cayenne pineapple obtained from Ajanla farms, Ibadan, Nigeria was used for the study. The fruits were hand peeled and sliced to spherical slices of 5 cm radius and 0.5 cm thickness. Pineapple
fruit slices were pretreated at three levels of sulphiting and sucrose/osmosis and two conditions of drying (50 oC at 16 h and 70 oC at 10 h), resulting in 18 treatments in a factorial experimental design. Changes in moisture were monitored hourly and Fick’s second law was used to describe the rate of moisture transfer to determine the Deff as the slices were dried using a cabinet dryer. Results showed that the Deff was strongly affected by sulphiting at 2500 ppm equalling 9.10+ 0.13 x 10-6 and 6.78 + 0.53 x 10-6cm/s for 70 oC and 50 oC drying temperatures, respectively. The osmotic pretreatment at 40% sucrose recorded 4.91 + 0.15 x 10-6cm/s and 6.93 + 0.03 x 10-6cm/s for 70 oC and 50 oC drying temperature respectively. The control samples had 3.14 + 0.23 x 10-6 and 4.19 + 0.21 x 10-6 at 70 oC and 50 oC drying temperature, respectively. The high values obtained from the pretreated samples may be due to the restructuring of the cell walls. The combination of sulphiting and osmotic pretreatments also exhibited significant impact on the Deff value, ranging between 5.13 to 8.42 x 10-6, though not as pronounced as with the single pretreatment method. Furthermore, drying at 70 oC influenced the Deff value more than drying at 50 oC with both pretreatment methods. The study, therefore, showed that pretreatment methods improved the Deff of the pineapple slices, with the sulphiting pretreatment at 2500 ppm having the highest value.
Key words: Sulphiting, osmotic, pretreatment, drying, kinetics
