OPTIMUM DEHYDRATION AND DRYING KINETICS OF THREE SELECTED VARIETIES OF TOMATO (Lycopersicum species) FROM SOUTHWESTERN NIGERIA.

Abstract

Tomato (Lycoperscium spp.) is an important component of daily dietary intake with seasonal and geographical variation in its production. They are usually in short supply in the dry season and effective storage in the fresh state still poses a challenge. Pre-treatment methods have been reported to improve drying characteristics of fruits and vegetables but there is dearth of information on drying of indigenous variety of tomato despite its high nutritional value. Pre-treatment methods for three varieties of tomato from southwestern Nigeria and drying conditions with mass transfer kinetics for the optimally pre-treated variety were investigated. Roma-VF (Lycopersicum esculentum Mill), Koledowo (Lycopersicum pimpinellifolium Mill) and Ibadan-Local (Lycopersicum esculentum CV) were used in this study. Samples were pre-treated in binary osmotic solutions (sugar and salt) at different concentrations (40/20, 45/15, 50/10oBrix/%), temperature (30, 40, 50oC) and time (30, 60, 90, 120, 180min) using fruit to solution ratio 1:10. Moisture Content (MC) was determined using the AOAC standard. Models for water loss (WL), solid gain (SG) and weight reduction (WR) were developed and optimal response (highest WL, WR, least SG and MC) was obtained and data were analyzed using ANOVA at p=0.05 within and across varieties. Mechanism of mass transfer phenomena was studied by drying at 40, 50 and 60oC. Five thin layer drying models (Exponential, Henderson and Pabis, Page, Modified Page and Logarithmic) were compared and fitted into the experimental moisture ratio. Adequacy of fit was based on highest R2, χ2 and least RMSE. Diffusion coefficient and activation energy were determined using Arrhenius equation. Water loss increased with increasing solution temperature and sugar/salt concentration. Ibadan-Local and Roma varieties had their highest WL (0.30) at 45/15 sugar/salt concentration, while Koledowo had its highest WL (0.26) at 40/20 sugar/salt concentration all at 50oC solution and 50oC drying temperatures which could possibly be due to its thicker outer skin impeding moisture migration. Water loss and SG were significantly different among the varieties. As temperature increased from 40-50oC, drying time reduced from 26-18.5 h (treated) and 35-25.5 h (untreated) respectively in Ibadan-Local variety. Drying occurred in falling rate period with better curves in pre-osmosized tomato. Exponential model fitted at 40oC with R2, χ2 and RMSE ranges of 0.83-0.90, 199.37-380.02 and 0.0797-0.1009, at 45oC 0.94-0.98, 735.49-2706.82 and 0.0464-0.3640 and at 50oC, Henderson and Pabis fitted at 0.85-0.90, 187.87-380.02 and 0.0798-0.0966 respectively for treated tomato. For untreated tomato at 40oC, Page model fitted with R2, χ2 and RMSE ranges of 0.95-0.98, 881.61-2938.62 and 0.0301-0.0538, Page and Modified Page at 45oC with 0.92-0.08, 246.71-607.28 and 0.0798-0.0966 and Modified Page fitted at 50oC with 0.83-0.92, 246.99-607.24 and 0.0778-0.1008 respectively. Moisture diffusivity was higher in pre-treated in Ibadan-Local samples ranging from 1.17-3.51x10-8 compared to untreated with 1.25-3.13x10-8 while the activation energies were respectively 46.81 kJ/mol and 52.61 kJ/mol implying faster drying with lower energy requirement in osmosized sample. Optimum pre-treatment conditions have been established for the three varieties of tomato. Effective moisture diffusivity and activation energy of pre-treated Ibadan-Local were within the range for most agricultural materials. Faster drying and lower energy requirement make osmosized pre-treatment a promising approach for drying of Ibadan-Local variety.