Green plantain solar dryer direct sun drying Functional, thermal Chemical properties Microbial load


Title: A Comparative Study of Solar and Direct Sun Drying of Pre-Treated Green Plantain

Affiliation: Food Science Technology Department
School of Engineering and Engineering Technology
Federal University of Technology, Owerri Imo State

Date: April, 2016

A comparative study of solar and direct sun drying of pre-treated green plantain was conducted. A distributed – type natural circulation solar dryer fabricated from locally available materials and direct open sun drying were used to investigate the drying characteristics of green plantain slices between January and February, 2012. The fingers harvested weekly from the 9th to 13th week were washed, peeled and cut into slices (1cm thickness). Each batch of the sliced plantain fingers were pre-treated by soaking in citric acid, sodium metabisuphite and blanching prior to direct sun and solar drying. The untreated plantain flour served as control. The thermal properties which included starch gelatinization temperature and enthalpy were by Rapid Visco Amylograph and Differential Scanning Calorimetry. The shelf stability of the flour samples was also investigated over a period of 6 months by monitoring the microbial load. Assessment of the functional properties showed no significant differences in the oil absorption capacity, gelling point and bulk density of the flour samples irrespective of harvesting time and pre-treatment. However, there were significant differences in water absorption (p< 0.05) with the sodium metabisuphite treated sample having the highest value of 2.74ml/g. Higher pH values were recorded for flours treated with sodium metabisulphite and blanched with values ranging from 6.87 – 7.54. Also, in the swelling index (P<0.05) the 13th week blanched sample had the highest value of 3.76 ml/ml. The minimum temperature of the solar dryer during the drying period was 31.50C and maximum gave 45.6.00C. Relative humidity minimum was 32% and maximum was 81%. The moisture content for the dried flours ranged from 9.68 to 13.53% with the lowest value from flour treated with combined citric acid and blanching and the highest 13.53% coming from flour treated by blanching alone.  Total starch showed 75.42 to 82.72% and no significant difference existed in using direct sun drying or solar dryer. The variations of moisture ratio with time were used to test twelve different one-layer drying models. Reduced chi square and root mean square error (RMSE) were used in testing the consistency of all the models and coefficient of determination (r2) was used to determine the best fit of the drying model. The Verma experimental model was found to be superior in terms of fitting performance. The Verma model gave maximum r2 of 0.9989 and RMSE minimum of 0.0008. The pasting temperature varied from    66.9 0C to 84.30C with direct sun dried samples showing the lowest value. The gelatinization enthalpy (∆H) varied significantly with the solar dried having the highest value of 11.81 (J/g). The microbial load count showed no species of yeast in all the dried flour samples and the staphylococcus counts were quite low 1 x 104 to 1 x 105 cfu/g for the flour samples. The properties of unripe plantain slices whether by direct sun or solar drying indicated possible application in food formulations.  
Keywords: Green plantain, solar dryer, direct sun drying, functional, thermal, chemical properties, and microbial load.