Dehydration Characteristics, Quality Evaluation and Consumer Assessment of Solar Dried Tomato
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Date
November, 2018
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Abstract
Tomato (Lycopersicon esculentum) is an important vegetable used in cooking most local foods in Ghana. At the peak season of harvesting, high loses are incurred because of the absence of facilities to store, process and extend the shelf life of fresh tomatoes. Solar drying has been proven to be a more efficient and low cost method of enhancing quality and adding value to tomato and other vegetables. However, there are concerns on the usage, functionality and sensory appeal of the dried products by consumers due to the methods of drying. In this study a natural mixed mode solar dryer suitable for drying tomato was adapted and used to investigate the dehydration characteristics, quality and consumer acceptability of the dried products. An initial baseline survey was conducted using semi structured questionnaires administered to 395 randomly selected respondents in the Accra Metropolis. Information was obtained on the demographics, consumption pattern, knowledge and acceptance of tomato processing technologies and assessment of quality attributes of tomato. The efficiency of a passive solar dryer was evaluated and used in the processing of fresh tomato to powder. The processing involved the pre-treatment of 6mm slices of fresh roma variety of tomato by dipping in (a) 1% potassium metabisulphite solution and (b) 1 % ascorbic acid solution (1:1) for 10 minutes. Untreated tomato slices served as control. Samples were then dried in the passive solar dryer and in the open sun, with the open sun drying serving as control. The moisture content, moisture ratio and dehydration rate of solar dried tomato was assessed. The quality of dried tomato was also assessed for their physicochemical, nutritional and microbiological characteristics. Physicochemical analysis involved the determination of pH, total titratable acidity, dry matter, ash, tristimulus colour, total soluble solids, water activity and moisture. Nutritional quality was assessed for lycopene, beta-carotene, total carotenoid acids using high performance liquid chromatography (HPLC). Flowability, smoothness and compressibility (or packing porosity), particle size and shape distribution of the tomato powder was determined using a Morphologi G3-ID. Scanning electron microscopy of pre-treated solar and sundried tomato samples were imaged with an FEI Quanta 3D FEG scanning electron microscope. Quantitative Descriptive Analysis (QDA) was carried out to compare the sensory descriptive profiles of solar dried tomato powder with existing products on the market using a trained descriptive panel of nine (9). Home Use Test (HUT) was conducted using a trained panel to assess the acceptability of solar dried tomato powder in local foods. The results showed that, most consumers (74%) preferred tomato powder that was conveniently packaged to retain the characteristic intense taste and the flavour. The 24 hr dryer efficiency of 24.2 % facilitated the drying process of tomato (final moisture content of 12-14%). The ash content was slightly higher in the sundried tomato (9.3 -10.14 %) compared with the solar dried tomato (9.4 - 9.68 %), an indication of potential contamination with extraneous materials from the environment. Water activity for solar dried tomato powder were significantly lower (0.35 - 0.38) than the sundried tomato powder (0.53 - 0.57). Generally, water activity lower than 0.6 is considered microbiologically safe for storage. Lower tristimulus colour L* values (37.81 - 40.31) observed for sundried tomato samples indicated that these samples were darker in colour than the solar dried samples with L* values (50.35 - 46.44). Aerobic mesophile counts were lower in solar dried tomato pre-treated with potassium metabisulphite (3.90 CFU/g) compared with sundried samples (4.85 CFU/g). Sulphur dioxide content of solar dried tomato pre-treated with potassium metabisulphite (740.8 metabisulphite (3.90 CFU/g) compared with sundried samples (4.85 CFU/g). Sulphur dioxide content of solar dried tomato pre-treated with potassium metabisulphite (740.8 ppm) was lower than the maximum legal limit (2000 ppm) recommended in fruits and vegetables. This indicates its safety for human consumption. A strong, negative correlation between sulphur dioxide concentration and microbial load was observed for solar dried tomato. QDA results indicated a strong tomato aroma intensity (scored 127 out of 150) for solar dried tomato powder, with the market samples having an extremely low intensity score of 0.7 out of 150 for tomato. Sensory profiling of the two products differed extremely in aroma, appearance and texture. Sensory characteristics of the reconstituted solar dried tomato powder was similar to that of fresh tomato and tomato paste (two products commonly used in cooking). It had a very coarse appearance and texture (as predicted by the particle size distribution and shape profile and parameters of convexity and circularity) and a strong boiled/cooked aroma compared to tomato paste which had a higher intensity of red colour, metallic and stewed tomato concentrate aroma and flavour. Most of the participants used tomato powder to prepare local dishes such as “jollof “rice, tomato stew and light soup because of the good swelling characteristics of the product. The mixed mode solar dryer developed in this study was efficient in processing tomato powder which appealed to consumers and had varied uses in food production. The dryer thus has the potential of enhancing post-harvest loses, extending the shelf life of tomato and creating an alternative processing method which is simple and convenient.
Description
A thesis submitted to the department of Food Science and Technology, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfilment of the requirements for the award degree of Doctor of Philosophy in Food Science and Technology.