Influence Of Hot-air, Microwave-vacuum, And Far-infrared Catalytic Drying On Drying Kinetics And Quality Of Tomato Slices

Drying is one of the oldest and commonest methods of preserving agricultural produce after harvest. The study investigated the influence of hot air, microwave-vacuum, and far-infrared catalytic drying on drying kinetics and quality of dried tomato slices. In the first study, a conventional hot air dryer was used to investigate the effect of four temperatures (50,60,70, and80℃) on the drying kinetics and quality changes in the lycopene content, non-enzymatic browning, colour and flavour of dried tomatoes. As expected, the drying time reduced from1140mins to540mins as the air temperature increased from50to80℃. The non-enzymatic browning increased with temperature from0.051in the fresh tomatoes to1.40after drying at80℃. The lycopene levels of the fresh tomatoes significantly (p=0.05) increased from11.46mg/100g dry matter to61.23,59.10,60.88, and65.28mg/100g dry matter when dried at50,60,70, and80℃respectively. Eleven out of the twelve sensors used in the electronic nose system indicated flavour degradation of all dried samples compared with the fresh tomatoes. The values of the hue angles recorded for the dried tomatoes ranged between51.81°and61.95°, revealing that the dried tomatoes were yellow hued, thus indicating less browning. The drying characteristics curves were evaluated against the Page, Henderson and Pabis, and the Logarithmic mathematical models but the Page model best described the drying of tomato slices. The effective moisture diffusivity coefficient increased with increasing air temperature and was5.13×10-10m2s-1,6.45×10-10m2s-1,8.44×10-10m2s-1, and10.26×10-10m2s-1at respective air temperatures of50,60,70, and80℃with activation energy for moisture removal of22.28KJ/mol.In the second study, a microwave-vacuum dryer was used to investigate the influence of four microwave powers (200,300,500,700W) and three vacuum pressures (0.04,0.05and0.06MPa) on drying kinetics and quality of dried tomato slices. The results showed that increase in microwave-vacuum drying conditions increased drying rates and resulted in decreased drying time from84to14min. The non-enzymatic browning index increased with microwave power whereas the overall flavour degradation was between18.99and20.80%. The brightness and the yellowness of the dried tomatoes generally increased but there was a slight reduction in redness when compared with the fresh. The effective moisture diffusivity increased with microwave power and was7.22×10-9m2s-1,9.10×10-9m2s-1,14.99×10-9m2s-1, and25.19×10-9m2s-1at respective microwave powers of200,300,500, and700W.Among the thirteen thin layer drying models that were used to fit the experimental data, the Midilli et al model gave the highest correlation coefficient, lowest residual sum of squares, root mean square error, and reduced chi-square, thus indicating that the model of Middilli et al best described the micro wave-vacuum drying of tomato slices. The highest ascorbic acid retention occurred in the samples dried at200W and0.06MPa, with a significant decrease (p<0.05) from an initial average value of2.74±0.29mg/g to1.87±0.13mg/g dry matter, representing a decrease of about32%in relation to the fresh tomato. On the other hand, the average lycopene content of the dried tomatoes significantly (p<0.05) increased from11.46mg/100g dry matter to a maximum value of25.44mg/100g dry matter after microwave-vacuum drying at700W and0.04MPa.In the third study, another hot air dryer assisted with a blower was used to dry tomato slices. In this study, the response surface method was used to investigate the results of experiment designed with the Box-Behken approach to make known the effects of air temperature, air velocity, and sample thickness, on drying time, lycopene content, ascorbic acid content, non-enzymatic browning, and colour of dried tomato slices. Three levels of temperature (40to60℃), air velocity (1.0to2.0m/s) and sample thickness (7to11mm) were used for the experiment. The desirability index technique was used to predict the ideal drying condition. At the best conditions of44℃air temperature,2.0m/s air velocity, and7.72mm sample thickness, drying time was527±76min, lycopene content was62.7±4.3mg/100g dry matter, ascorbic acid content was3.07±0.14mg/g dry matter, brightness value was62.92±2.18, ratio of redness to yellowness was,0.78±0.05, and the non-enzymatic browning index was0.55±0.06absorbance unit.In the fourth study, a catalytic far-infrared radiation (FIR) dryer was designed by us and used co dry the tomato slices. The three-level factorial response surface method was used to investigate the effects of distance between FIR emitter and the surface of tomato slices, and sample thickness on drying time, non-enzymatic browning, colour brightness, ratio of redness to yellowness, ascorbic acid content, and lycopene content of the dried tomato slices. Three levels of distance (38to50cm), sample thickness (7to11mm) were used for the experiment. The desirability index technique was used to determine the ideal drying conditions that yield minimum drying time and nonenzymatic browning and maximum brightness colour, redness to yellowness ratio, lycopene content, and ascorbic acid of drying of tomato slices. At the best conditions of40.29cm distance and9.04mm sample thickness, the drying time was108±4min, the non-enzymatic browning index was0.338±0.12Abs unit, the brightness colour was40.43±2.29, the ratio of redness to yellowness was0.92±0.13, the ascorbic acid content was3.76±0.27mg/g dry matter, and the lycopene content was72.34±19.87mg/100g dry matter. The second part of the fourth study investigated the effect of four levels of distances (i.e.38,44,50to56cm) and three-levels of sample thickness (7,9,11mm) on the drying kinetics and flavour of dried tomatoes. The drying kinetics was dependent on the distance between FIR emitters and the heat-irradiated surface, and thickness of the samples as well. The effective moisture diffusivity from the tomatoes increased and correlated to a third order polynomial relationship with decreasing sample thickness and distance between FIR emitter and surface of tomatoes. Only the falling drying rate period was observed with catalytic FIR drying of tomato slices without the constant drying rate period. Relativity’s of far-infrared energy dried samples to the fresh tomatoes revealed that,58.3%of the dried products had flavour enhancement between3.0to36.6%whereas41.7%had flavour degradation between3.1and14.1%. Among the thin-layer drying models fitted to the FIR drying data, the Midilli et al model gave the best fit. These results demonstrate that microwave-vacuum and FIR should be considered as efficient drying methods for tomato with respect to colour and ascorbic acid preservation, minimization of brown pigment formation and increment in lycopene content. Higher microwave powers (i.e.700W) should be avoided in microwave-vacuum drying of tomatoes. The study thus provides useful information for optimization of drying conditions for industrial processing of quality products.