Effects Of Ultrasound Pretreatments On The Osmotic Dehydration And Frying Of Sweet Potato(IPOMEA BATATAS)

Sweet potato is a highly nutritious crop,which is rich in calorie,fibre,carotenoid,vitamin C and other minerals.Consumption of Sweet potato has numerous health benefits such as good eye sight,high immune body system,low glycemic index,antioxidants and anti-inflammatory activities.China is the largest producer of sweet potato in the world and US is the largest exporter of sweet potato in the world.Sweet potato can be consumed as boiled,fried,roasted,puree,syrup,noodles and so on.Due to the perishable nature of sweet potato after harvest,they are dried in order to preserve it for a very long time.The use of conventional dryer to dry sweet potato has some side effects like loss of nutritional contents,disruption of structure or texture,expensive to operate and long drying time.Therefore,this necessitates the application of osmotic dehydration of sweet potato in a bid to preserve the nutritional contents and other physicochemical properties.Ultrasound is a new technology that is simple,efficient and environmentally friendly that has been used in the food industries for the processing of foods such as extraction,hydrolysis,cutting,degassing,freezing and emulsification.Ultrasound enhances the mass transfer of molecules,shortens process time,minimizes use of raw materials and increases the product yield.The pretreatment methods employed in this work include sweet potato samples immersed in distilled water with ultrasound?UD?,osmotic dehydration without ultrasound?OD?and ultrasound-assisted osmotic dehydration?UOD?.The part one of this work has the following objectives;to determine the effects and optimization of process parameters on the ultrasound assisted osmotic dehydration of sweet potato;to determine the effects of ultrasound pretreatments on the mass transfer kinetics and diffusion coefficients during osmotic dehydration of sweet potato;to investigate the effects of ultrasound pretreatment on the texture,colour,structure,carotenoid and vitamin C of osmodehydrated sweet potato.The part two of this work has to do with pretreatment of sweet potato slices using the pretreatment methods?UD,OD and UOD?prior to frying and it contains the following objectives;to investigate the possibility of reducing oil uptake during deep fat frying of ultrasound pretreated sweet potato;to investigate the mass transfer kinetics of ultrasound pretreated sweet potato during deep fat frying;to determine the effects of ultrasound pretreatment on the structure,texture and colour kinetics of sweet potato during deep fat frying;to investigate the effects of ultrasound pretreatment on the sensory and nutritional contents?carotenoid and vitamin C?of deep fat fried sweet potato.In the optimization of ultrasound assisted osmotic dehydration of sweet potato,the process parameters investigated were ultrasound frequency of 20 to 50 k Hz,ultrasound pretreatment time of 10 to 30 min and sucrose concentration of 20 to 60%?w/v?at constant temperature of 30?.Ultrasound probes having diameter ranging from 1 to 3 cm depending on the frequencies were used for this experiment.The response parameters were water loss,solid gain and weight reduction.The results showed that water loss and weight reduction increased as the ultrasound frequency increased from 20 to 35 k Hz and decreased as the ultrasound frequency decreased from 35 to 50 k Hz.The ultrasound frequency,pretreatment time and concentration were found to have significant effect on water loss.Ultrasound pretreatment time and sucrose concentration had significant effect on weight reduction,while only sucrose concentration had significant effect on solid gain.The optimized values that would give maximum water loss,minimum solid gain and maximum weight reduction were frequency of 33.93 k Hz,time of 30 mins and sucrose concentration of 35.69%?w/v?to give predicted values of 21.62,4.40 and 17.23% for water loss, solid gain and weight reduction,respectively.This implied that low ultrasound frequency favoured the osmotic dehydration of sweet potato and also reduced the use of raw material?sucrose?needed for the osmotic dehydration of sweet potato.The mass transfer kinetics,structure,carotenoid and vitamin C of osmodehydrated sweet potato as affected by ultrasound pretreatments were also determined.Ultrasound probe having frequency of 28 k Hz,diameter of 2 cm and maximum power of 300 W for different pretreatment times of 20,30,45 and 60 min at room temperature of 30? were employed for the experiment.Azuara model was used to determine the kinetic parameters and it fitted well the experimental data with high coefficient of correlation ranging from 0.92 to 0.98,low values of chi square?<0.6?,root mean square error?<0.9?and percent mean relative deviation?<10%?.Samples pretreated by ultrasound assisted osmotic dehydration?UOD?significantly had the highest mass transfer coefficient?0.0690 min-1;0.0456 min-1?,equilibrium value for water loss?33.11%?,solid gain?6.5487%?and structure compared to ultrasound with distilled water?UD?and osmotic dehydration without ultrasound?OD?.The highest carotenoid retention was found in samples pretreated by OD compared to UD and UOD.However,the ultrasound was found to enhance the retention of vitamin C?>70%?in samples pretreated by UD and UOD.Further experiments were carried out on diffusion coefficient,texture and colour of osmodehydrated sweet potato as influenced by ultrasound pretreatments.The moisture and solute diffusivities of all the pretreatments ranged from 2.89 to 4.95 x 10-10 and 1.17 to 1.67 x 10-10 m²s^-1,respectively.The moisture diffusivity of samples treated in UOD was significantly?p < 0.05?higher than that of OD and UD by 30.82 and 41.62%,respectively.The highest reduction in texture was also obtained in samples pretreated by UOD and was 92.47% softer than untreated sample.The results further showed that UD increased the brightness?L*?and reduced the redness?a*?of sweet potato while UOD enhanced the yellowness?b*?and chroma?C*?of sweet potato.The highest total colour difference??E?was found in samples pretreated by UOD.The results further showed that the average hue values?ho?of the untreated sample was 66.24 o and that of the UD,OD and UOD ranged from 64.49 to 71.9o,63.41 to 72.62 o and 54.68 to 61.39 o,respectively.The various pretreated samples were also subjected to frying at 130,150,170? for 2,4,6,8 and 10 min.The first order kinetic model used for the mass transfer rate of moisture loss and oil uptake fitted the experimental data well by high coefficient of determination?0.82-0.98?.The lowest moisture content was found in fried samples pretreated in UOD and OD while the lowest oil uptake was obtained in fried samples pretreated in UD having 65.11 and 71.47% oil reduction at temperature of 150 and 170?,respectively,compared to the untreated sample.The k values of all pretreated samples?UD,OD and UOD?were higher than that of the untreated at 150 and 170?.This result therefore showed that ultrasound pretreatment can be a good technique in lowering the oil uptake during deep fat frying.The texture,colour,structure,carotenoid,vitamin C and sensory analysis of fried pretreated sweet potato as affected by ultrasound were investigated.The samples tested for texture and colour were subjected to frying at 130,150 and 170? for 2,4,6,8 and 10 min,except for samples tested for structure,carotenoid,vitamin C content and sensory analysis,which was carried out at 150? for 6 min.The results showed that the normalized maximum force?F*?of samples pretreated by UD had no significant difference with that of the untreated,while samples pretreated by UOD and OD were significantly higher than that of the untreated.Also,the rate of hardening of sweet potato was faster in sweet potato pretreated by UOD and OD compared to untreated.Samples pretreated by UOD also showed high lightness,low redness,moderate yellowness and low total colour difference compared to other samples.The texture and colour kinetics of sweet potato were temperature dependent and expressed as activation energy.Although,there was a significant difference?p<0.05?between the carotenoid of samples treated by OD and other samples,but there was no significant difference?p>0.05?between the untreated samples and samples pretreated by UD and UOD.Similar results were obtained for vitamin C.The results further showed that the presence of oil was observed in the structure of untreated samples,while the structure of samples pretreated by UD had little or no oil.This confirms the results from previous work that samples pretreated by UD had little oil compared to other pretreated samples.The sensory analysis also showed that there was no significant difference between the untreated samples and the samples pretreated by ultrasound.From the foregoing,it is very clear that the use of ultrasound in the osmotic dehydration and frying of sweet potato could be a very good technique in the food industry to enhance drying of sweet potato,and reduce the oil uptake during frying.