Influence Of Ultrasound And Osmotic Pretreatments On Drying Kinetics And Bioactive Composition Of Sweet Potatoes(Ipomea Batatas L.)

Sweet potatoes?Ipomoea batatas L.?represent one of the most important food crop the world.World-wide production of sweet potatoes estimated around 100 million tons in 2017,and China ranked first followed USA among largest sweet potato produces and as well as exporter in the world..It is enriched with numerous nutritional compounds such as vitamin A&C and carotenoids,diet fibers,potassium,fatty acids and several others bioactive antioxidant compounds which are beneficial for human health.Other than cooked or baked sweet potatoes consumption,it can be processed into flour,flakes,starch,snacks or bakery products.Due to high moisture content,sweet potatoes are very sensitive to microbial spoilage,even at ambient storage conditions.In addition,sprouting of sweet potatoes at ambient or normal atmospheric conditions and chilling injuries at low or cold temperature during storage conditions adds an additional problem for long term preservation of sweet potatoes.Thus,an effective means of preservation is needed for sweet potatoes.The drying of sweet potatoes not only helps to prolong its storage life but the dried form is characterized with more concentrated source of calories enriched with high bioactive phytochemicals like phenolic contents,dietary fibers,carotenoids contents,and ascorbic acid than regular sweet potatoesDrying can be simply done by conventional approach or by numerous modern approaches like hot air drying with or without some pretreatments as additional energy to reduce the drying time and maintain the quality of the dried product.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 ultrasound and osmotic dehydration for drying of sweet potatoes in a bid to preserve the nutritional contents and other physicochemical properties.Drying with ultrasonic pretreatments is a new technology that is simple,efficient and environmentally friendly and has been used in the food industries for the processing of different foods items,but there is scarcity of literature available on drying of sweet potatoes regarding using different frequencies of ultrasounds along with wide range of hot air and infrared drying temperatures.Ultrasound enhances the mass transfer of molecules,thus shortens the processing time,and minimizes the use of raw materials and maintains the quality of the dried product.In this work,ultrasound,osmotic dehydration and or distilled water with hot air drying and infrared drying were combined together in order to determine the effects of ultrasound application on drying of sweet potato.The main research contents and results are as follows:?1?The main aim of the study was to shorten the total drying time and to improve the quality of sweet potato slices.The effect of ultrasound?US?frequency?20 kHz?and osmotic agent?glucose?using two different concentrations?10%,20%?on quality and drying kinetics with pretreatment times?10,20,30,and 45 min?at hot air drying 60^oC were investigated.The results obtained showed that drying kinetics of sweet potatoes were improved by sonication,with high mass transfer coefficient and drying rate while the osmotic solution had no any significant effect on moisture diffusivity.The logarithmic model showed the best fit to the experimental data for all treatments.For ultrasound pretreated samples,the quality parameters shown significant changes in comparison to control ones and as well as with osmotically dehydrated pretreated samples of sweet potatoes.?2?The influence of ultrasonic frequency?20 kHz?and glucose pretreatments?10%,20%?either alone or in combination were tested on drying of sweet potatoes using hot-air dryer at60^oC for studying the phytochemicals,antioxidant activities and structural changes?FTIR,SEM?of the final dried product.The results obtained showed that total phenolic content?TPC?,total flavonoid content?TFC?were significantly?P<0.05?higher in glucose pretreated samples while the antioxidant activities were higher in combined ultrasound/glucose pretreated samples.For vitamin C,much degradation occurred in the glucose-pretreated samples when compared with other pretreated samples with the exception of the control.FTIR spectrum reveals the presence of glucose,phenols and flavanol compounds in dried samples.The microstructure confirmed the much hardness?N?of the samples in glucose-pretreated samples due to glucose layers and less cell damaged that increased the hardness level of the final product.?3?In this study,our focus was to investigate the bioactive,phytochemicals and antioxidant activity with ultrasound pretreatments?40 kHz and 60 kHz?at two different hot air-drying temperatures?70^oC and 80^oC?using HPLC,FTIR,SEM and spectroscopic analysis.The results revealed that ultrasound pretreatment at 40 kHz with 70^oC maintained the phytochemical compounds in the dried sweet potatoes.Ellagic acid and chlorogenic acid were found as predominant phenolic acids using HPLC analysis,while identification of two new bioactive compounds quercetin-3-rhamnoside and quercetin 3-?-D-glucoside were the novel findings of the current study.A short new band appeared in FTIR in all treatments from2164 to 2041 cm^-1 which refers to C=C alkenes functional group.?4?The study aims to determine drying of sweet potatoes using ultrasound?US?pretreatments?20,40 and 60 kHz?at three different infrared?IR?drying temperatures?60^oC,70^oC,and 80^oC?to evaluate the phytochemical and textural quality of the dried product.The results in this study revealed that,drying time was significantly decreased in moderate US frequency?40 kHz?at 70^oC with the increasing drying temperature.The antioxidant activity of the samples increased especially at 60 kHz and 80^oC,while US-IR treatments shown a positive impact on total carotenoids contents?TCC?and?-carotene.For phenolic compounds,Ellagic acid and Rutin were quantified in higher amount while Quercetin-3-rhamnoside and Quercetin 3-?-D-glucoside were two new compounds identified for the first time in sweet potatoes.FTIR spectra showed the successful synthesis of OH group and phenolics in samples treated with the US at 20 kHz.?5?Different ultrasound frequencies as pretreatments with different infrared drying temperatures were tested for drying of sweet potatoes to study the drying kinetics,mathematical modeling,activation energy and thermodynamic properties of dried sweet potatoes.The results revealed that the treated samples showed a significant reduction in drying time by 110 to 60 min as compared to the control ones.Hii model,Page model,and Silva models found satisfactorily described the drying kinetics among 13 different tested mathematical models.The enthalpy and Gibbs free energy decreased with the increase in drying temperature while entropy decreased and obtained as negative during drying.In addition,the moderate ultrasound frequency of 40 kHz positively and significantly?p<0.05?influenced,enzyme inactivation,enzymatic browning and microstructure of sweet potatoes.