| Drying is one of the oldest and most conventional methods used to preserve foods.Unique traditional methods have been applied to the drying of plant-based food products.However,these methods have several drawbacks,such as low drying rate,high energy and processing costs accompanied by loss of food product's nutritional quality.In this study,we have used the Victoria plum fruits to investigate the drying characteristics and behaviour of mass energy transfer.Plum is the one of the fruits that belongs to family of the Rosaceous and the genus prunus.It has many health benefits because it contains numerous health promoting components,antioxidants,minerals,fiber,flavonoids,vitamins and polyphenols such as lutein,cryptoxanthin,and zeaxanthin in adequate amount.Due to these compounds,plums are used as medicine to treat leukorrhea,irregular menstruation and miscarriage.Beside this,plums are also been used in diabetes,heart disease and cancer,muscular atrophy,constipation,and in promoting bone density.The plum fruit is consumed as fresh fruit,juice and dried as well.The dried plum is called as prunes in the market.From long back it has been dried by traditional drying methods.The convectional drying methods have various disadvantages,slow drying rate,long time,and low product quality,diminish nutritional properties and increase chances of contamination.Recently,a number of other techniques,freeze drying,spray drying,vacuum drying etc have been applied to dry the fruit and vegetables.But they have various disadvantages like high processing labour cost,long drying time,slow drying rate and diffusivity,deteriorates nutritional values and high specific energy consumptions.In some cases,they require special packaging for dried product.Therefore,developments of promising drying techniques are needed to overcome these problems.In this regard,the nonthermal technology,pulsed electric field?PEF?andultrasound?US?are modern and promising techniques that have various applications in food science.Recently,they have become an area of innovation in order to meet increasing industrial demands.Application of PEF and US during drying and osmotic dehydration has beneficial impacts on extension of shelf-life,increases in storage stability and improvement of nutritional quality.The PEF treatment during drying process has been found to exhibit several beneficial effects,such as increasing the shelf-life and enhancing drying rates along with improving nutritional quality of plum fruit PEF is used to increase the cell permeability and drying rate by facilitating the water diffusion coefficient.Drying of food products is the most promising area in food industry which needs improvement in terms of achieving energy efficiency and cost effectiveness to dry food products with maintenance of nutritional properties to adequate levels.In first part,we applied the PEF with intensities of 1-3 kV/cm,30 pulses and 70? drying temperature to determine the convective drying kinetics,cell disintegration,color and micro-structural changes of dried plums.The total specific energy consumption produced by PEF treatment was 10-90 kJ/kg.It was observed that disintegration index?Zp?value improved from 0.147 to 0.572 by increasing PEF intensity.Result showed that high disintegration index led to enhanced drying rate and reduced the drying time.The convective water diffusion coefficient was increased from 0.27 to 16.47 9 10-9 m2/s with increasing PEF intensities.PEF pre-treatment improved the color properties of dried plum as compared to untreated.Additionally,the microscopic investigation by SEM explored that PEF pre-treatment causes shrinkage in dried plum.The higher PEF intensity creates more shrinkage that might be responsible for maximum water diffusion in plum.In second part,we have studied the influence of ultrasound-assisted osmotic dehydration and convective drying to measure the textural properties,retention of bioactive compounds,nutritional quality and metabolites of plum.The osmotic dehydration was performed in osmotic solvent such as 50%glucose and sucrose with ultrasound?25kHz?for 30 and 60 min,and samples were dried by convective hot air oven at 55?.The solute diffusion coefficient was measured by Fick' s law of diffusion The study explored that ultrasound-assisted osmotic dehydration decreases moisture content with enhancing water loss and solid gain from the plum.The texture profile analysis by 20%compression stated that osmotic process led to increase softness and decrease hardness of plum treated in glucose.Likewise,the increase in antioxidant and phenolics was observed in plum treated in 50%sucrose for 30 and 60 min,and plum treated in 50%glucose for 30 min.The chemical shift of 1H NMR analysis reveals that ultrasound-assisted dehydration has little or negligible effect on metabolites found in plum.Color of ultrasound-assisted osmo-dehydrated plum was also affected with treatment time osmotic solution usedIn third part,we have studied the combined treatment of PEF and US on plum to observe their effect on drying kinetics,mass transfer and convective water diffusion coefficient.The PEF pre-treatment of 1,2 and 3 kV/cm,50 pulses followed by US,40kH with 99%frequency for 30 minutes at 27±2? was applied.Electric conductivity was measured to calculate the cell disintegration index and drying of plum was performed at 55? using hot air oven for 180 minutes.Results reveals that plum treated at 3kV/cm showed higher cell disintegration index value followed by 2 and 1 kV/cm at equal pulse number.Combined application of PEF followed by US reveals that it increases drying rate and reduces drying time with increase PEF intensities at uniform US treatment.Similarly,convective water diffusion coefficient increases from 8.75×10-9 to 14.7×10-9 with increasing PEF intensity followed by US at uniform temperature.Overall investigations of the project proposed that application of PEF and US pre-treatment increases drying rate and shorten drying time.It also increases convective water diffusion coefficient and helps to improve the retention of bioactive compound in dried product.It improves the textural properties and color of dried product.This study also stated that application of PEF treatment alone would be more effective to increase drying rate and diffusivity by reducing processing energy as compared to combined treatment of PEF and US.The application of these techniques may reduce labour cost by decreasing specific energy consumption i.e.an urgent need of the processing industries. |
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