| Oranges are one of the widely consumed citrus fruits in the world and contain many bioactive substances such as polyphenols,flavonoids,carotenoids,vitamin C and limonin.Although the processing and sales volume of oranges increase year by year,there are still problems of total excess and seasonal excess.Additionally,the orange products in the world trade are mainly fresh fruit,canned fruit and juice.However,the deep processing products of oranges are relatively single,which cannot meet the increasing demand of consumers for diversified and personalized products.Therefore,this study planned to develop a high qulity semi-dried and low-sugar orange slice product using Gannan navel orange as raw material.It mainly studied and optimized the novel osmotic dehydration process,low temperature drying process and the stability of the product properties during storage.The specific research content and main conclusions are as follows:(1)The effects of four osmotic dehydration methods(osmotic dehydration(OD),vacuum osmotic dehydration(VOD),ultrasound osmotic dehydration(UOD)and ultrasound assisted vacuum osmotic dehydration(UVOD))and four osmotic agents(sucrose,xylitol,sorbitol and maltitol)on the mass teansfer rate and quality of orange slices were explored.The results showed that ultrasound damaged cell wall and vacuole of orange slices,resulting in more water loss in cell wall and the transformation of free water from vacuole to cytoplasm and intercellular space,and the synergism of ultrasound and vacuum resulted in the increase of mass transfer area.Therefore,compared with the other three osmotic dehydration methods,UVOD had the highest mass transfer rate,the lowest hardness and the lowest water activity(p<0.05).Peleg model was suitable for simulating the mass transfer process of orange slice.In addition,the color change of UVOD treated samples was minimal due to the enzyme-inactivation effect of ultrasound and the air removal effect of vacuum.In terms of flavor and chemical groups,there was no significant difference between all the osmotic dehydrated orange slices and the fresh one(p>0.05).Under the same osmotic dehydration method,compared with the other three osmotic agents,xylitol treated samples had the highest mass transfer rate and the lowest water activity because of its low molecular weight(p<0.05).Therefore,UVOD and xylitol were selected as osmotic dewatering methods and osmotic agents in this study.(2)The single factor experiment was used to determine the effect of vacuum pressure,ultrasound power,ultrasonic vacuum time,concentration of osmotic pressure and osmotic time on the water loss rate(WL),solid gain rate(SG),hardness,and(35)E value.The vacuum pressure,ultrasonic vacuum time and osmotic time were optimized by unitary linear regression analysis and response surface method.The results showed that under the conditions of xylitol as osmotic agent,temperature of 25℃,solid-liquid ratio of 1:4 and no mechanical agitation,the optimal parameters of osmotic dehydration were:osmotic concentration of 50°Brix,ultrasound power of 300 W,vacuum pressure of 0.042 MPa,ultrasonic vacuum time of 22.817 min,and osmotic time of 3 h,where the treated orange slices had good WL,SG,hardness,and(35)E value,(37.44%,16.299%,28.50 g and 6.81,respectively).(3)The orange slices treated using optimal UVOD were dried at low temperature.The effects of low-temperature convective drying(CD-35℃,CD-45℃and CD-55℃)and vacuum freeze-drying(FD)on physicochemical and sensory qualities of orange slices were compared.The relationship between water activity and water content was tested to determine the final water content(24%).The results showed that the Page model well fit the low temperature drying process of orange slices well,and the drying time of different samples was ranked from high to low as the control(direct convective drying of fresh orange slices at 45℃)>FD>CD-35℃=CD-45℃>CD-55℃.Compared with the control group,the drying time of the orange slices obtained by UVOD-CD treatment decreased by 58.62%to 89.66%.At the same time,the products obtained by this treatment possessed the lower product hardness,smaller color changes,higher content of ascorbic acid,and total phenol content,stronger antioxidant resistance,and higher sensory scores.Compared with the orange slices dried by UVOD-CD treatment,the orange slices obtained by FD had the lowest hardness,better retention of ascorbic acid and total phenolics,however,its antioxidant activity was lower and its drying time was longer.Among the UVOD-CD groups,compared with CD-35℃and CD-45℃groups,CD-55℃treatment saved 62.5%of drying time,and orange slices dried using this method had the lowest(35)E value(p<0.05),and the highest hardness,ascorbic acid content,total phenolics content,and sensory scores(p<0.05).What’s more,there was no significant difference in antioxidant activity between CD-55℃drying samples and CD-35℃drying samples with highest antioxidant activity(p>0.05).Above all,CD-55℃was selected as the low-temperature drying process for semi-dried orange slices.(4)The storage and shelf-life model of semi-dried orange slices prepared by UVOD combined with CD-55℃were studied at different temperatures(4℃,25℃and 37℃),whose qualities were also compared with two commercially available products.The results showed that with the increase of storage time and temperature,the water content and water activity of all samples declined first and then rose during the entire storage period.(35)E value increased continuously,while ascorbic acid,total phenolics and sensory evaluation indexes decreased continuously.When water activity was lower than 0.60,the total number of colonies,coliform and mold remained within the safe range throughout the storage period.The R~2 value of the shelf-life model obtained by combining the zero-order dynamics model with Arrhenius model was higher than 0.90,which meant it can well predict the shelf life of semi-dried orange slices stored at the temperature range of 4℃～37℃.Based on the shelf-life prediction model,the shelf life of semi-dried orange slices stored at 4℃,25℃and 37℃was 398 days,208 days and149 days,respectively.In this study,xylitol was used to prepared self-made products.Xylitol was easier to form hydrogen bonds with water than sucrose,helping to bind water activities.Therefore,compared with commercially available orange slices,self-made orange slices had lower water activity while had higher water content,which was beneficial to prepare products with lower hardness and higher edible safety.In addition,although having higher costs,because the processing time of self-made orange slices was short(only 4.5 h)and the preparation process were always kept at low temperature,the content of ascorbic acid and total phenolics,antioxidant activity and sensory quality of self-made orange slices were significantly better than those of commercially available products. |
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