| As an above-ground by-product of sweet potato,the yield of sweet potato leaves is comparable to that of sweet potato and rich in nutritional value,but the exploitation rate of sweet potato leaves in China is low,except for some of them used for making feed,most of them are discarded in the fields,thus causing environmental pollution.Many domestic and foreign researchers have shown that sweet potato leaves are rich in nutrients and have various health benefits.Based on the above problems,this paper evaluated the physicochemical properties of sweet potato leaves in three drying methods,and determined hot air drying as the best drying method,and on this basis,analyzed the effect of sweet potato leaf powder addition on the quality of dough and bread.At the same time,emulsifier,enzyme preparation and gluten powder were selected to improve the quality of sweet potato leaf bread,and the best improved formulation was determined by response surface optimization experiment.Finally,the content of carotenoid fraction agents was analyzed in different drying methods and processes,and the relationship between them and antioxidant activity was investigated.The main research contents and findings are as follows:(1)In order to investigate the effects of different drying methods on the quality of sweet potato leaves,three drying methods were employed,including hot air drying(HAD),steamed green combined with hot air to dry(SD+HAD),and steamed green combined with vacuum freeze-dried(SD+VFD).Basic nutritional components,water-holding capacity,color,content of active ingredients,carotenoid components and their content,antioxidant capacity,and other factors were evaluated as the examination indicators to study the effects of the three drying methods on the nutritional quality of sweet potato leaves.The results showed that the fat content of sweet potato leaf meal with SD+VFD treatment was the highest at 7.66 g/100 g DW,followed by SD+HAD treatment at 6.31 g/100 g DW and the lowest at 3.70 g/100 g DW for HAD;the protein content of HAD treatment was the highest at 29.32 g/100 g DW,followed by SD+HAD at 25.70 g/100 g DW and the lowest at 25.18 g/100 g DW for SD+VFD.The water holding capacity and oil holding capacity of SD+VFD treated sweet potato leaf meal were the strongest at 5.52% and 2.71%,followed by HAD at 4.91% and 2.48%,respectively,while the solubility and swelling capacity of HAD treated sweet potato leaf meal were the strongest at 29.33% and 7.56%,followed by SD+HAD at 27.48% and 2.88%.Moreover,SD+VFD treatment had the best retention of total phenol,total flavonoid and carotenoid contents in sweet potato leaves,followed by HAD treatment,and the loss of total phenol,total flavonoid and carotenoid contents in sweet potato leaf powder by SD+HAD treatment was relatively large.In conclusion,the HAD treatment was less costly and had better retention of nutrients.(2)In order to determine the key technology of sweet potato leaf bread making,the effects of sweet potato leaf flour on dough quality and the effects of sweet potato leaf flour on bread quality were investigated respectively,so as to determine the key factors affecting sweet potato leaf bread,and the composite improver was selected to optimize the process of sweet potato leaf bread making by response surface optimization method for the key factors affecting the quality of sweet potato leaf bread.The results were as follows:Firstly,the experimental studies of flour characteristics,blowing characteristics and dynamic rheological characteristics were compared,and the results showed that the addition of sweet potato leaves led to the increase of water absorption and weakness of dough,and the formation time,stabilization time,energy storage modulus,loss modulus,loss angle tangent value tan δ and dough ductility all showed a trend of first decreasing and then increasing,and the baking quality W value of dough decreased,indicating that the addition of destructive sweet potato leaves would destroy the dough The dough quality W value decreased,indicating that the addition of sweet potato leaves would destroy the dough tenderness and reduce the dough quality.The results of pasting characteristics experiments showed that the addition of sweet potato leaves reduced the dough viscosity and the regeneration value decreased;in the fermentation characteristics experiments,the addition of sweet potato leaves had a negative effect on the dough volume,and the p H of the dough continued to decrease with the increase of time,and the sweet potato leaves reduced the air-holding capacity of the dough and caused the dough to collapse.At the same time,the increase of sweet potato leaves content in the dough led to the decrease of wet gluten content in the dough,and the dough tenderness was weakened.Meanwhile,when the content of sweet potato leaves was 10%,the disulfide bond content in the dough decreased by 53.9%,the bound water in the dough disappeared,and the weakly bound water and free water increased by 18.83% and 0.27%,respectively,and the dough microstructure was consistent with the above trend.Secondly,sweet potato leaf powder was added to the bread system at(0%,1%,3%,5%,7%,10%,based on flour mass)to investigate the effects of sweet potato leaf addition on water holding and solubility of the mix,basic nutrient composition of the bread,specific volume,color,textural properties,internal texture structure,sensory scores and active ingredient content.The results showed that the addition of sweet potato leaf powder did not significantly differ in sensory scores between(0% and 1%),and when sweet potato leaf was added at ≥3%it significantly reduced the bread quality and the acceptability of the bread.The addition of sweet potato leaf powder led to an increase in water-holding power and a decrease in solubility of the mix;compared with wheat bread,the specific volume of the bread decreased by 40.8%,the brightness of the crust and core decreased by 59.44% and 65.07%,respectively,and the hardness increased by 1.88 times,and the sensory score was significantly reduced to unacceptable when the sweet potato leaf addition was 10%.Finally,based on the reasons for the decrease in bread and dough quality caused by sweet potato leaves,six improvement agents including sucrose fatty acid ester,α-amylase,glucose oxidase,guar gum,cellulase,and transglutaminase were selected to optimize bread quality.The results showed that cellulase,glucose oxidase,and guar gum had a significant effect on the sensory evaluation of bread,and the size order of the effect was glucose oxidase >cellulase > guar gum.Based on the steepest slope experiment results as the response surface center point,the composition and dosage of the composite improvement agent were determined to be: glucose oxidase 15 mg/kg,cellulase.26%,guar gum 3%.The improved effects of sweet potato leaf bread were compared in terms of volume,texture characteristics,and internal structure.The results showed that compared with before improvement,the volume of the improved sweet potato leaf bread increased by 36.6%,the number of gas holes increased,and the hardness of the bread significantly decreased,indicating that the composite improvement agent was suitable for the processing of sweet potato leaf bread.(3)To clarify the changes in carotenoid fraction agent content during drying and processing,and also to analyze the correlation between it and antioxidant activity.The results showed that the highest lutein content was found in sweet potato leaves treated with SD+VFD during drying,followed by HAD treatment,and the lowest lutein in sweet potato leaves treated with SD+HAD.In addition,β-carotene in sweet potato leaves treated with SD+VFD and SD+HAD were not significantly different before and were significantly higher than those treated with HAD,which indicated that temperature had a significant effect on the stability of carotenoids;during processing,the differences in lutein and β-carotene before and after fermentation were not significant,but after baking the lutein and β-carotene in the bread were significantly lower compared to before and after fermentation,further indicating that the temperature treatment had a greater effect on the carotenoid fraction and content.In addition,the correlation analysis revealed that the main components that exerted antioxidant activity after different drying treatments were carotenoids,lutein,β-carotene,total phenols and total flavonoids;during the processing,high temperature treatment improved the antioxidant capacity of the samples.The negative correlation between antioxidant activity and carotenoids,lutein and β-carotene,and the positive correlation between total phenols and total flavonoids indicated that the main components exerting antioxidant activity during processing were total phenols and total flavonoids. |
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