Effect Of Plant Polysaccharides On The Freeze-thaw Cycles Of Starch Gel And Their Application In Pre-baked Frozen Bread

As a kind of frozen dough technology,pre-baking technology greatly meets the needs of the current baking market by virtue of its advantages of saving labor and trouble.However,in the process of processing,storage,transportation and consumption,due to the fluctuation of ambient temperature,pre-baked bread will undergo many freeze-thaw cycles,which has an adverse effect on its quality.Starch is the largest component in pre-baked bread.The freeze-thaw process destroys the structure and function of starch,which will lead to the decline of the quality of pre-baked bread and affect the industrialization of pre-baked bread.Therefore,in this paper,two plant polysaccharides,mulberry leaf polysaccharide and laminaria japonica polysaccharide,were selected to study their effects on the physicochemical properties of starch and the freeze-thaw stability of starch gel,and further explore the effects of these two polysaccharides on the freeze-thaw process of pre-baked bread,providing theoretical support for the application of mulberry leaf polysaccharide and laminaria japonica polysaccharide in pre-baked bread.The main research contents are as follows:Firstly,mulberry leaf polysaccharide and laminaria japonica polysaccharide were taken as experimental objects to explore the effects of polysaccharide on the transparency,solubility,dilatability and gelatinization characteristics of starch.The results showed that the light transmittance of starch paste was decreased by mulberry leaf polysaccharide and laminaria japonica polysaccharide,and the transparency of starch was much lower than that in blank group.The two polysaccharides increased the solubility of starch and decreased the swelling degree of starch.The peak viscosity,peak-valley viscosity and final viscosity of wheat starch decreased significantly with the increase of mulberry leaf polysaccharide and laminaria japonica polysaccharide,and the decay value and recovery value also decreased.Secondly,the effects of mulberry leaf polysaccharide and laminaria japonica polysaccharide on the freeze-thaw stability of starch gel were studied.The results showed that mulberry leaf polysaccharide and laminaria japonica polysaccharide significantly reduced the water extraction rate of wheat starch gel during freeze-thaw period,and improved the freeze-thaw stability of wheat starch gel during freeze-thaw period.The recovery value of all wheat starch gels containing mulberry leaf polysaccharide and laminaria japonica polysaccharide was lower than that of wheat starch during freeze-thaw process（1.84 J/g）.And the higher the proportion of polysaccharide addition,the stronger the effect of inhibiting regenerative.Similarly,the crystallinity of starch was also correlated with the retrogradation of starch,and the crystallinity degree of starch increased after multiple freeze-thawing,while mulberry leaf polysaccharide and laminaria japonica polysaccharide reduced the crystallinity of starch（16.18%）.Fourier infrared spectroscopy showed that the two polysaccharides and starch were bonded by hydrogen bond,and the two polysaccharides reduced the R_1047/1022 value of starch gel during freeze-thaw process,and inhibited the double helix structure and the increase of starch order degree during freeze-thaw process.NMR results showed that polysaccharide decreased the water migration rate and increased the water holding capacity of starch gel.In addition,mulberry leaf polysaccharide and laminaria japonica polysaccharide also reduced the hardness and gel strength of starch gel during freeze-thaw process.The G’of wheat starch gels containing mulberry leaf polysaccharide and laminaria japonica polysaccharide was lower than that in blank group,and the tanδvalue decreased at a lower rate than that in blank group.Scanning electron microscopy showed that mulberry leaf polysaccharide and laminaria japonica polysaccharide reduced the pore structure of gel network,stabilized the matrix thickness around starch gel,and improved the microstructure of gel.In vitro digestion experiments showed that the increase of starch recrystallization led to the enhancement of anti-enzymatic hydrolysis.Mulberry leaf polysaccharide and laminaria japonica polysaccharide reduced the content of rapidly digestible starch gel,and the two polysaccharides could reduce the digestibility of wheat starch gel.Finally,the effects of mulberry leaf polysaccharide and laminaria japonica polysaccharide on freeze-thaw process of pre-baked bread were studied.The results showed that with the increase of polysaccharide concentration,the hardness,chewability and cohesiveness decreased during freeze-thaw process,and the texture properties improved well.The addition of mulberry leaf polysaccharide and laminaria japonica polysaccharide also inhibited the increase ofΔH caused by freezing and thawing.Compared with the blank group,ΔH of bread decreased by 44.10%when mulberry leaf polysaccharide was added at 3%,and 35.90%when laminaria japonica polysaccharide was added at 2.5%after 3 weeks of freeze-thaw.The results showed that both polysaccharides could delay starch regeneration and thus reduce the aging of bread caused by freeze-thaw.With the addition of mulberry leaf polysaccharide and laminaria japonica polysaccharide,the proportion of bound water decreased,the proportion of multilayer water increased,and the proportion of free water decreased.Mulberry leaf polysaccharide and laminaria japonica polysaccharide can bind free water molecules,reduce the proportion of free water in bread,and improve the freeze-thaw stability of bread.In conclusion,both mulberry leaf polysaccharide and kelp polysaccharide can effectively improve the freezing-thawing stability of wheat starch gel,significantly inhibit water dissolution and starch recovery during freezing-thawing process,and also significantly improve the quality of pre-baked bread after freezing-thawing.Therefore,this paper can provide theoretical guidance for the research mechanism of improving the stability of starch freeze-thaw,and provide experimental basis for promoting the development and application of mulberry leaf polysaccharide and kelp polysaccharide in improving the quality of frozen dough bread.