Study On The Effect Of Milk Protein On The Quality Of Gluten-Free Rice Bread And Its Mechanism

Gluten is an important substance that forms the tissue structure of bread,but it is also one of the common allergens.Gluten allergy accounts for about 1% of the population and cannot be cured at present.It is necessary to adhere to a gluten-free diet to avoid the disease.Rice flour is one of the commonly used gluten free raw materials,but because the quality of bread depends largely on the function of gluten,there are many problems in rice bread,such as high hardness,easy aging,white color and insufficient flavor.In addition,with the continuous deepening of research,the issue of nutritional imbalance in gluten free bread has become increasingly prominent,so additives that can simultaneously improve the sensory quality and nutritional value of bread are currently a research hotspot.In this study,two kinds of milk proteins with both functional and nutritional properties,namely whey protein isolate(WPI)and sodium caseinate(Na Cas),were added to bread to explore the role of different gradient milk proteins(0,4% WPI,8% WPI,12% WPI,16% WPI,2.5% Na Cas,5% Na Cas,7.5% Na Cas,10% Na Cas)in each stage of bread processing,and clarify the reasons.The main content is as follows:First,the effects of two kinds of milk proteins on the hydration,gel,pasting,thermodynamics and infrared characteristics of rice flour were studied to provide a theoretical basis for the study of dough and bread.The results showed that the water holding capacity and water binding capacity of rice flour decreased with the increase of WPI addition,while Na Cas could significantly increase the water holding capacity of rice flour and reduce the oil holding capacity at a higher addition level.From the perspective of rice flour gel,WPI and Na Cas both led to a significant reduction of its water absorption index,more free water in the system,which reduced the hardness of the gel,and had a negative impact on its elasticity and resilience.The results of pasting and thermodynamics showed that both WPI and Na Cas significantly reduced the pasting viscosity of rice flour protein system,but could increase the initial gelatinization temperature and pasting stability,and inhibit the cooling retrogradation of amylose.Fourier transform infrared analysis shows that two types of milk proteins interact with starch through non covalent forces such as hydrogen bonds,thereby affecting starch properties.Secondly,study the effects of milk protein on the rheological,fermentation,and microstructure characteristics of rice dough,and explore the reasons for the improvement of bread quality.Due to the dilution effect of protein on the starch system,the storage modulus(G’)and loss modulus(G")of rice dough continued to decline with the increase of WPI addition,while Na Cas could improve the water holding capacity of rice flour,and the dough G’ and G" continued to decline and then rebounded at a high addition(10%).In addition,comparing the rheological frequency scanning results of WPI and Na Cas,it can be found that Na Cas dough has a higher viscosity,which is more conducive to gas retention during the fermentation process.Rheological temperature scanning showed that both WPI and Na Cas led to the decrease of G’ and G" in rice dough during the heating process,and the overall strength of dough gel was weaker,but both of them showed lower tanδ value during the temperature maintaining and cooling stages,better network structure of the system.Scanning electron microscopy and fermentation rheological results showed that WPI was distributed between the gaps of starch granules and not tightly bound with starch,so the dough was more fluid and produced more gas for fermentation,but it was easy to leak and had a low retention coefficient,and the final height of dough fermentation was low,while Na Cas dough had an excellent sheet-like film structure that could well accommodate the gas produced,so the dough fermented for a longer time before it leaked and the fermentation The final height was maximum.Next,the effects of milk protein on bread quality were evaluated in terms of texture,specific volume,color,appearance and section morphology,volatile flavor substances and sensory aspects,and the reasons were explained in relation to the above studies.After adding WPI and Na Cas,the specific volume of rice bread increased from 1.49 m L/g to 1.98 m L/g and 2.32 m L/g,respectively,which is related to the better gas holding and structural characteristics of milk protein dough.Both milk proteins can significantly reduce the hardness,chewiness and adhesiveness of bread,and increase the elasticity,which is similar to the texture change law of gel.Milk protein bread shows more soft,elastic and refreshing characteristics,so the texture preference in sensory evaluation is significantly improved.Milk protein can deepen the color of the bread shell,improve the disadvantage of the white color of rice bread,and increase the variety and content of volatile flavor compounds in rice bread,resulting in a stronger aroma and significantly increased preference for appearance and aroma.When the amount of WPI added increases to 12% and 16%,cracking occurs in the upper part of the bread,which is related to the microstructure of the dough that is not strong enough.Na Cas dough has excellent membrane structure,better continuity and ductility,and the bread does not show cracking.In summary,milk protein can comprehensively improve the quality of rice bread.Finally,the effect of milk proteins on rice bread in short-term storage was investigated to reveal the reason why milk proteins delayed the deterioration of bread quality.The addition of WPI and Na Cas increased the hardness of bread at a much lower rate than that of pure rice flour bread,indicating that milk protein was beneficial to the quality maintenance of bread during storage.On the one hand,milk protein significantly reduced the rate of moisture loss during storage,which was beneficial to maintain the softness of bread.On the other hand,the DSC results showed that both milk proteins caused a significant decrease in the aging enthalpy of bread after storage,but no inhibition of starch recrystallization was observed in the FTIR and XRD results,suggesting that the inhibition of bread aging by adding milk proteins was due to its ability to cause a dilution effect on the starch system,leading to a decrease in the concentration of starch crystallization in bread.