| Drying extends the shelf life of wheat-products by reducing the water content.Wheat starch and gluten are the main components of wheat-products,and their ability to bind with water affects the drying characteristics of wheat-products.Moisture migration from products to environment is the process of decreasing free water.The rate of water migration is proportional to the difference between the internal and external water potential.The difference between the internal and external water potential can be characterized by the difference between the water activity of the sample and the relative humidity of the environment.Excessive water loss rate will cause rapid shrinkage of the sample,resulting in unbalanced stress,making the product deformed,and even structural damaged,such as distortion and surface cracks.In this study,the wheat variety Ningchun No.4 was used as the raw material.Wheat starch and gluten powder were extracted from wheat flour.According to different ratio of starch and guten,blended powders,extrudates with household noodles machine?extrudates with noodles machine?and extrudates with twin screw extruder?extrudates?were made.Based on the study of moisture absorption and desorption characteristics of powder and extrudates with noodles machine with different ratios of starch and gluten powder.The effect of different water activity?aw?and relative humidity?RH?values?aw-RH?on water loss rate changes and deformation characteristics of exturdates were studied.It is expected to provide a theoretical basis for regulating moisture migration and deformation of noodle productsThe results show that:1.The moisture adsorption and desorption rate of starch powder are 2.6×10-6?g/g?/s,2.1×10-6?g/g?/s,the moisture adsorption and desorption rate of gluten powder and 3.2×10-6?g/g?/s,1.2×10-6?g/g?/s.The moisture adsorption and desorption rate of starch extrudates with noodles machine are3.3×10-6?g/g?/s,2.4×10-6?g/g?/s,the moisture adsorption and desorption rate of gluten extrudats with noodles machine are 1.0×10-6?g/g?/s,0.6×10-6?g/g?/s.There is no significant difference between the rate of moisture adsorption and desorption of the starch powder and starch extrudaes with noodle machine.The moisture adsorption and desorption rate of gluten extrudates with noodle machine are significantly lower than that of gluten powder.That indicates structure hinders the migration of moisture.2.Fitting adsorption moisture isotherms of starch and gluten powder by GAB equation,the result shows,the monolayer water content?M0?of starch and gluten powder are 13.4 and 9.8.The moisture binding energy of starch is higher than that of gluten.When aw is higher than 0.9,the moisture binding energy is is closed to 0 J/mol,the minimum moisture content of extrudates with noodles machine is 23%.It shows that water exists as free water when moisture content is beyond 23%.While the minimum moisture content of extrudate is 27%,which shows that water exists as free water when moisture content is beyond 27%,and exists in bound state when moisture content below 27%.3.Compared the water loss rate of difference between water activity?aw?and relative humidity?RH??aw-RH?gradually decreasing,basically unchanged and gradually increasing,find that when?aw-RH?gradually increased,the average water loss rate is the largest,reaching to 3.0×10-3?g/g?/min when the average moisture content of extrudate decreased from 35.2%to 16%.Increased starch content will increase water loss rate.When the starch content decreases from 100%to 0%,the time from less than 300 minutes increase to more than 800 minutes when water content is the same.4.When the aw changes from 0.35 to 0.11,the apparent density of the pure starch extrudate with noodle machine decreases by 57 kg/m3,and the pure gluten extrudate with noodle machine decreases by only 5 kg/m3.The change value of apparent density of starch extrudates with noodlle machine is higher than that of gluten.That indicates the extrudates of starch and gluten with noodle machine shainks after losing water.When using increasing?aw-RH?value to deal with extrudates,the deformation degree of of the extrudates more severe.Such as,the changing rate of the number of cells,the wrapper length and the area of slice gradually increase,the changing rate of total concavity decrease to a negative value,and the proportion of area of cells and coarse/fine clustering increase.Those indicates that the degree of deformation of the extrudates increased,and the number of surface cell decreases,the surface of extrudate is rougher and uneven after the water loss.The conclusions are:The monolayer water content of starch is higher than that of gluten.The rate of moisture adsorption is lower than that of gluten,but the rate of desorption is higher than that of gluten because the structure formed by gluten powder hinders water migration.The increase in the value of difference between the water activity of the sample and the relative humidity of the environment will increase the rate of water loss and the deformation characteristics of the sample.An increase in the proportion of starch will increase the water loss rate of the sample,and increase the shrinkage and deformation of the sample.So,the moisture migration rate and deformation degree can be controlled by the difference between the water activity of the sample and the relative humidity of the environment. |
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