Rheological properties of raisins: Part II. Effect of water activity
Rheological properties of raisins: Part II. Effect of water activity
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葡萄干的流变特性：第二部分。
DOI：
10.1016/0260-8774(94)00036-9
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发表时间：
1995
影响因子：
5.5
通讯作者：
W. Wolf
中科院分区：
文献类型：
--
作者：
P. Lewicki;W. Wolf
采用压缩-松弛试验测定了汤普森无核葡萄干的流变学特性与水分活度的关系。根据COST 90程序测定葡萄干的吸水等温线。已经发现，葡萄干的流变特性强烈地依赖于水活度。当水分活度低于0.25时，葡萄干表现为脆性体。据推测，干葡萄干中的糖是无定形的，玻璃的性质对材料的特性至关重要。在这种状态下，压缩过程中断裂的可能性很高。当水活度高于0.3时，水的增塑作用就显现出来。葡萄干变成粘弹性体，但直到aw= 0.4，由于浆果内部的高粘度，弹性组分不强。水的进一步吸收和aws的增加导致糖溶液的稀释和皮肤弹性的增加。压缩葡萄干的松弛行为在aw= 0.4附近变化。表皮的弹性足够高，以防止葡萄干在低于0.3MPa的应力下的压缩测试期间破裂。水分活度高于0.7会削弱皮肤，并减轻稀释糖溶液的流动。浆果容易变形，破碎的可能性大大增加。
Rheological properties of Thompson seedless raisins were measured in relation to the water activity of the berry using the compression-relaxation test. The water sorption isotherm of raisins was determined according to the COST 90 procedure. It has been found that the rheological properties of raisins are strongly dependent on water activity. At water activities lower than 0.25, the raisin behaves as a brittle body. It is inferred that sugars in dry raisins are amorphous and the properties of the glass are pivotal to the characteristics of the material. In this state the probability of breakage during compression is high. At water activities above 0.3, a plasticizing effect of water is manifested. The raisin becomes a viscoelastic body but until aw= 0.4, the elastic component is not strong because of the high viscosity of the interior of the berry. Further uptake of water and increase of awleads to dilution of the sugar solution and increased elasticity of the skin. The relaxation behaviour of compressed raisins changes around aw= 0.4. The elasticity of the skin is sufficiently high to prevent breakage of raisins during the compression test at stresses lower than 0.3 MPa. Water activity higher than 0.7 weakens the skin and eases of the flow of diluted sugar solution. Deformation of the berry is easy and the probability of breakage increases strongly.