Rheological Properties of Chymosin-Induced Skim Milk Gels as Affected by Milk Storage Time and Temperature

Fundamental dynamic rheological tests were used to measure the rigidity (|G*|) and viscoelasticity (tan δ) of gels made from skim milk samples that were (1) never cooled, (2) stored at 4°C, or 3) stored at 4°C and then rewarmed to 25°C. Milk cooled and held at 4°C prior to gel formation produced gels with lower |G*| and higher tan(δ) than fresh milk gels. Chilled milk samples that were rewarmed to 25°C for 16h prior to gel formation produced gels with rheological properties similar to fresh milk gels. The solubilization of β-casein during cold storage was hypothesized to be the cause of observed differences in gel rheology.     

Rheological Properties and Microstructure of Acid Milk Gels as Affected by Fat Content and Heat Treatment

Gels were made from recombined milks containing 0, 1.5 or 3.5% fat that were heated at 75, 80 or 90°C for 30 min, followed by acidification with glucono-8-lactone at 30°C. The rheological and microstructural properties of acid gels were investigated using dynamic low-amplitude oscillatory rheology and confocal scanning laser microscopy (CSLM). Heating milks, at temperatures ≥80°C, increased the storage moduli (G′) and decreased the gelation time. Recombined milks containing high fat (3.5%) had higher G′ than gels made from low-fat or skim milk. Milk heat treatment resulted in gels with a cross-linked microstructure. Recombined fat globules appeared to be embedded in the protein matrix.     

电感耦合等离子体质谱法测定鲜奶、酸奶和奶粉中稀土元素

建立了电感耦合等离子体质谱法(ICP-MS)测定乳及乳制品中16种稀土元素的方法。采用微波消解前处理技术消解样品,优化了仪器工作参数,选择¹⁰³Rh、¹¹⁵In和²⁰⁹Bi元素作为内标,校正基体干扰。结果表明,16种稀土元素质量浓度在0.2~5 ng/mL范围内线性关系良好,线性相关系数r≥0.9995,检出限在0.03~0.52 μg/kg之间,平均回收率在92.3%~109.0%之间,相对标准偏差(RSD)为2.21%~5.36%。采用该方法测定国家标准物质柑橘叶(GBW10014),其测定值均在标示值范围内。对市售鲜奶、酸奶和奶粉中稀土元素进行检测,结果显示,3类乳品中均检出稀土元素,含量为0.04~19.3 μg/kg。该方法具有快速、准确、操作简便,能够满足鲜奶、酸奶和奶粉中稀土元素的测定。     

离子色谱法测定奶粉中的葡萄糖、蔗糖和乳糖

建立高效阴离子交换色谱-脉冲安培检测法测定奶粉中葡萄糖、蔗糖和乳糖的方法。以METROSEP CARB 1(150mm×4.0mm)阴离子交换柱为分离柱,脉冲安培检测,37mmol/L NaOH溶液为淋洗液,以柠檬酸作为蛋白质沉淀剂。葡萄糖、蔗糖和乳糖的线性范围分别为1～40、1～50、1～50mg/L,相关系数分别0.9966、0.9968、0.9985,检出限分别为0.014、0.091、0.083mg/L,精密度为1.10%～4.96%,回收率为90.13%～104.83%。该方法分析时间短,前处理简单,适用于快速测定奶粉中的葡萄糖、蔗糖和乳糖。     

快速测定婴幼儿配方乳粉中的维生素B1及B2

建立高效液相色谱-荧光法快速测定婴幼儿配方乳粉中维生素B₁及维生素B₂。方法优化了前处理条件,采用高压酸提法提取目标化合物,样品沉淀蛋白后直接测定维生素B₂,取部分清液用碱性铁氰化钾衍生后测定维生素B₁。目标物在对应的浓度范围内呈现良好的线性关系（R²≥0.9998）,样品加标回收率在92.4%～102%之间。该方法快速,准确,适用于婴幼儿配方乳粉中维生素B₁、B₂的测定。     

毛细管气相色谱内标法同时测定奶粉中亚麻酸、ARA、EPA和DHA

建立了以二十一烷酸甲酯为内标物,同时测定奶粉中亚麻酸ARA、EPA和DHA的毛细管气相色谱方法。试样经氢氧化钾-甲醇甲酯化后、生成相应的脂肪酸甲酯,再经气相色谱分析、内标法定量。结果表明,奶粉中亚麻酸、ARA、EPA、DHA和内标物均获得良好的分离;4种被测组分色谱峰面积与其质量浓度有良好线性关系,相关系数均大于0.998;样品加标回收率为95.4%～104.7%,变异系数为2.168%～4.368%,检出限为1.0mg/100 g。该方法操作简便、快速、准确,适合批量奶粉中亚麻酸、ARA、EPA和DHA的测定。     

Acid Diffusion into Rice Boluses is Influenced by Rice Type,Variety, and Presence of α‐Amylase

Breakdown of rice during gastric digestion may be influenced by rice structure, presence of salivary α‐amylase, and hydrolysis by gastric acid. During mastication, saliva is mixed with rice, allowing α‐amylase to begin starch hydrolysis. This hydrolysis may continue in the gastric environment depending on the rate at which gastric acid penetrates into the rice bolus. The objective of this study was to determine the acid uptake into rice boluses with and without α‐amylase in saliva. Two types each of brown and white rice (medium and long grain), were formed into a cylindrical‐shaped bolus. Each bolus was sealed on all sides except one to allow one‐dimensional mass transfer, and incubated by immersion in simulated gastric juice at 37 °C under static conditions. Acidity of the boluses was measured by titration after 1 to 96 h of incubation. Effective diffusivity of the gastric juice through the bolus was estimated using MATLAB. Average acidity values ranged from 0.04 mg HCl/g dry matter (medium grain white rice, no incubation) to 10.01 mg HCl/g dry matter (long‐grain brown rice, 72 h incubation). The rice type, presence of α‐amylase, and incubation time all significantly influenced rice bolus acidity (P < 0.001). Effective diffusivity of gastric juice into the bolus was greater in brown rice than in white rice. These results indicate that starch hydrolysis by α‐amylase may continue in the stomach before the gastric acid penetrates the rice bolus, and the rate of acid uptake will depend on the type of rice consumed.