Influence of homogenisation and the degradation of stabilizer on the stability of acidified milk drinks stabilized by carboxymethylcellulose

The present work deals with the influences of both homogenisation and the degradation of carboxymethylcellulose (CMC) on the stability of two kinds of acidified milk drinks (AMDs), directly acidified milk drinks and yoghurt drinks. The effect of homogenisation pressure for direct acidification process was investigated and evaluated. The experimental results showed that homogenisation was required to achieve a significantly small particle size (0.7 μm in the present work) and to prevent sedimentation and serum separation. However, homogenisation at too high pressures was not beneficial for the stability of the colloidal systems. The occurrence of degradation of CMC during homogenisation weakened the stabilisation effect of CMC. A qualified homogenisation pressure of 20 MPa should be chosen to achieve a good stability when a usually practical pressure range of 0–30 MPa was applied. In addition, the stability of directly acidified milk and yoghurt drinks prepared under the same homogenisation pressure was also investigated. While their stability increased with increasing CMC concentration, the degradation of CMC at low pH during storage gave rise to instability of the final products.     

Influence of molecular weight and degree of substitution of carboxymethylcellulose on the stability of acidified milk drinks

The influence of molecular weight (M_w, 250,000, 700,000) and degree of substitution (DS, 0.7, 0.9 and 1.2) of carboxymethylcellulose (CMC) on the diameter and ζ-potential of casein micelles during acidification in diluted dispersions and on the stability of acidified milk drinks was investigated. The experimental results suggested that CMC with high M_w or low DS would result in thick adsorbed layer onto casein micelles. The ζ-potential of CMC-coated casein micelle increased with increasing the M_w of CMC with the same DS while at a fixed M_w the ζ-potential for CMC with high DS (1.2) increased in comparison with those for CMC with low DS (0.7 and 0.9). Both M_w and DS of CMC influenced the stability of acidified milk drinks. CMC with high M_w increased the viscosity of acidified milk drinks significantly and therefore contributed to the stability. CMC with high DS resulted in high ζ-potential of CMC-coated casein micelles, increasing the electrostatic repulsion between casein particles, which prevented the phase separation in acidified milk drinks. It was also found that the amount of CMC needed for efficient coverage of casein micelles increased with increasing the M_w of CMC. Above the efficient coverage concentration, the long-term stability of acidified milk drinks with high M_w CMC was better than that with low M_w CMC.     

果胶对酸性乳饮料稳定作用的研究

通过测定添加不同含量的果胶对酸性乳饮料的沉淀率、粘度、粒径分布、水分子流动性及Zeta电位的影响,研究了果胶对酸性乳饮料的稳定作用。结果表明,果胶对酸性乳饮料的稳定作用表现为使产品的沉淀率降低,粘度升高,在微观性质上表现为使产品的粒径减小,水分流动性减弱,Zeta电位的绝对值升高。      

果胶对酸性乳饮料稳定性的影响

酸性乳饮料的稳定性与稳定剂的含量、均质压力、均质温度、pH有关。选取果胶作为稳定剂,探讨了工艺条件对酸性乳饮料稳定性的影响。结果表明：果胶添加量0.4%、均质压力20MPa、均质温度50℃、pH4.3时酸性乳饮料的稳定性最好。     

大豆水溶性多糖对酸性乳饮料稳定作用的研究

通过测定添加不同质量分数的大豆水溶性多糖对酸性乳饮料的沉淀率、黏度、粒径分布、水分子流动性及Zeta电位的影响,研究了大豆水溶性多糖对酸性乳饮料的稳定作用.结果表明,大豆水溶性多糖对酸性乳饮料的稳定作用表现为使产品的沉淀率降低,黏度升高,在微观性质上表现为使产品的粒径减小,水分流动性减弱,Zeta电位的绝对值升高.其中大豆水溶性多糖对产品的粒径分布有较大影响,对产品的Zeta电位和水分子的流动性影响较小.     

羧甲基纤维素钠对酸性乳饮料稳定作用的研究

通过测定添加不同含量的羧甲基纤维素钠(CMC)对酸性乳饮料的沉淀率、粘度、粒径分布、水分子流动性及Zeta电位的影响,研究了CMC对酸性乳饮料的稳定作用。结果表明,CMC对酸性乳饮料的稳定作用表现为使产品的沉淀率降低,粘度升高,在微观性质上表现为使产品的粒径减小,水分流动性减弱,Zeta电位的绝对值升高。     

调配型酸乳饮料中最优稳定剂的选择

在制作酸性乳饮料时需添加稳定剂。探讨了稳定剂与酪蛋白的作用机理。选用果胶、CMC、PGA、瓜尔豆胶、黄原胶5种稳定剂,通过离心沉淀率的测定,选取其中稳定效果最优的稳定剂。结果表明：在酸乳饮料体系中,果胶是最优稳定剂。     

Water mobility in acidified milk drinks studied by low-field 1H NMR

Low-field nuclear magnetic resonance (LF-NMR) transverse relaxation (T₂) was used to characterise acidified milk drinks (AMDs) with varying composition. Pectin was added to the AMDs at concentrations of 0.0%, 0.1%, 0.3% or 0.5% and the protein used was either skim milk powder (SMP) or SMP with added whey protein concentrate. Distributed exponential analysis of the T₂ relaxation revealed that the AMDs contained a single water component and that the T₂ relaxation times and distributions differed significantly with respect to pectin concentration and protein type. Temperature profiles (5–25–5 °C) showed that pectin concentrations of 0.3% or 0.5% prevented phase separations in the AMDs as a consequence of heating and cooling, whereas a new free water component emerged in the samples with pectin concentrations of 0.0% or 0.1% pectin. Overall, LF-NMR provides a powerful tool for elucidating industrially relevant texture problems such as whey separation.     

可溶性大豆多糖在酸性乳饮料中的应用工艺条件研究

目的研究不同工艺条件对含可溶性大豆多糖的酸性乳饮料的稳定性的影响。方法以产品离心沉淀率、粒径分布以及稳定性扫描结果为指标,测定可溶性大豆多糖浓度、调酸温度、调酸终点p H和调酸前后均质工艺对产品稳定性的影响。结果样品稳定性随着可溶性大豆多糖浓度增高而上升,当大豆多糖浓度大于0.3%时稳定性随大豆多糖浓度升高变化不大;样品稳定性随着调酸温度升高而降低;样品在调酸终点p H为3.75时最稳定;对于相同大豆多糖添加量的体系,仅在调酸后进行均质的样品稳定性较好。结论对于蛋白质含量约为0.7%的酸性乳饮料,大豆多糖最优添加量为0.3%,调酸温度应在20℃以下。如果需要在调酸前增加一道均质工艺可能需要调整大豆多糖用量。酸性乳饮料稳定性受调酸终点p H影响。     

Zeta potential of pectin-stabilised casein aggregates in acidified milk drinks

High-methoxy (HM) pectin is commonly used to improve the stability of acidified milk drinks (AMDs) and zeta potential measurements can aid in elucidating the mechanisms responsible for the improved stability. Pectin was added to an AMD model system in concentrations of 0.0%, 0.1%, 0.3% or 0.5% and the samples were subjected to heat treatment at 75 °C for 20 min; control samples were not heated. A change in zeta potential from positive to negative values with addition of pectin was observed. Increasing the pectin concentration resulted in a more negative zeta potential. Heat treatment caused a decrease in the numeric value of the zeta potential, which indicates a partial desorption of pectin from the protein aggregates.     

酸性乳体系中果胶对酪蛋白胶粒稳定作用的研究进展

酪蛋白胶粒在牛乳中可稳定存在,但当pH降低时易发生聚集而使体系失稳。果胶可用于酸性乳饮料中作为稳定剂。在低pH值条件下,果胶吸附于酪蛋白上,由于产生静电排斥作用和空间位阻作用而使乳体系稳定。同时,乳体系中存在的自支持弱网络结构,也对体系有稳定作用。     

Monitoring of acidified milk gel formation by ultrasonic shear wave measurements. High-frequency viscoelastic moduli of milk and acidified milk gel

In the present paper we describe the application of the thickness shear mode resonator technique for the measurement of viscoelastic parameters of milk and acidified milk gels in the megahertz frequency range. The technique provides information on the viscoelasticity of milk and milk gels in the time scale of 10(-8) to 10(-7) s. The length scale of the measurements, determined by the depth of penetration and the wavelength of the shear wave, falls between the submicron and micron range. In milk acidified by glucono-delta-lactone we observed an increase in the high- and low-frequency loss modulus, G", below pH 5.1, indicating aggregation of casein particles into clusters. There was a sharp rise in high- and low-frequency storage (G') and loss (G") moduli between pH 4.85 and 4.7, as a result of gel network formation in acidified milk. Both G" and G' of milk gel in the megahertz frequency range are several orders higher than those we obtained at low frequencies (0.02 to 10 Hz) using dynamic rheology. The high-frequency (5 to 25 MHz) viscosity of milk was found to be the same as at low frequencies. Overall, our results demonstrate the high sensitivity of the ultrasonic shear wave measurements to the changes in the rheological parameters in acidified milk during gelation.     

自动标准加入法测定乳粉、乳饮料及果汁中的钠含量

目的建立自动标准加入法测定乳粉、乳饮料和果汁中钠含量的方法。方法采用自动标准加入法对全脂奶粉、脱脂奶粉、浓缩乳清粉、婴幼儿配方奶粉以及市场上各类果汁饮料及乳饮料进行钠含量的测定,研究样品中钠离子的提取方式和滴定剂浓度对检测结果的影响,并将测定结果与火焰原子发射光谱法进行对比。结果自动标准加入法用于乳粉、浓缩乳清粉、果汁饮料及乳饮料中钠含量的测定结果重复性好,相对标准偏差均小于5%;而且与火焰原子发射光谱法的测定结果之间无显著性差异,两者之间的相对偏差小于5%。结论自动标准加入法重复性好、准确度高,可用于食品与饮料中钠含量的检测。     

乳味饮料中5种农药的色谱法测定

低温冷冻萃取结合高效液相色谱法,测定乳味饮料中的苯磺隆、氟嘧磺隆、氟铃脲、氟苯脲、杀铃脲。反相C18柱分离,以V(甲醇)∶V(水)=73∶27为流动相,紫外可变波长检测。乙腈为萃取剂,-18℃冷冻萃取,考察了盐效应、冷冻时间、萃取剂与样品比例等条件对萃取结果的影响。优化条件下,5种农药的线性关系良好,相关系数大于0.999 4,相对标准偏差小于2.33%,加标回收率在92.3%⁹8.7%。     

The determination of milk powder added to whole milk

The objective was to determine the adulteration of fresh milk with reconstituted full fat milk powder. The ultra violet and visible spectra (700 to 240 nm) indicated two empirical parameters which are used to detect and quantify adulteration. Each parameter was calibrated against standards of adulteration and then tested against samples of known adulteration. One parameter, tested against two known series (5, 10, 20, 40 and 80% adulteration) gave the best correlation (coefficient of variation, 2.9%) between 20 and 60% adulteration. Correlation decreased at 80% adulteration (coefficient of variation 6.7%) and at 0 to 20% adulteration (coefficient of variation 35.9%). The second parameter, used in the range of 0 to 10% adulteration, when tested against 14 known samples gave results with an average of 12.5% below the true values. Sensitivity was 2.5% added reconstituted milk powder. The effect of low fat values (less than 2.3%) is discussed and a correction factor derived. The method when tested against 12 known samples gave the correct result in all cases within the limitations of the method; 4 commercial samples of milk were also analyzed.     

The mechanism of determining the adulteration of whole milk with milk powder by spectrophotometry

The spectrophotometric method for determination of the reconstituted milk adulteration in the raw milk has been suggested by empirical experiments. However, the mechanism is unclear. In this study, we followed and modified their methods and found that the mechanism is the turbidity difference between homogenized milk and non-homogenized milk. The equation of turbidance S=KBC˙ (d³/d⁴ + λ⁴) is offered as an explanation of this observation. There are linear relationships between transmittance (T[%]) and the amount of there constituted milk added (r=0.99) to the raw milk, non-homogenized milk, and homogenized milk. This method is recommended for detecting the addition of reconstituted milk to raw or non-homogenized milk. The result of the empirical methods showed that the detection rate of adulteration can be as accurate as 2.5%, but this method is not recommended for detecting the adulteration of homogenized milk.     

Stabilisation of acidified skimmed milk with HM pectin

HM pectin was used to stabilise acidified milk drinks (AMD) against flocculation of milk protein. Two temperatures (16 and 27 °C) were used for acidification in order to obtain AMD of different average particle size. Stable samples were diluted with either serum simulating buffer (SSB) or water, and for these samples the adsorption of the pectin onto the casein aggregates was measured using HPLC. The particle diameter, viscosity and sedimentation, as well as the viscoelastic properties, were used to evaluate the amount of pectin needed to stabilise AMD. Additionally, confocal laser scanning microscopy was used with fluorescence recovery after photobleaching (FRAP) to assess protein mobility in samples. Upon diluting with water an increased adsorption of pectin onto the casein aggregates was observed and the stability decreased. In both water and SSB, a certain amount of pectin was firmly-possibly irreversibly-bound to the casein aggregates, and it could be inferred that the adsorbed pectin formed a multilayer on the casein aggregates. Even though the presence of a weak gel network was expected, no gel could be demonstrated in stable AMD by FRAP or the applied rheological measurements. We thus considerer it unlikely that formation of a network is solely responsible for the stability in AMD and propose a stabilising mechanism resembling steric stabilisation combined with secondary adsorption.     

高甲氧基果胶在稳定调配型酸乳体系中的研究

研究了高酯果胶在稳定调酸乳体系中的作用,并从粒径,黏度,沉淀率和热稳定性几个方面来评价稳定调酸乳饮料体系的果胶用量。研究发现不同的酸化温度对果胶和酪蛋白相互作用有影响,这对长货架期和短货架期酸乳饮料的研发都有实际指导作用。     

Physical and functional properties of whole milk powders prepared from concentrate partially acidified with CO2 at two temperatures

Effects of carbonation of whole milk concentrate on spray dried powder properties were investigated. Concentrate acidification by CO₂ addition (2000 ppm) was found to strongly modify the functional properties (solubility, dispersibility) and structural/physical properties (porosity, free fat) of the resulting powders. For concentrates treated at low temperature (where the majority of emulsified fat is in a solid state at 4 °C), colloidal calcium phosphate (CCP) release, casein dissociation and fat coalescence were observed. For warm CO₂ treated concentrates (30 °C) only CCP release was observed. The best functional properties (higher solubility and dispersibility) were found for powders produced from the warm treated concentrates, which were possibly due to the high porosity and better fat globule preservation.