现代物化方法在干酪品质研究方面的应用

近年来,世界干酪产量一直保持上升趋势。由于商业竞争及消费者要求的提高,对干酪品质的要求越来越高。要准确判断干酪的品质,就需要一些能够准确判断其品质的仪器。主要探讨了现代的一些物化方法在研究干酪品质方面的应用,主要有:(1)电子显微镜(ElectronMicroscope);(2)差示扫描量热仪(Differentialscanningcalorimetry);(3)核磁共振(NuclearMagneticresonance)。     

现代物理化学方法在油脂质量控制中的应用

介绍了现代物理化学方法(差示扫描量热法、电子显微镜、核磁共振法、X-射线衍射法)的原理及其在油脂质量控制中的应用,并预测了油脂质量控制研究的发展方向。     

Crystallization behavior of milk fat obtained from linseed-fed cows

Milk with an increased content of unsaturated fatty acids was obtained by incorporating 60% of extruded linseed into the concentrate of cows. Two groups of Holstein cows (3 animals/group) were fed a concentrate (control or linseed enriched) together with the same roughage diet (ad libitum). After an adaptation period of 3 wk, evening and morning milk samples were collected every 7 d for 3 wk. Milk was decreamed and anhydrous milk fat (AMF) was isolated from the fat fraction by using the Bureau of Dairy Industries method. The objective of this study was to investigate if the crystallization mechanism of milk fat changed when the content of unsaturated fatty acids was increased. Therefore, the crystallization behavior of a milk fat enriched with unsaturated fatty acids was compared with that of a control milk fat. Nonisothermal crystallization was investigated with differential scanning calorimetry, and 1-step and 2-step isothermal crystallization behaviors were investigated using pulsed nuclear magnetic resonance, differential scanning calorimetry, and x-ray diffraction. A higher content of unsaturated fatty acids in AMF resulted in an increased proportion of low melting triglycerides. These triglycerides lowered the solid fat content profile, particularly at refrigerator temperatures. Furthermore, they induced some changes in the crystallization and melting behaviors of milk fat compared with a control AMF, although no fundamental changes in the crystallization mechanism could be revealed. Even though a lower melting point could be observed for milk fat with a higher content of unsaturated fatty acids, a similar degree of supercooling was needed to initiate crystallization, resulting in a shift in onset temperature of crystallization toward lower temperatures. In addition, slower crystallization kinetics were measured, such as a lower nucleation rate and longer induction times, although crystallization occurred in a similar polymorphic crystal lattice. During melting, a shift in offset temperature toward lower temperatures could be observed for the 3 melting fractions of AMF in addition to a higher proportion of low melting triglycerides. These results demonstrate that a higher content of unsaturated fatty acids has some effect on the crystallization behavior of milk fat. This knowledge could be used to produce dairy products of similar or superior quality compared with conventional products by intervening in the production process of dairy products.     

Glass formation and dormancy in bacterial spores

Summary Differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) techniques were used to obtain evidence whether a glassy state exists within fully hydrated dormant Bacillus subtilis spores. DSC thermograms of hydrated dormant spores contained two major transitions; one at ∼60 °C assigned to be associated with the initial 'activation' stage for germination, and the other at ∼120 °C assigned to be associated with the 'inactivation' process (i.e. killing of the spores). The peak at ∼60 °C had characteristics which were all consistent with at least some region of the compartmentalised structure of fully hydrated dormant spores being in a glassy state. Solid (CPMAS) and liquid state (SPMAS) mobility resolved ¹³C NMR spectroscopy studies on the calcium dipicolinic acid (CaDPA) resonances, which are only present in the central protoplast of bacterial spores, showed it to be present in an amorphous solid-like environment. The DSC and NMR results were both found to be consistent with the hypothesis that the dormancy of bacterial spores is due to at least the central protoplast region being maintained in a low moisture content glassy state.     

Effect of Calcium Maltobionate on the Glass Transition Temperature of Model and Hand-made Hard Candies

Glass transition temperature (T_g) is an important parameter for the physical quality control of hard candies. In order to understand the applicability of calcium maltobionate to hard candy, effect of calcium maltobionate addition on the T_g of model and hand-made hard candies was investigated. Freeze-dried calcium maltobionate-sugar (sucrose containing a small amount of glucose-fructose mixture) and calcium maltobionate-reduced isomaltulose mixtures were prepared as model candies, and their anhydrous T_g was evaluated using a differential scanning calorimetry. The anhydrous T_g increased linearly with the molar fraction of calcium maltobionate. From these results, it was expected that calcium maltobionate can improve the physical stability of normal and sugarless candies. For comparison, various commercial candies were employed, and their T_g was evaluated using a thermal rheological analysis. The T_g values were in the range of 28–49 °C. The T_g values were higher than 25 °C, which is significant with respect to the physical stability of the candies. Calcium maltobionate-sugar and calcium maltobionate-reduced isomaltulose candies were prepared as hand-made candies. The calcium maltobionate-reduced isomaltulose candies had higher T_g than the calcium maltobionate-sugar candies at each calcium maltobionate content, although reduced isomaltulose has a lower T_g than sugar. At a high calcium maltobionate content, calcium maltobionate-reduced isomaltulose candy had an equivalent T_g to the commercial sugarless candies, and thus practically acceptable stability was expected. In the case of calcium maltobionate-sugar candies, there was a possibility that the hydrolysis of sugar reduced their T_g. Vacuum-concentration will be useful to improve the T_g of the candies.     

Effect of TiO2 nanoparticles on basalt/polysiloxane composites: mechanical and thermal characterization

In the present investigation, a novel technique has been developed to fabricate composite materials containing TiO₂ nanoparticles, polysiloxane resin, and basalt fabric. A high-intensity ultrasonic probe was used to obtain a homogenous molecular mixture of TiO₂ nanoparticles and polysiloxane resin, thus the nanoparticles were infused into the resin through sonic cavitation. The loading effect of TiO₂ nanoparticles on the thermal and mechanical properties of basalt fabric reinforced polysiloxane composite materials has been investigated. Composite samples were prepared, each using two layers of basalt fabric with TiO₂ nanoparticles loading from 0.5, 1, 1.5, 2, 2.5, and 3% by weight. Size distribution of nanoparticles was observed by particle size analyzer and the prepared fabric nanocomposites were characterized by scanning electron microscopy, dynamic mechanical analysis (DMA), thermo gravimetric analysis (TGA), and differential scanning calorimetry (DSC). Tensile testing was performed as per American standard for testing of materials (ASTM) standards. The dependence of dynamic mechanical parameters E′, E′′, tan (delta), T _g, and heat distortion temperature (HDT) are associated with the filler content and can be controlled by the curing conditions. Tensile results show that 1.5 wt.% loading of TiO₂ nanoparticles in the nanocomposites resulted in highest improvement in tensile modulus compared to the neat system. DMA studies also revealed that 1.5 wt.% doped system exhibits highest storage modulus as compared to the neat and other loading percentages. DSC and TGA studies show that T _g and HDT of the composite increases with the increase in wt.% of nanofillers in the composite. Based on these results, it is clear that miscibility of nanoparticles in the resin is of prime importance with regard to performance.     

Hydration of heat-treated soy protein isolate and its effect on the molten flow properties at an elevated temperature

The relationship between the flow properties of soy protein isolate (SPI) at 140°C, measured using an extrusion viscometer, and changes in the hydration state were investigated. The state of the water in SPI was analysed by measurement of the freezable water content using differential scanning calorimetry (DSC) and by spin-spin relaxation rate (R₂) measurements using pulsed nuclear magnetic resonance (NMR). The dependence of the flow properties on the state of water was indicated by the correspondence between the water content at which the flow characteristics of molten SPI changed, and the critical water content indicated by DSC measurement of the minimum freezable water content, and by the inflection point in NMR measurement of the spin-spin relaxation rate (R₂).     

Effect of water and guar gum content on thermal properties of chestnut flour and its starch

Thermal properties of chestnut flour and chestnut starch at several water content (40, 50, 60 and 95%, flour basis, f.b.) as well as the influence of guar gum (0.5, 1.0, 1.5 and 2.0%, f.b.) on both raw materials at fixed water content (50%, f.b.) were determined by differential scanning calorimetry (DSC). Thermal properties of guar gum–water systems at several guar content (0.5, 1.0, 1.5 and 2.0%, w/w) were also obtained by DSC. Results indicated that the water content and the presence of guar gum had a significant impact on the thermal properties of chestnut flour and its starch. For each endothermic curve, the values of onset (T_o), peaks (T_p1, T_p2) and final (T₁) temperatures decreased linearly with increasing water content. Experimental data were successfully (R² > 0.997) described following the Flory equation. A reverse trend was observed in the enthalpy values. Thermal properties of chestnut flour and chestnut starch were suppressed by the presence of guar gum even at the lowest concentrations employed (0.5%, f.b.). The guar gum addition to the assayed systems promoted a starch gelatinization delay and the enthalpy values showed a threshold content above 1.0% of guar gum. Analyses of aqueous guar gum mixtures showed that the existence of a second transition in chestnut starch systems can be successfully explained by means of hydrocolloid–starch interactions, whereas in the flour other interactions should be taken into account.     

热分析技术在食品热物性研究中的应用

热分析技术是研究物质在加热或冷却过程中发生的某些物理变化和化学变化的技术。影响食品热物性的因素很多,目前唯有通过实测才能真实反映食品热物性的真实情况。本文综述了近年来国内外热分析技术在食品物性研究领域的应用,例举了国内外的一些用热分析技术进行食品物性分析的方法和特点。      

Calorimetric study of the glass transition occurring in aqueous glucose: Fructose solutions

Glass transition temperatures (T_g) for dilute and concentrated glucose: fructose: water solutions have been determined by differential scanning calorimetry. The supplemented phase diagrams (glucose: fructose 1:4, 1:2, 1:1, 2:1, 4:1 (w/w)) are presented and from these the temperature (T_g') and concentration (C'_g) of the maximally freeze-concentrated glass have been determined. Theoretical predictions of T_g for binary (glucose: fructose) and tertiary (glucose: fructose: water) systems according to the Couchman–Karasz and Gordon-Taylor equations are given and discussed. Annealing vitrified glucose: fructose glasses above their T_g allows the glass concentration to approach C'_g as a result of ice formation. Viscosity measurements of fructose and fructose: glucose mixtures allow for a calculation of T_g of the non-aqueous solutions.     

Influence of sodium chloride and sodium tripolyphosphate on the quality of UK-style grillsteaks: relationship to freezing point depression

Grillsteaks were prepared from lean beef flaked at –4°C and subsequently mixed at nominal temperatures of either –3°C or +1°C with each combination of five sodium chloride levels (0, 0.5, 1.0, 2.0 and 4.0%) and three levels of sodium tripolyphosphate (Na5P3O10) (0, 0.25 and 0.50%).
Sodium chloride was more effective than sodium tripolyphosphate in lowering the initial freezing point (ifp) of meat, as measured by differential scanning calorimetry which also showed that the freezing point depression was proportional to the solute concentration.
Cooking losses generally were lower with higher levels of sodium chloride and sodium tripolyphosphate, and when the meat was mixed at +1°C. It is argued that localized salt concentrations can influence product quality, as can the relative proportions of ice and water when meat is processed below the ifp.     

Moisture‐Absorption and Water Dynamics in the Powder of Egg Albumen Peptide,Met‐Pro‐Asp‐Ala‐His‐Leu

Moisture absorbed into the powder of Met‐Pro‐Asp‐Ala‐His‐Leu (MPDAHL)—a novel egg albumen antioxidant peptide—profoundly affects its properties. In this study, we elucidated water dynamics in MPDAHL using DVS, DSC, and low‐field ¹H NMR. Based on the DVS data, we found that MPDAHL sorption kinetics obey a parallel exponential model. DSC results indicated that both water and heating could change the microstructure of MPDAHL. The T₂ parameters of NMR reflected the different phases of moisture absorption revealed that there were 4 categories of water with different states or mobility in the MPDAHL during the moisture absorption process. The fastest fraction T_2b mainly dominated the hygroscopicity of MPDAHL and the absorbed water significantly changed the proton distribution and structure of MPDAHL. Thus, this study shows that DVS, DSC, and low‐field ¹H NMR are effective methods for monitoring water mobility and distribution in synthetic peptides. It can be used to improve the quality assurance of functional peptides.     

Thermal stability and transition temperatures of monovalent salts of high‐acyl gellan gels

Thermogravimetry/derivative thermogravimetry (TG/DTG), rheometry and differential scanning calorimetry (DSC) were used to study the thermal stability and determine the transition temperatures of the sodium and potassium salts of high‐acyl gellan (HAG) in the presence of 0–100 mm NaCl and KCl, respectively. TG/DTG revealed the potassium gellan (KHAG) gels to be more stable than those of sodium gellan (NaHAG), regardless of external cation concentration. Rheometry and DSC showed the melting (T_m) and gelling (T_g) temperatures to increase with cation concentration. The DSC peak temperatures showed thermal hysteresis contrary to rheometry. In most cases, DSC revealed KHAG to exhibit higher T_m and T_g than NaHAG. Consequently, thermal characteristics of NaHAG and KHAG gels depend on the size of the external cation and its ability to coordinate water molecules. Cation salts of HAG exhibit significantly lower transition temperatures than the commercial preparation from which they were produced.     

New insights into the microencapsulation properties of sodium caseinate and hydrolyzed casein

The aim of the present study was to describe the fundamental physical characteristics of spray-dried carrier matrices based on sodium caseinate and casein hydrolyzate and microcapsules as well as their impact on the stability of a microencapsulated functional ingredient. Spray-dried carrier matrix was characterized by different physical methods (helium pycnometry, nitrogen displacement for surface area analysis, X-ray photoelectron spectroscopy, positron annihilation lifetime spectroscopy). Surface viscoelasticity of protein-stabilized oil–water interfaces was analyzed using dynamic pendant drop tensiometry. Fish oil was microencapsulated and microencapsulation efficiency as well as oxidative stability over time was monitored.     

基于LFNMR及成像技术分析牛肉贮藏水分含量变化

为研究牛肉贮藏水分含量变化,基于低场核磁共振(LF-NMR)技术测定牛肉横向弛豫时间T₂、峰面积,运用差示扫描量热法(DSC)测定玻璃化转变温度(T_g)。结果表明,肉样中存在4种不同水分群,-10℃贮藏牛肉T₂明显降低,自由水含量显著下降,在-14、-18、-22℃贮藏牛肉T₂未发生明变化。冷却过程中不易流动水(pT₂₂)前期呈下降趋势,后期较稳定,说明随贮藏时间的延长,不易流动水逐渐转移为自由水。自由水(pT₂₃)先减小后增大后又极显著降低(p<0.01)。通过对牛肉品质指标与核磁共振参数T₂的相关性分析,发现蒸煮损失与T₂₀、T₂₂、pT₂₃显著相关(p<0.05),相关系数分别为0.777、0.745、0.783,因此可根据T₂₂、pT₂₃表征牛肉在贮藏过程中保水性的变化。通过核磁成像实验发现,在-10℃贮藏牛肉,干耗现象严重、肉的保水性差,不利于牛肉的长期贮藏。     

Effect of water content on the glass transition,caking and stickiness of protein hydrolysates

Abstract

Protein hydrolysates play an important function in many special dietary foods, e.g. for infants, those with genetic disorders, athletes and geriatrics. Because of hydrolysis these materials are more hygroscopic than the initial intact protein. In this reset the moisture sorption isotherms and the glass transition profile as a function of moisture content for fish protein hydrolysates (FPH), whey protein hydrolysates (WPH) and casein hydrolysates (CH) were determined. The properties of each hydrolysates were related to molecular weight with a lower glass transition curve at lower molecular weight, the Tg was proportional to degree of hydrolysis. These hydrolysates may be as important as sugars in lowering Tg. Fish moisture sorption was related to hydrolysis. CH was less susceptible to caking than FPH and WPH.     

亲水多糖对鲜湿面货架期内水分迁移及老化进程的影响

利用低场核磁共振、差示量热扫描、Avrami数学模型研究瓜尔胶、可溶性大豆多糖、卡拉胶对鲜湿面贮藏期间水分迁移、热力学参数、老化动力学的影响。结果表明：贮藏7 d的鲜湿面结合水含量：瓜尔胶＞可溶性大豆多糖＞卡拉胶＞空白组（P＜0.05）;不易流动水含量：瓜尔胶＞卡拉胶＞可溶性大豆多糖＞空白组（P＜0.05）;自由水含量：空白组＞可溶性大豆多糖＞卡拉胶＞瓜尔胶（P＜0.05）。多糖主要作用于淀粉及面筋蛋白表面极性基团所吸引的结合水;同时多糖对3 种水分流动性的束缚并非呈单一的线性关系,且能抑制鲜湿面淀粉老化过程中重结晶融化起始温度（T0）、重结晶融化终止温度（Tc）、老化焓（?H）的上升速率;老化动力学方程：Y空白组=0.732x－0.946,Y卡拉胶=0.744x－1.192,Y瓜尔胶=0.791x－1.328,Y可溶性大豆多糖=0.752x－1.114。     

不同解冻条件对滩羊肉水分的影响

利用低场核磁共振设备,研究了冷冻滩羊肉分别经过25、10、5℃静止空气解冻后,水分分布和保水性的差异。结果表明,与冷鲜滩羊肉对比,经冻融处理的滩羊肉中,不易流动水的弛豫时间显著降低,其相对含量明显提高,自由水的弛豫时间明显提高,其相对含量显著降低。对比不同温度解冻的滩羊肉发现,随解冻温度的降低,滩羊肉中自由水的弛豫时间明显增大。通过相关性分析结合系水力和加压失水率的变化得出,滩羊肉的保水性在一定范围内随解冻温度的降低而提高。实验表明核磁成像图可以快速表征滩羊肉的水分含量。      

低场核磁共振技术测定石花菜对酸奶水分迁移及超微结构的影响

利用低场核磁共振及其成像技术对比了石花菜酸奶和酸奶水分迁移特性;并采用环境扫描电镜对石花菜酸奶超微形貌特征进行观察分析。结果表明,石花菜酸奶在发酵及储藏过程中的水分含量变化幅度小于不添加石花菜的酸奶,持水性增强;确定了前期发酵最优时间为6 h,冷藏后熟最优时间为24 h;通过环境扫描电镜观察发现石花菜酸奶中酪蛋白胶束与石花菜琼胶分子间相互缠结,蛋白链球颗粒粘结在致密的琼胶网络结构之中。