Texture of nonfat yoghurt as influenced by whey protein concentrate and Gum Tragacanth as fat replacers

Seven batches of nonfat yoghurt stabilized with different concentrations of whey protein concentrate (WPC) and Gum Tragacanth (GT) were produced to study the effects of WPC and GT as fat replacers on the rheological properties of yoghurt. By increasing the WPC up to 15 g/L, storage modulus (G'), loss modulus (G") and complex dynamic viscosity (η*) values were increased and sensory impressions of texture and appearance were improved when compared with the control nonfat yoghurt. Addition of gum above 0.5 g/L led the decrease of G', G", η* and resulted in lower scores for sensory attributes than control nonfat yoghurt.     

搅拌型酸奶应力松驰行为的研究

目的以搅拌型酸奶为研究对象,建立一种应力松驰行为评价的测试方法。方法对酸奶施加应变,研究其松驰模量随时间的变化规律,并对方法的实验参数进行优化,并对市面上流行的7类搅拌型酸奶进行应力松驰行为的测试。结果通过应变扫描确定了酸奶的线性粘弹性区域,结合不同应变大小时的应力松驰实验结果,最终选择1%作为酸奶应力松驰实验特性研究时的应变值;进一步研究了不同松驰时间对松驰模量的影响,最终选择60 min作为松驰时间。酸奶在松驰初始阶段及松驰结束时的松驰模量差异明显,可以评判酸奶的弹性大小与松驰时间内松驰模量的损耗情况。酸奶的应力松驰行为可以通过6个Maxwell模型进行表征,不同松驰时间下起主要作用的应力松驰特征模量与特征时间的Maxwell模型数存在明显差异。结论通过应力松驰行为的测试可以评价搅拌型酸奶的粘弹性特性。     

乳清浓缩蛋白对搅拌型酸奶品质特性影响的研究

研究了以全脂奶粉和乳清浓缩蛋白（WPC-3503）为原料，按照全脂奶粉：乳清浓缩蛋白为100：0，90：10，80：20，70：30的质量配比生产乳固形物含量为12％的酸奶，对含有不同乳清蛋白比例的酸奶在贮藏过程中理化特性、乳酸菌总数的变化以及感官特性进行了比较分析。结果表明，WPC-3503代替10％-20％全脂奶粉生产酸奶时，在贮藏过程中可减缓pH值及酸度的变化速度，提高酸奶的黏度和保水率，改善感官特性，但对乳酸菌总数无明显的影响。     

Effects of various whey proteins on the physicochemical and textural properties of set type nonfat yoghurt

In this study, the changes during storage in the physicochemical, textural and sensory properties of nonfat yoghurts fortified with whey proteins, namely whey protein concentrates (WPC), whey protein isolates and whey protein hydrolysates, were investigated. Enrichment of nonfat yoghurt with the whey protein additives (1% w/v) had a noticeable effect on pH, titratable acidity, syneresis, water‐holding capacity, protein contents and colour values on the 14th day of storage (P < 0.01). The addition of whey proteins to the yoghurt milk led to increases in the hardness, cohesiveness and elasticity values, resulting in improved textural properties. The addition of WPC improved the texture of set‐type nonfat yoghurt with greater sizes in the gel network as well as lower syneresis and higher water holding capacity. This study suggests that the addition of whey protein additives used for fortification of yoghurt gave the best textural and sensory properties that were maintained constant during the shelf life.     

Approaches to minimise yoghurt syneresis in simulated tzatziki sauce preparation

The phenomenon of syneresis becomes more profound in increased moisture yoghurt‐based products. Such a product is the traditional Greek appetizer tzatziki that contains cucumber, a vegetable with a high moisture content. During the manufacture of tzatziki, the addition of cucumber causes the protein network to break up, provoking an increase in syneresis. The aim of this study was to investigate a tzatziki manufacturing procedure that will lead to significantly decreased syneresis. Two different manufacturing procedures were compared: the extra moisture coming from cucumber was introduced before or after fermentation. The effect of adding whey protein concentrate (WPC), albumin, sodium caseinate or a mixture of these was also studied. The results show that the addition of extra moisture before yoghurt fermentation leads to a significantly lower syneresis (7.5%) and higher consistency (2000 cp) than those obtained in the case of addition after fermentation (25% and 1500 cp, respectively). The use of albumin, WPC or a mixture of albumin, WPC and sodium caseinate further decreased the phenomenon of syneresis to below 5%, without altering the colour of the product (ΔΕ* < 2.3).     

乳清浓缩蛋白在酸奶生产中的应用

以鲜奶，奶粉，乳清蛋白等乳成分为主要原料，研究了乳清浓缩蛋白在酸奶生产中的制做方法，对乳清浓缩蛋白代替部分高档脱脂奶粉生产酸奶产品的保水率，粘度，口感及组织状态进行了比较分析。结果表明，在酸奶生产中，添加一定的浓缩乳清蛋白代替高档脱脂奶粉是可行的，产品较为理想。     

Enhancement of emulsifying properties of whey proteins by controlling spray-drying parameters

Whey protein concentrate is the main source of globular proteins in food products which are principally used as emulsifying, foaming and gelling ingredients. These whey proteins are commonly used in powder form obtained by a spray-drying process. It is well known that β-lactoglobulin, the major protein component in whey, is greatly affected by heat treatments, with consequences on its adsorption properties at fluid–fluid interfaces. This study concerned four whey protein powders obtained using spray-drying at four different air inlet temperatures (from 170 to 260 °C), leading to different levels of protein solubility, denaturation and end-use properties. After evaluation of the protein denaturation by HPLC, the emulsifying properties were studied through particle size parameters and rheological properties in relation with spray-drying parameters. Our results indicated that oil-in-water emulsions, stabilized by 5% (w/w) protein samples, exhibited a shear-thinning flow behaviour, and the harsher the spray-drying conditions (the higher the protein denaturation), the less viscous were the emulsions. The apparent viscosity of emulsions measured at 20 °C and 50 s⁻¹ shear rate was around 0.08 Pa s when containing whey proteins before drying, and around 0.05–0.018 Pa s after drying at air inlet temperatures from 170 to 260 °C. These differences in emulsion rheological properties were related to particle size effects, in regards to analysis of particle size distributions which showed a finer emulsion according to spray-drying intensity. Our results will be presented and discussed in terms of optimization of spray-drying process relative to globular protein surface activity.     

Graininess and roughness of stirred yoghurt as influenced by processing

The aim of this work was to study how heat treatment, the type of starter culture, incubation temperature, and storage time can affect the physicochemical characteristics of stirred yoghurt. A four-factor experimental design was used for data analysis. Yoghurt milk was heated at 95 °C for 5 min or 130 °C for 80 s. Yoghurts were produced with three different starter cultures that had been incubated at 37, 42 or 45 °C and stored at 4 °C for 15 days. Visual roughness, number of grains, perimeter of grains, storage modulus, and yield stress all decreased when heating temperature was increased, when an exopolysaccharide-producing starter culture was used, or when incubation temperature was decreased. Storage time did not affect any of the physicochemical properties of yoghurt, except for the pH.     

Antioxidant properties of Maillard reaction products obtained by gamma-irradiation of whey proteins

The radiation processing of sugar–amino acid solutions results in formation of Maillard reaction products (MRPs). In the present study, the efficacy of gamma-irradiation in the formation of MRPs from whey protein powder was investigated. The formation of MRPs in whey protein suspension was studied by monitoring spectrophotometeric and chemical changes. A dose-dependent increase in UV absorbance and development of fluorescence was observed. Formation of brown pigments was established by increased A_420 nm and Hunter colour upon irradiation. These MRPs exhibited antioxidant activity as measured by 1,1-diphenyl-2-picrylhydrazyl and β-carotene bleaching assays. Reducing power and iron-chelating abilities of MRPs also increased upon irradiation. These MRPs were able to scavenge hydroxyl and superoxide anion radicals under in vitro conditions. Dose-dependent decrease in free amino groups and lactose suggested the formation of glycated proteins upon irradiation treatment. SDS–PAGE analyses indicated formation of crosslinked proteins upon irradiation.     

The effect of supplementing goats milk with whey protein concentrate on textural properties of set-type yoghurt

Yoghurt was manufactured from goat's milk and supplemented with 30 g L^?1 of whey protein concentrate (WPC). The textural properties of the yoghurt were evaluated during the shelf‐life of the product and the textural characteristics of yoghurt made from cow's milk were used as a reference. The instrumental analyses used were the puncture test, stress relaxation test and texture profile analysis. The addition of WPC to goat's milk enhanced the textural characteristics of yoghurt. These advantageous attributes included increased firmness, hardness and adhesiveness. These attributes were quantitatively similar (P > 0.05) to those obtained from yoghurt made from cow's milk. In addition, the textural properties were maintained constant throughout the shelf‐life of the product.     

Cold,gel-like whey protein emulsions by microfluidisation emulsification: Rheological properties and microstructures

Novel cold, gel-like whey protein concentrate (WPC) emulsions at various oil fractions (φ; 0.2–0.6) were formed through thermal pretreatment (at 70 °C for 30 min) and subsequent microfluidisation. The rheogical properties and microstructures, as well as emulsification mechanism of these emulsions were characterised. The rheological analyses indicated that the gel-like emulsions exhibited shear-thinning and predominantly elastic gel behaviours, and the apparent viscosities and the mechanical moduli of the emulsions remarkably and progressively increased with increasing the φ from 0.2 to 0.6. Confocal laser scanning microscopy analyses confirmed close relationships between rheological properties and gel network structures at various φ values. The formation of the gel-like network structure was closely related to the high emulsifying efficiency by microfluidisation. This kind of novel gel-like emulsion might exhibit great potential and be applicable in food formulations, e.g. as a kind of carrier for heat-labile and active ingredients.     

The effect of chitosan on the properties of emulsions stabilized by whey proteins

The influence of the cationic amino polysaccharide chitosan content (0–0.5%) on particle size distribution, creaming stability, apparent viscosity, and microstructure of oil-in-water emulsions (40% of rapeseed oil) containing whey protein isolate (WPI) (4%) at pH 3 was investigated. The emulsifying properties, apparent viscosity and phase separation behaviour of aqueous WPI/chitosan mixture at pH 3 were also studied. The interface tension data showed that WPI/chitosan mixture had a slightly higher emulsifying activity than had whey protein alone. An increase in chitosan content resulted in a decreased average particle size, higher viscosity and increased creaming stability of emulsions. The microstructure analysis indicated that increasing concentration of chitosan resulted in the formation of a flocculated droplet network. This behaviour of acidic model emulsions containing WPI and chitosan was explained by a flocculation phenomenon.     

Oxidation promotes cross-linking but impairs film-forming properties of whey proteins

The objective of the study was to investigate the impact of oxidation on the film-forming properties of whey protein isolate (WPI). Sequential heating (70–90 °C) then oxidation (0.1 mM FeCl₃/1 mM ascorbate/0–20 mM H₂O₂) (H → O) or vice versa (O → H) were conducted to oxidize/unfold WPI at pH 6.8 and 8.0 before casting. The resulting films were characterized through mechanical, microstructural, and protein electrophoretic analyses. Oxidation promoted protein cross-linking mainly through disulfide bonds. Tensile strength (TS) and elongation at break (EAB) of films decreased for WPI oxidized by higher concentrations of H₂O₂. Film solubility (protein leachability) at pH 3–7, ranging from 20 to 40%, was unaffected by H₂O₂ up to 5 mM but reached almost 100% at above 5 mM H₂O₂ except at pH 4–5. β-Lactoglobulin dimers and its complex with α-lactalbumin were abundant in O → H WPI films and polymers of WPI dominated in H → O films. Microstructural images confirmed that oxidation promoted crumbly structures thereby explaining the reduced film-forming capability.     

Influence of tribomechanical micronization on the physical and functional properties of whey proteins

Because of their high nutritional value and commercially important functional properties, whey proteins are of high interest nowadays. Whey protein concentrates (WPCs) containing 60% and 80% proteins (WPC-60 and WPC-80, respectively) were treated using laboratory equipment for tribomechanical micronization and activation (TMA) with three different rotor speeds: 16 000, 20 000 and 22 000 r.p.m. The results showed that the TMA treatment causes a significant decrease (  P < 0.05) in particle size, a change in particle size distribution and an increase in specific area of WPC. As a consequence of the TMA process, rupturing of protein globules and the appearance of a greater number of protein fragments occurred. The solubility, dispersibility, foaming and emulsifying properties of tribomechanically treated whey protein concentrates (TWPCs) were also modified. The results of particle analysis confirmed that a decrease in particle size correlates with an improvement of the foaming properties. These changes in the treated materials depend on the type of the WPCs used and the rotor speed of the TMA equipment. The most obvious changes were observed in samples treated at the maximum rotor speed of the TMA equipment.     

Effect of whey protein concentrate addition on the physical properties of homogenized sweetened dairy creams

The influence on their whipping properties of homogenization at first and second stage pressures of 3.5/1.5 MPa and addition of whey protein concentrate (WPC) powder at three different (0.7, 1.4, and 2.1 wt percentage) concentrations to sweetened and homogenized creams was studied. Homogenization of cream significantly decreased maximum overrun and made the foam microstructure less open, while increasing whipping time, cream and foam lightness (Hunter L -value) and apparent viscosity. It also resulted in a less elastic foam structure with an increased drainage. Addition of WPC decreased the amount of maximum overrun, foam drainage and its lightness in parallel with developing a more compact microstructure. It increased the whipping time, apparent viscosity of unwhipped creams and foams, and resulted in a less elastic foam structure. The apparent viscosity of whipped cream with 2.1 wt percentage WPC, however, was lower than that of whipped cream with 1.4 wt percentage WPC, due most probably to the start up of gel formation at 2.1% WPC concentration in sweetened cream when it was sheared. Fresh foam whipped from sweetened cream with 2.1 wt percentage WPC also tended to have a slightly but not statistically significant lower elastic modulus (G') than fresh foam whipped from sweetened cream with 1.4 wt percentage WPC. This concentration can be considered as the critical value for gel formation in sweetened creams enriched by whey proteins when sheared. This study indicated the potential of WPC powder for reducing foam drainage from whipped homogenized sweetened cream.     

搅拌型酸奶稳定剂的改良研究

在酸奶制品中使用稳定剂的目的是提高酸奶的粘稠度并改善其质地、状态与口感。本研究是以搅拌型酸奶作为目标产品，首先选择若干单体稳定剂按不同比例进行复配，在复与稳定剂添加量、培养时间和培养条件相同的情况下，取终产品的感官和理化指标筛选出最佳配比；然后以市售稳定剂作为对比样，在培养时间和培养条件相同的情况下，根据黏度和感官指标筛选最添加量。结果表明，在保证产品质量不变的情况下，用单体稳定剂可以替代市售稳定剂。这样能为企业节省大量的生产投入，并带来较大的经济效益。     

谷氨酰胺转胺酶对搅拌型酸奶品质的影响

因为谷氨酰胺转胺酶对乳蛋白有交联作用,所以在酸奶制作中加入谷氨酰胺转胺酶可以提高酸奶的品质,将0.2g/L牛乳的谷氨酰胺转胺酶使用不同的添加方法(对酶灭活和不灭活)应用到酸奶制作中,通过酸奶硬度、持水力、流变特性、乳酸菌等指标的变化及综合感官评价,评定谷氨酰胺转胺酶的最适添加方法。结果表明两种不同的酶添加方法都可以减弱酸奶后酸化,提高酸奶硬度、持水力和剪切应力。TGase酶与发酵剂同时加入显著提高了酸奶的理化特性(p<0.05),硬度在第21d达到最大,与对照组相比提高8.48g,持水力随贮存时间的延长,不断增大,增幅为5.41%。在贮存后期TGase酶活力减弱,对酸奶的质构无不良影响,而且这种方法不需要额外的酶反应时间,热处理工序及设备,符合实际生产的需求。      

Modulation of rheological properties by heat-induced aggregation of whey protein solution

Heat-induced protein aggregation at low protein concentrations generally leads to higher viscosities. We here report that aggregated protein can yield weaker gels than those from native protein at the same concentration. Aggregated protein was produced by heating a solution of whey protein isolate (WPI) at 3% and 9% w/w. The higher protein concentration resulted in a larger aggregate size and a higher intrinsic viscosity. The protein fraction in native WPI had the smallest size and the lowest intrinsic viscosity. The same trend was observed for the shear viscosity after concentrating the suspensions containing aggregates to around 15% w/w. Suspensions containing aggregates that were produced from a higher concentration possessed a higher viscosity. After reheating the concentrated suspensions, the suspension from the 9% w/w aggregate system produced the weakest gel, followed by the one from 3% w/w, while the native WPI yielded the strongest gel. Reactivity of the aggregates was also an important factor that influenced the resulting gel properties. We conclude that aggregation of whey protein solution is a feasible route to manipulate the gel strength of concentrated protein systems, without having to alter the concentration of the protein.     

Preconcentration of yoghurt base by ultrafiltration for reduction in acid whey generation during Greek yoghurt manufacturing

The feasibility of applying ultrafiltration (UF) to concentrate a yoghurt base prior to fermentation during Greek yoghurt manufacturing was studied as a way of minimising acid whey (AW) generation. Two Greek yoghurts were prepared by concentrating a milk base through UF to 13.8% and 17.5% (w/w) (GY‐2, GY‐3). Production of GY‐3 resulted in zero AW discharge, while the GY‐2 production discharged 78% lower AW compared to the control (GY‐1). GY‐2 exhibited a hard gel structure, low syneresis, maximum viscosity properties and a high protein and fat content. Applying both, UF and straining, resulted yoghurts with required structural attributes while substantially reducing the generation of AW.