Effect of pH adjustment at heating on the rheological properties of acid skim milk gels with added potato starch

The rheological properties of acid skim milk gels, prepared from milk with added potato starch and pH adjusted (pH 6.5–7.1) prior to heat treatment and acidification, were investigated. The storage modulus, G′, of the final acid gels was increased by heating the milk at higher pH and further increased by adding starch. The effect of pH at heating and addition of starch appeared to be additive and independent of each other up to a starch addition level of 1%. Above this starch level, the pH at heating had a lesser effect. This may have been due to the increased viscosity of the aqueous phase as a result of starch gelatinization or to direct contributions of the starch to the gel network structure. Confocal microscopy showed that milk proteins developed fewer but broader protein clusters at higher pH than at lower pH. Starch addition resulted in an increased density of the protein network.     

Effects of different components in skim milk on high-pressure-induced gelatinisation of waxy rice starch and normal rice starch

The gelatinisation of starch in skim milk required higher pressures than did the gelatinisation of starch in water. This study examined the effects of various milk components on the pressure-induced gelatinisation of waxy rice starch and normal rice starch, in order to understand the differences between the gelatinisation characteristics of starch in skim milk and starch in water. Gelatinisation was retarded in skim milk, which was attributed to the effects of soluble milk minerals and lactose. The presence of these components may reduce the plasticising ability of the suspension medium. Direct interactions between the milk components and starch molecules may also contribute to retarded gelatinisation. Milk proteins (casein and whey protein) did not affect the degree of pressure-induced gelatinisation at the concentrations of these components in skim milk, at 10% total solids.     

脱脂乳的不同热处理对其酸奶凝胶流变学性质的影响

研究了复原脱脂乳经不同加热处理后其酸奶凝胶形成过程中流变学特性的变化,复原脱脂乳经７０～９０℃处理１０～２５ｍｉｎ,用Ｄ－葡萄糖－δ－内酯酸化成胶,用流变学方法动态监测凝胶性能。并监测酸化过程和成胶时间,结果显示,不同条件下自理后的脱脂乳在酸化过程中,ｐＨ值并未改变,但产储存模量（Ｇ′）和损失模量（Ｇ″）随受质变程度的增加而增加,只是当加热温度为９０℃时反而随加热时间的延长而降低,在８５℃加热２５     

Effects of different milk substitutes on pasting, rheological and textural properties of puddings

Commercial pudding powders are usually composed of starch, hydrocolloids, sugars, colorings and aromas, and are intended to be dissolved in milk. However, the use of milk substitutes is mandatory for people suffering from lactose intolerance and milk allergies, but these ingredients have an effect on pudding structure and rheological behavior that has been little studied. This work investigates the pasting, rheological and textural behavior of two commercial pudding powders dissolved in rice drink, light soy drink, partially skimmed milk (reference sample), and water (model system). The eight different puddings were evaluated both during preparation (heating and cooling phases) and a 3 day refrigerated storage period, by means of viscoamylographic tests, rotational and oscillatory measurements, and penetration tests. The two pudding powders, apparently similar in formulation, resulted in final products with large structural differences, that could be ascribed to different types and quantity of starch and carrageenans present in the formulation. Soy and rice drinks can not be indifferently used for pudding production, as the optimal texture of the final product is not always achieved. Generally, the properties of soy drink-based puddings were similar to those of the reference samples, while the characteristics of rice drink-based products were comparable to those of the water systems.     

Effect of ultrasonication on the properties of skim milk used in the formation of acid gels

Skim milk was ultrasonicated for times up to 30 min either with or without temperature control. Ultrasonication (US) without temperature control resulted in the generation of considerable heat, with the milk reaching  95 °C within 15 min of treatment. The whey proteins were denatured. Changes to the casein micelle size were observed, with decreases during the early stages of US and increases (because of aggregation) on prolonged treatment. Significant κ-casein dissociated from the micelles. Acid gels prepared from these ultrasonicated samples increased in firmness (final G′) up to a maximum final G′ after  15 min of US, followed by a decrease from this maximum on prolonged treatment. US with temperature control demonstrated that the denaturation of the whey proteins was entirely due to the heat generated during US, although the casein micelle size was still reduced. Acid gels prepared from ultrasonicated skim milk in which the temperature remained below the denaturation temperature of the whey proteins had low final G′, although a small increase was observed with increasing US time. Acid gels prepared from the samples that were ultrasonicated at temperatures above the denaturation temperature of the whey proteins had higher final G′, which could reach values similar to those obtained by the conventional heating of milk. The results of this study indicate that, in skim milk, most of the effect of US can be related to the heat generated from the treatment, with US itself having only a small effect on the milk when the temperatures are controlled.

Industrial relevance

The control and the manipulation of the firmness of acid skim milk gels are important in many dairy food applications such as yoghurts and some types of cheese. US is an emerging technology that could be used to process skim milk for use in acid gelled products. This study has demonstrated that acid gel firmness can be substantially manipulated when skim milk is ultrasonically treated before acidification; however, most of the effect is due to the heat generated during US treatment. As the effects of US are similar to those obtained through conventional heating processes, and as US can control spoilage microorganisms, using US under controlled temperature conditions could be an alternative to conventional heating to give desired functional properties and storage stability to milk products. However, the temperature/denaturation/aggregation would need to be carefully controlled to minimize the detrimental effects of excessive heating.     

Effect of aqueous ozonation on the pasting,flow and gelatinization properties of wheat starch

Wheat starch–water suspensions were exposed to ozone gas for 15, 30, and 60 min at 5 °C at a concentration of 0.00042 g dissolved ozone/100 g. Pasting properties were determined by using a Rapid Visco Analyzer (RVA) while a Bohlin rheometer was used to obtain flow behaviour (n) and consistency (K) indices. An enzymatic method using glucoamylase was applied to calculate percent starch gelatinization. Pasting temperature (P_T) of ozonated starch samples decreased significantly (P < 0.05) as compared to that of native starch. Cooking stability starch samples increased upon ozonation indicated by lower breakdown viscosity (B_V) values. The retrogradation tendency of wheat starch samples was reduced after ozonation which was indicated by lower setback viscosities (S_V). Native and ozonated starch samples exhibited a shear-thinning behaviour with values of flow behaviour indices (n) between 0.57 and 0.76. The greater n values obtained for ozonated starch samples as compared to native starch showed that ozonation increased the shear-thinning behaviour of wheat starch. Percent gelatinization for all ozonated starch samples were significantly higher than the control starch samples (P < 0.05), probably due to, weakened hydrogen bonding between the starch molecules and partial depolymerization of starch molecules by ozonation.     

Influence of milk proteins on the pasting behaviour and microstructural characteristics of waxy maize starch

Gelatinisation of 5 wt% waxy maize starch (WMS) under shear (16.8 s⁻¹), alone and in mixtures with 5 wt% α-lactalbumin (α-lac), β-lactoglobulin (β-lg), α-caseinate or β-caseinate, showed reinforcement of the starch granule structure by both caseinates, but not by the whey proteins (α-lac and β-lg). Reinforcement was evident from (i) later onset of increase in viscosity on heating; (ii) higher gelatinisation temperature by differential scanning calorimetry; (iii) micrographs showing reduced swelling during heating and in the final pastes obtained on cooling; and (iv) elimination of a characteristic “secondary swelling peak” observed for WMS immediately after completion of heating to 95 °C and attributed to fracture of a restricting layer of lipid and protein at the surface of the granules. A likely mechanism of reinforcement is binding of caseinate to the lipid–protein layer. Images from confocal laser scanning microscopy with fluorescent labelling of protein showed attachment of aggregated β-caseinate to the surface of WMS granules in mixtures that had been heated (under shear) to 70 °C. Corresponding images for mixtures with α-caseinate (which is less aggregated) showed penetration of protein to the interior of the granules, which would allow binding to occur on the inside of the surface layer as well as the outside. The inability of the more hydrophilic whey proteins to reinforce the WMS granules suggests that binding of caseinates to the lipid–protein layer occurs predominantly by hydrophobic association. The understanding that caseinates make gelatinised WMS granules smaller and tougher could be useful in product formulation.     

Effects of Alcalase/Protease N treatments on rice starch isolation and their effects on its properties

The effects of different protease treatments on rice starches and their properties were studied. The rice starches produced from protease N exhibited higher pasting viscosities than those produced from alcalase. The hot pastes of the starches produced from protease N also showed higher elastic moduli, zero-order Newtonian viscosities and yield stresses than those produced from alcalase. No differences were found in the crystalline pattern, thermal properties, granules appearance, and average molecular weight (M_w) of the rice starches between the two protease treatments. But the M_w of the pasted starch produced from protease N was higher than that produced from alcalase. When additional protease was added to the isolated starches and the mixture pasted, the M_w of the starches pasted with added alcalase was significant lower than that of the starches pasted with added protease N. The reduction in molecular weight suggested that alcalase had modified the starch molecules during pasting.     

Effect of damaged starch on the rheological properties of wheat starch suspensions

The influence of damaged starch content on particle size distribution and rheological properties of unheated starch suspensions and pasting properties were investigated. Four samples containing different amounts of damaged starch were studied. Particle size distribution curves shifted toward higher diameters, and a greater overlap of both populations of particles (A- and B-type granules) and a decrease of both peak heights were produced. The flow curves of unheated starch suspensions were fitted using the power law model. The flow behavior indexes were higher than the unit. The consistency coefficient increased with the increment of damaged starch content. Unheated starch suspensions showed time-dependent rheological behavior and were described by the Weltman model. The unheated suspensions exhibited a thixotropic behavior. With regard to the pasting process, the increment of damaged starch content produced gradual reductions in peak viscosity, final paste viscosity, breakdown and setback. Results demonstrated the importance of the presence of physically damaged granules in wheat starch rheological characteristics.     

Effect of milk solids concentration on the gels formed by the acidification of heated pH-adjusted skim milk

Reconstituted skim milk of 10–25% total solids was adjusted to pH values between about 6.2 and 7.1 and heated at 80 °C for 30 min. Gels were formed from the heated milks by slow acidification to pH 4.2 and the gelation process and final gels were analyzed for their rheological properties. At each milk concentration, the final acid gel firmness (final G′) and breaking stress could be changed markedly by manipulation of the pH during heating. The final gel firmness and breaking stress could also be modified by changing the concentration of the milk solids prior to heating and acidification. The results indicated that similar gel firmness and breaking stress could be achieved over a range of milk concentrations by control of the pH of the milk during heating. When expressed as a percentage change in final G′ or breaking stress relative to that obtained at the natural pH, plots of the change in final G′ or breaking stress versus pH fell close to a single curve, indicating that the same mechanism may influence the gelation properties at all milk concentrations. The final G′ and breaking stress were related to the denaturation and interaction of the whey proteins with the casein micelles, and the formation of non-sedimentable casein when the milk was heated.     

Prediction of sweetpotato starch physiochemical quality and pasting properties using near-infrared reflectance spectroscopy

A rapid predictive method based on near-infrared reflectance (NIR) spectroscopy (NIRS) was developed to measure sweetpotato starch physiochemical quality and pasting properties. The starch samples were scanned by NIRS and analyzed for quality properties by reference methods, respectively. Results of statistical modeling indicated that NIRS was reasonably accurate in predicting amylose content (AC), amylose percent (AP), total starch content (TSC), protein content (PRC), phosphorus content (PHC), solubility (SOL), swelling power (SP), average granule diameter (AGD), big granule percent (BGP), small granule percent (SGP), crystallinity (CRY), peak viscosity (PKV), hot paste viscosity (HPV), setback (SB), and pasting temperature (P_temp) with high coefficients of determination (RSQ = 0.85–0.92) and relatively low standard errors of prediction. The results showed that NIR analysis was sufficiently accurate and effective for rapid evaluation of starch physicochemical properties in sweetpotato. The NIR-based protocol developed in this study can be used for screening large number of starch samples in food enterprises and sweetpotato breeding programs.     

Effect of amylose content and rice type on dynamic viscoelasticity of a composite rice starch gel

Amylose content is an important indicator to determine the utility of raw milled rice. Indica type rice with high amylose content is usually used for manufacturing rice noodles, while Japonica rice may be mixed partially to adjust the noodle texture. The effect of amylose and rice type on dynamic viscoelasticity of rice starch gel was investigated using a model starch composite in this study. The information will be helpful to control and obtain the required noodle texture by combination of different rice types. The results show that nonwaxy Indica and waxy Japonica rice starches in a composite mixture were incompatible and demonstrated their individual gelatinization behavior during heating. High amylose starch showed higher moduli and lower loss tangent values, as well as higher retrogradation rate. The starch gel made from Japonica rice starch showed a slow retrogradation rate even containing a similar amount of amylose to Indica starch. The storage modulus of the gel made from higher amylose rice was shown to be more independent of frequency. Not only amylose content but also chain length distribution in amylopectin affected the dynamic viscoelasticity of rice gel. Japonica rice starch, with fewer super-long chains in amylopectin, retrograded slower after gelatinization than Indica rice, thus the paste is too sticky for production of rice noodles.     

剪切法研究脱脂奶粉的抗结块性能

采用剪切仪测定了纯脱脂奶粉及含有添加剂的脱脂奶粉在不同处理时间下的剪应力,并用此来评价脱脂奶粉的抗结块性能。研究了处理时间和组成对脱脂奶粉抗结块性能的影响。     

Relationship between physical properties of casein micelles and rheology of skim milk concentrate

The properties of casein micelles in milk concentrates are of interest for the use of ultrafiltered (UF) skim milk concentrates in dairy products, and for the general understanding of colloidal stability and behavior of the casein micelle. The rheological behavior of UF skim milk concentrate with a casein concentration of 19.5% (wt/wt) was investigated at different pH and NaCl concentrations by analyzing flow viscometry and small amplitude oscillatory shear measurements. Viscometric flow curves were fitted to the Carreau-Yasuda model with the aim of determining values for the viscosity at infinite high shear rates and thereby estimate the voluminosity of the casein micelles (ν_casein) in the UF concentrate. The voluminosity of the casein micelles increased with addition of NaCl and decreased when pH was decreased from 6.5 to 5.5. At pH 5.2, ν_casein increased because of acid-induced aggregation of the casein micelles. The changes in ν_casein could be interpreted from transmission electron microscopy of freeze-fractured samples of the UF concentrate and partly from dynamic light scattering measurements. Altered interactions between casein micelles due to different pH and NaCl concentrations are proposed to occur due to collapse of the κ-casein layer, changed ionic strength, and altered distance between casein micelles.     

Effect of controlled kappa-casein hydrolysis on rheological properties of acid milk gels

An experimental method based on the controlled chymosin-induced kappa-casein hydrolysis of milk was proposed to modify micellar reactivity. Milk samples with a degree of kappa-casein hydrolysis of 19, 35, and 51% were obtained. The physicochemical properties of partially converted casein micelles were determined. The net negative charge of casein micelles was reduced with increasing degree of kappa-casein hydrolysis and a small but significant decrease in hydrodynamic diameter and micellar hydration were noted. Dynamic low amplitude oscillatory rheology was used to monitor the rheological properties of acid milk gels (GDL) made with partially chymosin-hydrolyzed milks in comparison with those of strictly acid and rennet gels. An increase in the gelation pH value was observed with increasing the degree of kappa-casein hydrolysis. The moduli values (G' and G') reached 2 h after the point of gel were, for all degrees of hydrolysis tested, significantly higher than those of strictly rennet and acid gels. Comparison of changes in delta G'/delta t with time indicated differences in gel formation that could be related to the increased values of G' obtained for acid gel made with chymosin-treated milk. At a given time after gelation (2 h), increasing the degree of kappa-casein hydrolysis in milk led also to an increase in the loss tangent and the serum holding capacity of acid milk gels suggesting a correlation between these two parameters.     

Effect of processing and storage on rheological properties of fortified milk and curd systems

Fortified milk systems were prepared by blending pasteurized standardized milk with skim milk powder, peanut flour or peanut protein isolate to 15, 18, 20, and 23% total solids followed by processing at 60°C and 80°C for 30min and storage at 4°C for 24h. Curds were prepared by the lactic fermentation of the processed milk systems. The rheological properties showed that all the systems exhibited pseudoplastic flow. The flow became less Newtonian with increasing total solids, heat treatment, and storage. Curd obtained from the fortified milk processed at 80°C showed increased yield stress and curd strength with the increasing concentration. Degree of heat treatment, total solids content and storage were shown to have a pronounced effect on the apparent viscosity, consistency index, and yield stress of the fortified milk systems.     

Rheological and structural properties of differently acidified and renneted milk gels

In this study we assessed the rheological and structural properties of differently acidified and renneted milk gels by controlling pH value and renneting extent. Skim milk were exactly renneted to 4 extents (20, 35, 55, and 74%) and then direct acidified to the desired pH (4.8, 5.0, 5.2, 5.5, 5.8, and 6.2), respectively. Rheological properties were assessed by dynamic rheological measurements, structural properties were studied by spontaneous whey separation and confocal laser scanning micrograph, and protein interactions were studied by dissociation test. Results showed that minimally renneted milk samples (20 and 35%) formed weak gels with low storage modulus, and the acidification range within which gels could form was narrow (pH ≤5.2). Highly renneted milk samples formed more gels with high storage modulus. The results of this study revealed that acidification determined the structural properties of highly renneted milk gels. As pH increased from 5.0 to 6.2, highly renneted milk gels had lower loss tangent, decreased spontaneous syneresis, and smaller pores. For both the low and high rennetings, divalent calcium bonds contributed less at low pH than at high pH. In conclusion, renneting increased the pH range suitable for gel formation; acidification determined the spontaneous syneresis and microstructure of highly renneted milk gels.     

Effect of pH on the gelation properties of skim milk gels made from plant coagulants and chymosin

The effect of three milk pH values, 6.0, 6.3 and 6.7, on gelation properties was monitored by small amplitude oscillatory rheology as well as a large deformation (yield) test for gels induced by the plant coagulants, Cynara cardunculus L. and Cynara humilis L., and chymosin. Gel microstructure was studied using confocal scanning laser microscopy. For each coagulant, a decrease in pH of milk resulted in a decrease in gelation time (tg), and an increase in the rate of increase in storage modulus (G'). At pH 6.0 and 6.3, plant coagulant-induced gels reached a maximum value in G' (G'max) followed by a decrease in G' values during the rest of the experiment, reflecting higher proteolytic activity of plant coagulants towards caseins as determined by SDS-PAGE. Gels produced at pH 6.0 and 6.3, exhibited a minimum in loss tangent (tan delta) followed by slight increase in tan delta during gel aging, that may have been associated with faster rearrangements of the gel network structure. In gels aged for approximately 6 h, the values for G', tan delta at low frequency (0.006 Hz) and yield stress were higher for chymosin than for plant-induced gels. For gels with the same pH value, no major differences were observed in microstructure between coagulants. Gels made at low pH values (6.3 and 6.0) appeared to have a denser or more interconnected structure than gels made at pH 6.7. Our results suggest that, at a low pH, the type of coagulant used in gelation is likely to have a considerably impact on gel/cheese structure.     

Influence of calcium salt supplementation on calcium equilibrium in skim milk during pH cycle

Calcium is a mineral essential for humans, especially for bone constitution. Yet most of the worldwide population does not satisfy their Ca needs. Hence, Ca supplementation is of major importance, even in western countries where some specific populations at risk do not satisfy the recommended daily intake of Ca. More than 70% of dietary Ca comes from dairy products. Calcium supplementation of naturally Ca-rich sources such as skim milk is then of special interest. To our knowledge, few data are available concerning milk Ca (MC) supplementation of milk, particularly when followed by pH cycle. In this paper, MC supplementation is studied and compared with Ca chloride (CC) supplementation as a well-known source of Ca. The effect of Ca salt supplementation followed by pH cycle was studied in reconstituted skim milk. Calcium supplementation was carried out with CC and MC at 25 mmol of Ca/kg of skim milk. Ionized Ca concentration and turbidity variations were followed in situ by Ca ion selective electrode and turbidimetry using light reflection. From normalized data on ionized Ca concentration and turbidity vs. pH, it appeared that hysteresis areas were smaller for CC-supplemented milk, whereas unsupplemented milk and MC-supplemented milk behaved similarly. For these 3 dairy systems, pH cycles to pH 5.0 led to a larger hysteresis area than pH cycles to pH 5.5. The shrinkage of the hysteresis area could be interpreted as a reinforcement of casein micelles with Ca ions over the pH cycle.     

Effect of addition of gelatin on microstructure of acidic milk gels and yoghurt and on their rheological properties

A study was made of the effect of the addition of gelatin on the microstructure of acid-heat-induced milk gels (90°C, pH=5.3) and yoghurt with and without the addition of 5% of milk solids, and a comparison was made with the microstructure of acidic milk gelatin gels obtained without heating (pH=5.3). It was seen that in the acid-heat-induced gels and in yoghurt the gelatin interacted with the network of milk proteins as a connection between the clusters formed, whereas it was the gelatin alone that was the basis of the formation of the gel when the milk did not reach the casein coagulation point (pH=5.3, unheated). The results of firmness tests indicated that the addition of 1.5% of gelatin developed fairly firm, deformable systems in all the cases studied, with a definite break point and almost total absence of syneresis. Dynamic rheology showed that the yoghurts with added gelatin exhibited more solid-like behaviour than the ones prepared without it.