Characterisation of dairy emulsions by NMR and rheological techniques

Droplet size distribution strongly affects macroscopic properties of emulsions. In this work rheological and NMR (nuclear magnetic resonance) analyses were performed to evaluate material properties as well as droplet size distribution and possible flow-induced alteration. The investigated materials were commercial dairy emulsions, based on milk, vegetable fats, a mixture of sugars, emulsifiers (hydrophilic or lipophilic) and natural stabilizers (carrageenan and guar gum). Rheological properties of three different dairy emulsions were related to the droplet size distribution obtained by NMR and dynamic light scattering (DLS) techniques. NMR seems to be a particularly useful technique because no previous sample treatment is necessary, whereas DLS requires a significant emulsion dilution that could change the droplet size distribution. Obtained results evidence the effect of two relevant ingredients (surfactants and fats) on rheological properties and droplet size distribution of tested emulsions. For sample containing sunflower oil and a hydrophilic emulsifier, rheo-NMR evidenced mechanical instabilities also confirmed by rheological tests. Comparison of results about the droplet size distribution evaluated by NMR and DLS evidenced that in this case sample dilution could affect size distribution. Finally, rheo-NMR was confirmed to be a powerful instrument capable of determining the material structure and alterations induced by the flow.     

Effects of protein oxidation on the rheological behaviour of different wheat flour

Wheat flour proteins are subject to oxidation reactions during production, processing and storage. The quality of protein and the rheological properties of wheat are crucial for the flour industry. However, the impact and mechanism of protein oxidised on wheat flour quality remain unclear. In this study, ozone was used to oxidise wheat grains, the secondary structure of protein in flour and the rheological properties of dough were analysed by FTIR and Mixolab. The proportion of α-helix and β-folding of protein were decreased significantly, as were the development time (DDT), stability time (DST) and protein weakening (C2) value of dough. Meanwhile, starch gelatinisation (C3), amylase activity (C4) and retrogradation (C5) were increased significantly, along with the elastic modulus (G′) and viscous modulus (G″). Microstructure analysis indicated that protein oxidation destroyed the gluten network structure in the dough. In addition, the L* value of dough was decreased and a* and b* values were increased significantly. The results showed that the oxidation of protein reduced the stability of protein secondary structure, weakened the structure and stability of the gluten network in dough, and changed the viscoelasticity and colour of dough. Overall, these findings provide a better understanding of rheological behaviour in wheat flour.     

Effect of concentrated flaxseed protein on the stability and rheological properties of soybean oil-in-water emulsions

Flaxseed protein concentrate containing-mucilage (FPCCM) was used to stabilize soybean oil-in-water emulsions. The effects of FPCCM concentration (0.5, 1.0, 1.5% w/v) and oil-phase volume fraction (5, 10, 20% v/v) on emulsion stability and rheological properties of the soybean oil-in-water emulsions were investigated. Z-average diameter, zeta-potential, creaming index and rheological properties of emulsions were determined. The result showed that FPCCM concentration significantly affected zeta-potential, creaming rate and emulsion viscosity. The increasing of FPCCM concentration led to a more negative charged droplet and a lower creaming rate. Oil-phase volume fraction significantly affected Z-average diameter, rheological properties, creaming index and creaming rate. With the increase of oil-phase volume fraction, both Z-average diameter and emulsion viscosity increased, while creaming index and creaming rate decreased. The rheological curve suggested that the emulsions were shear-thinning non-Newtonian fluids.     

Influence of green banana pulp on the rheological behaviour and chemical characteristics of emulsions (mayonnaises)

The influence of green banana pulp (GBP) on rheological behaviour and chemical characteristics of emulsions (mayonnaises) was investigated in this work. Five formulations were developed, using an experimental design for constrained surfaces and mixtures, with the following proportions (water/soybean oil/GBP): F1 (0.10/0.20/0.70), F2 (0.20/0.20/0.60), F3 (0.10/0.25/0.65), F4 (0.20/0.25/0.55) and F5 (0.15/0.225/0.625). Rheological properties of the emulsions were obtained with a rotational Haake Rheostress 600 rheometer and a cone and plate geometry sensor (60 mm diameter, 2° cone angle), with a gap distance of 1 mm. The samples presented high values for mineral salts and low calories. The emulsions showed pseudoplastic behaviour at 10 and 25 °C and rheological data were well described by the Herschel-Bulkley model. Formulations with high GBP content (F1 and F3) presented higher yield stress and apparent viscosity values. The results obtained with the response surface methodology, which were adjusted by the quadratic model, showed that GBP had a significant influence on the increase of yield stress and apparent viscosity. The consistency coefficient increased with an increase in soybean oil concentration and water contributed to high values of flow behaviour index in all emulsions samples.     

The interaction of milk sphingomyelin and proteins on stability and microstructure of dairy emulsions

The interaction between dairy proteins [micellar casein (MC) vs. whey protein isolate (WPI)] and phospholipids [PL; soy phosphatidylcholine (PC) vs. milk sphingomyelin (SM)] in an oil-in-water emulsion system was investigated. Sole PC–stabilized emulsion (1%, wt/vol) showed a significantly larger mean particle diameter (6.5 μm) than SM-stabilized emulsions (3.8 μm). The mean particle diameters of emulsions prepared by the combination of protein (1%, wt/vol) and PL (1%, wt/vol) did not significantly differ from the emulsions prepared with a single emulsifier (MC, WPI, and SM). Emulsion instability differed significantly among samples by a centrifugation-mediated accelerated stability test. Emulsion instability increased in the order of MC+SM < MC+PC, WPI+SM < WPI+PC < MC < SM < WPI < PC. Protein surface load determined by aqueous phase depletion was significantly decreased only in WPI+PC emulsion, whereas no significant difference was found between the MC+SM and WPI+SM emulsions. Topographic and phase images of emulsion surface by atomic force microscopy showed surface layers prepared by protein+PL combinations were composites with different mechanical properties, and PL formed a more compact domain than proteins. A smoother phase image was observed in MC+PL combinations than in WPI+PL counterparts. Based on the microstructure analysis using confocal laser scanning microscopy, combination and MC+SM formed a uniform and thick surface coating of fat droplets. More PC aggregates were observed in the emulsions containing PC (sole PC, MC+PC, and WPI+PC) compared with their SM counterparts. Based on these results, the appropriate selection of the PL matrix is important to modulate the emulsion stability of dairy emulsion products.     

两种不同芯材制备的乳化液稳定性研究

以乳清蛋白和变性淀粉为壁材,氢化棕榈油和高油酸葵花籽油为芯材制备O/W乳化液,讨论了不同p H和均质压力对乳化液稳定性的影响。结果表明,乳化液在中性条件下乳液液滴平均粒径最小,体系的Zeta电位的绝对值高达42 m V,静电稳定性好。均质条件为35 MPa均质两次,得到的乳化液粒径可以维持在400 nm,均质两次后得到的乳化液稳定性最高。随着存储时间的延长,氢化棕榈油的乳化液体系油脂氧化程度高于高油酸葵花籽油,包埋在一定程度上可以延缓不饱和脂肪酸含量高的芯材的氧化。      

乳状液成分对O/W乳状液性质的影响

采用单因素实验设计,通过机械搅拌方法制备O/W乳状液。通过乳状液的离心稳定性、粘度和乳状液的显微结构,研究不同HLB值的复合乳化剂及含量、脱脂乳粉溶液的浓度以及油和水比例对乳状液性质的影响,最终确定较佳的乳状液成分。实验结果表明:当以Span-80和Tween-80为复合乳化剂,其HLB值为9.6、复合乳化剂含量为16%(w/w)、脱脂乳粉溶液浓度为25%(w/v)、油与水比为1∶1(w/w)时,可以获得状态较好的乳状液,此时乳状液的离心稳定性最高,可以达到97.5%。     

Effects of bacteriostatic emulsifiers on stability of milk-based emulsions

For milk-based emulsion products such as canned coffee or tea, the addition of bacteriostatic emulsifiers is necessary to inhibiting the growth of heat-resistant sporeformers. Since bacteriostatic emulsifiers often cause the destabilization of emulsions, other type of emulsifiers, such as stability-enhancing ones, are necessary for the long-term stability of emulsions. Four milk-based emulsions were prepared from powdered milk combined with several types of emulsifiers. The long-term stability of emulsions, which was detected by the occurrence of a creaming layer after 3 months of storage, differed according to the composition of emulsifiers. To understand the reason for the differences in the stability of emulsions, particle size, distribution, ζ-potential, and the amount of proteins and phospholipids present in the cream layer (separated oil droplets) in the emulsions were measured. Only the amount of proteins adsorbed onto oil droplets was found to be closely related to the difference in emulsion stability, that is, the more proteins adsorbed, the higher the emulsion stability. SDS–PAGE analyses revealed that κ-casein and β-lactoglobulin play an important role in emulsion stability by adsorbing onto the oil droplet surface.     

两性离子型乳化剂对高能量乳液稳定性的影响

对比分析两性离子型乳化剂——大豆卵磷脂和非离子型乳化剂分子蒸馏单甘酯对高能量乳液体系的作用影响。研究表明,高温杀菌前,大豆卵磷脂和单甘酯都有较好的乳化作用,离心沉淀率、稳定系数、粒径电位和脂肪聚结率等都反应出较好的稳定度。高温杀菌后,单甘酯热稳定性不如大豆卵磷脂,甚至加重了乳液体系失稳,大豆卵磷脂和单甘酯最终离心沉淀率分别为2.89%和10.32%,稳定系数分别为0.995和0.875,平均粒径分别为402和813 nm,脂肪聚结率分别为29.6%和34.23%,结合共聚焦显微呈像,反应出大豆卵磷脂热稳定性更好,质量分数为0.06%就能够达到较好的乳化稳定效果,继续添加提升不明显。     

Effect of the temperature on the release of aroma compounds and on the rheological behaviour of model dairy custard

The influence of temperature was pointed out on the release of aroma compounds from a model dairy dessert and on its rheological behaviour. The recipe chosen for inter-laboratory studies within the programme of COST Action 921 was tested. The custards were flavoured with three aroma compounds belonging to strawberry note. The partition coefficients of three compounds (ethyl butyrate, ethyl hexanoate, cis-3-hexenol) were determined at three temperatures (12 °C: tasting temperature of refrigerated dairy products, 20 °C: room temperature, and 37 °C: mouth temperature). Two textures of the custards were obtained using kappa- and iota-carrageenan. Penetrometry tests characterized the rheological properties of the custards. Whatever the temperature, aroma compounds had a greater affinity for the custard rather than for the water. Moreover, the temperature modified the rigidity character of the custard, which should affect the repartition of the aroma compounds between vapour phase and custards.     

Manipulating oral behaviour of food emulsions using different emulsifiers

This study aims to investigate the assumption that the presence of α-amylase in the human saliva will interact instantly with starch and will lead to very different oral behaviour and enhanced flavour release. Hence, orange oil flavoured emulsion was prepared with whey protein isolates (WPI) and modified starch (MS). The stability and flavour release of emulsions were examined through in vitro and in vivo studies. MS emulsion mixed with artificial saliva containing α-amylase resulted in more pronounced changes in mean particle size from 0.185 to 2.35 μm and a significant increase in viscosity. Morphology and turbidity revealed strong flocculation, coalescence and creaming. However, WPI emulsion exhibited very little changes in stability and behaviour. Similar results were observed during oral digestion (in vivo) for both emulsion systems. Moreover, a higher intensity of flavour release (37%) was observed in MS emulsion than WPI. This work demonstrated that a starch-stabilised emulsion has a very different oral behaviour to that of a protein-stabilised emulsion.     

Effects of chlorophyll photosensitisation on the oxidative stability in oil-in-water emulsions

Effects of chlorophyll photosensitisation on the oxidative stability of oil-in-water (O/W) emulsions were determined by analysing headspace oxygen content, lipid hydroperoxides, and headspace volatiles. The roles of transition metals and singlet oxygen were tested by adding ethylenediaminetetraacetic acid (EDTA) and sodium azide, respectively. Emulsions with chlorophylls and visible light irradiation had significantly high lipid hydroperoxides and headspace volatiles and low headspace oxygen content (p < 0.05) after 32 h while samples without light irradiation did not show any significant changes (p > 0.05). Sodium azide did not show clear antioxidant capacities in O/W emulsion systems rather showed prooxidant properties at some concentration. Addition of EDTA, a metal chelator, accelerated the rates of lipid oxidation in a concentration dependent manner. EDTA may enhance the stability of chlorophylls in O/W emulsions and the resulting higher chlorophyll concentrations may generate more singlet oxygen thus accelerating the rates of lipid oxidation.     

The role of rheological behaviour in flavour perception in model oil/water emulsions

Decreasing the fat content of a food produces changes in a range of physical, chemical and sensory properties that are important in flavour perception. The aim of this paper was to study the role of rheological behaviour in flavour perception in model oil/water emulsions, using a series of samples with different hydroxypropyl methyl cellulose (HPMC) and oil content. The composition of the emulsions was adjusted to deliver iso-release aroma in vivo and to produce samples with the same viscosity in-mouth (measured as the Kokini oral shear stress). The results showed that there was no significant difference in perceived thickness between the three samples providing further evidence that the Kokini oral sheer stress is well correlated with in-mouth thickness. However, there was a significant difference in perceived volatile fruity flavour and sweetness. Samples containing the highest oil content were perceived as significantly less fruity and sweet despite having the same volatile release and in-mouth thickness.     

Fortification of cheese with vitamin D(3) using dairy protein emulsions as delivery systems

Vitamin D is an essential vitamin that is synthesized when the body is exposed to sunlight or after the consumption of fortified foods and supplements. The purpose of this research was to increase the retention of vitamin D(3) in Cheddar cheese by incorporating it as part of an oil-in-water emulsion using a milk protein emulsifier to obtain a fortification level of 280 IU/serving. Four oil-in-water vitamin D emulsions were made using sodium caseinate, calcium caseinate, nonfat dry milk (NDM), or whey protein. These emulsions were used to fortify milk, and the retention of vitamin D(3) in cheese curd in a model cheesemaking system was calculated. A nonemulsified vitamin D(3) oil was used as a control to fortify milk. Significantly more vitamin D(3) was retained in the curd when using the emulsified vitamin D(3) than the nonemulsified vitamin D(3) oil (control). No significant differences were observed in the retention of vitamin D(3) when emulsions were formulated with different emulsifiers. Mean vitamin D(3) retention in the model system cheese curd was 96% when the emulsions were added to either whole or skim milk compared with using the nonemulsified oil, which gave mean retentions of only 71% and 64% when added to whole and skim milk, respectively. A similar improvement in retention was achieved when cheese was made from whole and reduced-fat milk using standard manufacturing procedures on a small scale. When sufficient vitamin D(3) was added to produce cheese containing a target level of approximately 280 IU per 28-g serving, retention was greater when the vitamin D(3) was emulsified with NDM than when using nonemulsified vitamin D(3) oil. Only 58±3% of the nonemulsified vitamin D(3) oil was retained in full-fat Cheddar cheese, whereas 78±8% and 74±1% were retained when using the vitamin D(3) emulsion in full-fat and reduced-fat Cheddar cheese, respectively.     

Effect of different starches on rheological and microstructural properties of (II) commercial processed cheese

A range of commercial processed cheese samples containing starch were prepared on a Rapid Visco Analyser (RVA) and on a pilot plant scale. This work clearly demonstrated that it was possible to manufacture processed cheese with part of the protein replaced with potato starch, while maintaining similar rheological attributes (firmness) to those of the control and an acceptable melt index. Sensory evaluation showed that, although the reduced‐protein cheese samples had a good, clean, fresh flavour that was comparable with that of the control, at high starch concentrations the starch‐containing processed cheese had a pasty texture and tended to stick to the wrapper.     

Effect of oil content and processing conditions on the thermal behaviour and physicochemical stability of oil-in-water emulsions

The destabilisation mechanism of oil-in-water (o/w) emulsions was studied as a function of oil content (20% and 40% o/w), homogenisation conditions and crystallisation temperatures (10, 5, 0, −5 and −10 °C). A mixture of anhydrous milk fat and soya bean oil was used as the lipid phase and whey protein isolate (2 wt%) as emulsifier. Crystallisation and melting behaviours were analysed using differential scanning calorimetry. Physicochemical stability was measured with a vertical scan macroscopic analyser. Emulsions with 20% oil were found to be less stable than those with 40% oil. For 20% o/w emulsions, the crystallisation was delayed and inhibited in emulsions with smaller droplets and promoted in emulsions with larger droplets when compared with 40% o/w emulsions. Depending on the droplet sizes in the emulsion, the formation of lipid crystals (in combination with the emulsifier) either stabilises (small droplets) or destabilises (big droplets) the emulsion.