Rheological Properties Of Yogurt Research Articles

Studies regarding the production of a novel yogurt using some local plant raw materials

In this study, we evaluated the effect of some Romanian local plant raw materials on the rheological, physicochemical, and sensory properties of natural yogurt. These plants were added as a 2% powder in the set yogurt recipe and the finished products obtained under the same conditions were compared to a control sample. By adding plant powders, the rheological properties of yogurt were improved, the syneresis was reduced, and the water holding capacity of the yogurt samples was increased. The highest values for rheological properties were obtained for yogurt with buckwheat flour (YBF) and the lowest values for the same parameters were obtained for control yogurt (YC). The lowest value of syneresis was obtained for YBF, followed by the yogurt with beetroot powder (YBP), and the yogurt with acorn flour (YAF). The best results from sensory and physicochemical point of view were recorded by the yogurt with added 2% buckwheat flour. PRACTICAL APPLICATIONS: The results of the study lead to determining the formulation of a novel yogurt, with the best sensory acceptance, rheological, and physicochemical properties. Because of its universal popularity, there is a high economic importance in diversifying yogurt products. The use of local vegetable resources as raw materials for obtaining yogurt leads to their superior capitalization, as well as obtaining finished products with beneficial effects on the health of the consumers. Scientific impact of this study is by obtaining innovative products using domestic raw materials that not only improve consumer health, but also increase final product shelf life. Socio‐economic impact of the project is to increase consumer confidence in finished products and reduce losses by lowering the number of market returned products and by also increasing turnover.

Individual and sequential effects of stirring, smoothing, and cooling on the rheological properties of nonfat yogurts stirred with a technical scale unit

Few studies have considered the impact of unit operations during stirred yogurt manufacture because their operational sequence is difficult to replicate at the laboratory scale. The aim of this study was to investigate the individual and sequential effects of stirring in the yogurt vat, smoothing, and cooling on the rheological properties of yogurts, using a technical scale unit simulating some industrial conditions. The yogurts were prepared from a milk mixture that was standardized to contain 14% total solids, 0% fat, and 4% protein, and then homogenized, heated (94.5°C, 5 min), and inoculated at 41°C with the same thermophilic lactic starter. The operating parameters under investigation were 2 stirring durations in the yogurt vat (5 or 10 min), 2 cooling systems (plate or tubular heat exchanger), and 2 smoothing temperatures (38°C for smoothing before cooling; 20°C for smoothing after cooling). Sampling valves were installed at critical points on the technical scale unit so that the effect of each operation on the properties of stirred yogurt could be quantified individually. Syneresis, apparent viscosity, firmness, and consistency were analyzed after 1 d of storage at 4°C. In general, as the yogurts moved through the technical scale unit, the properties of the yogurts (evaluated after 1 d) changed: viscosity increased but syneresis, firmness, and consistency decreased. The individual effects of the operations showed that smoothing and cooling, compared with stirring duration, made the greatest contribution in terms of modifying yogurt properties. The stirring parameters (5 or 10 min) had similar effects on the yogurts. The use of a plate heat exchanger promoted a decrease in syneresis, whereas a tubular heat exchanger had a greater effect in terms of increasing firmness and consistency. The type of cooling system had no effect on stirred yogurt viscosity. Smoothing at 38°C had a greater effect on the increase in firmness, whereas smoothing at 20°C contributed more to a decrease in syneresis and increases in viscosity and consistency. This study confirms that each unit operation has a defined effect on the rheological properties of a nonfat stirred yogurt, which also depends on the operation sequence.

Effect of γ‐aminobutyric acid on the physicochemical, rheological and sensory properties of yoghurt

This study aimed to investigate the impacts of dairy processing conditions on the γ‐aminobutyric acid (GABA) content and the effect of GABA on the quality of the resulting yoghurt. Serial concentrations of GABA (0.05–1.0%, w/w, milk basis) were designed during the preparation of yoghurt. Colour and physicochemical and rheological properties of yoghurt were also examined. The results indicated that the fermentation process had no significant influence on GABA content. The GABA level greatly improved the texture properties of the resulting product at a concentration of 0.5% (w/w), whereas higher GABA concentrations impaired the texture. The lightness (L) value of yoghurt also increased in line with the GABA concentration.

Comparative rheological study of goat milk yoghurt and cow milk yoghurt

Goat is popularly known as poor man’s cow (or mini-cow) because of its immense contribution to the poor man’s economy. Like milk, yoghurt is also very nutritious. It has been demonstrated that acid milk is better to digest than normal milk.The most important benefits of yoghurt consumption cover the reduction of blood cholesterol level, anti-cancer effects and the improvement of antimicrobial activity and immunity in the human body.From the point of view of rheology, yoghurt is a non-Newtonian, rheological unstable, viscoelastic and pseudoplastic fluid.Beside the sensory quality, another important factor for the consumer's acceptance of the product are the rheological properties of yoghurt, such as apparent viscosity and flow behaviour.An important role is also played by the composition and physicochemical properties of milk which yoghurt is prepared from. Because of the differences in composition and physicochemical properties of goat and cow milk differences in the rheological properties of yoghurt from these types of milk can be expected.Texture analysis of goat and cow milk yoghurt was carried out to compare the quality of yoghurt using instrument namely TA.XT Plus texture analyzer. Study revealed that goat milk yoghurt was having lower firmness, consistency, cohesiveness and index of viscosity than cow milk yoghurt

Effects of Camel Casein Hydrolysate Addition on Microbiological and Rheological Properties of Yoghurt

Camel milk plays an important source of nutrients in arid lands and consumed as fresh ,pasteurized and fermented. This study aimed to evaluate the effects of camel and cow casein hydrolysates by trypsin enzyme on rheological and sensory properties and growth of starter culture of yoghurts made from cow milk. Caseins were separated from camel and cow milk ,and hydrolyzed by trypsin enzyme. The hydrolysates were added at concentration of (0.5; 1.0 and 1.5%) to yoghurt made from cow milk.
 Results showed that casein hydrolysates strongly decreased the fermentation and coagulation time of the yoghurts. The rate of pH decrease was higher with camel casein hydrolysate in comparison with cow casein hydrolysate at all concentrations (0.5; 1.0 and 1.5%) Viscosities of the yoghurt made with hydrolysates significantly (p蠄0.05) decreased than control samples. The addition of the hydrolysates significantly (p蠄0.05) increased the hardness and adhesiveness of the yoghurts. No significant differences in water holding capacity of control and treated samples were observed. However, increasing casein hydrolysate addition to 1.5% decreased water holding capacity of yoghurt samples. The sensory evaluation scores of the yoghurts were significantly (p蠄0.05) improved with the addition of casein hydrolysates.

Effect of exopolysaccharides from lactic acid bacteria on the texture and microstructure of buffalo yoghurt

To examine the effect of exopolysaccharides (EPS) on the microstructure and textural and rheological properties of yoghurt, purified EPS were added to buffalo milk to make yoghurts. The results showed that the viscosity, water-holding capacity, hardness, and microstructure of yoghurt were distinctly affected by the level of addition of EPS. With the increasing amount of supplemented EPS, the viscosity of buffalo yoghurt showed a downward trend with the increasing shear velocity, and an increasing degree of recovery of viscosity with the decrease in shear velocity, indicating that the steric barrier formed between the EPS and protein enhanced the viscoelastic properties. The microstructure showed more dense interspaced voids with EPS addition.

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.

Effects of Incubation Temperature, Starter Culture Level and Total Solids Content on the Rheological Properties of Yogurt

The effects of incubation temperature (35-45oC), starter culture level (0.003-0.006%, w/v), and milk total solids content (12.32-15.68%, w/v) on the rheological properties of yogurt were investigated using central composite rotatable design (CCRD) and response surface methodology (RSM). Gelation time during milk fermentation and apparent viscosity of yogurt stored for 3 days were the rheological parameters studied. The results revealed that the gelation time and apparent viscosity of yogurt were significantly affected (P < 0.1) by incubation temperature, starter culture level, and total solids. Incubation temperature had the greatest influence on the gelation time. Higher temperature led to shorter gelation time, but resulted in lower apparent viscosity of yogurt. Apparent viscosity of yogurt was mainly affected by total solids. Higher apparent viscosities were also obtained at lower incubation temperature.

RHEOLOGY AND TEXTURE OF SET YOGURT AS AFFECTED BY INULIN ADDITION

ABSTRACT The effect of inulin addition on rheology and texture of nonfat yogurt (NFY) was studied by the large (texture analyzer) and small (dynamic oscillatory rheometry) deformation tests. Skim milks (12% solids) were supplemented with three commercial chicory inulins with different chain lengths at 4% level and inoculated with mixed cultures of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus. Textural and rheological tests were conducted after an overnight storage at 4C and during a shelf life of 28 days. A good correlation was observed between the two tests. During storage, all inulin‐containing yogurts in comparison with the control NFY were characterized by lower values of firmness, apparent viscosity, yield stress, complex viscosity, storage modulus and loss modulus. NFYs supplemented with long‐chain inulin demonstrated a rheological behavior closer to that of control full‐fat yogurt.PRACTICAL APPLICATIONSThe use of inulin and oligofructose in the food industry has grown recently because of their interesting nutritional and technological properties. In the work presented here, three commercial inulin products with different chemical composition and functions were added to yogurt milk, and changes in rheological properties of yogurt were investigated in comparison with nonsupplemented control yogurts. According to the manufacturer's specifications, Raftiline HP and Raftiline GR are suitable for fat replacement because of the presence of long‐chain inulins, while Raftilose P95, being rich in mono‐ and disaccharides, is highly recommended for sugar replacement. The knowledge obtained from this study may be applied for the development of nonfat yogurts to satisfy the nutritional expectations of ever‐increasing health conscious consumers and at the same time to match the sensory and textural characteristics of full‐fat yogurt.

Effect of Temperature and Total Solid Contents on the Apparent Viscosity of Coconut Milk Yoghurts

Procedures for the preparation and rheological characterisation of coconut milk yoghurts are described. Coconut milk was fortified with soymilk, 50:50 (v/v/)% (coconut milk:soymilk), Nonfat dry milk, 60:40 (coconut milk:Nonfat dry milk), while whole coconut milk served as control. Milk preparations were homogenized, heated to 40, 60 and 80oC for 30 mins, inoculated with Streptococcus thermophilus and Lactobacillus bulgaricus and incubated at 43oC to attain pH 4.6. Rheological properties of yoghurts were studied at temperatures ranging from 10 – 80oC. All samples exhibited a pseudo plastic behaviour and described by a power law model. The effect of temperature on apparent viscosity was explained by a polynomial equation and gave a better fit than the Arrhenius relationship. The consistency coefficient exhibits strong dependence on temperature for which the activation energy of flow ranged from 4.57– 6.47kJ/mol. The flow behaviour index was low and nearly constant (~ 0.78) up to 40oC and then decreased with further increase in temperature.

Physical properties of yogurt fortified with various commercial whey protein concentrates

AbstractThe effects of whey protein concentrates on physical and rheological properties of yogurt were studied. Five commercial whey protein concentrates (340 g kg−1 protein nominal) were used to fortify milk to 45 g protein kg−1. Fermentation was performed with two different starters (ropy and non‐ropy). Resulting yogurts were compared with a control yogurt enriched with skim milk powder. The water‐holding capacity of the yogurt fortified with skim milk powder was 500 g kg−1 and ranged from 600 to 638 g kg−1 when fortified with whey protein concentrates. Significant rheological differences have been noticed between the yogurts fortified with different whey protein concentrates, independent of the starter used. Three whey protein concentrates generated yogurts with a behavior similar to the control. The two others produced yogurt with lower firmness (15 g compared with 17 g), lower Brookfield viscosity (6 Pa s compared with 9 Pa s), lower yield stress (2 Pa compared with 4 Pa), lower complex viscosity (13 Pa s compared with 26 Pa s), and lower apparent viscosity (0.4 Pa s compared with 1 Pa s) than the control, respectively. The yogurts with the lowest firmness and viscosity were produced with concentrates which contained the highest amount of non‐protein nitrogen fraction (160 g kg−1 versus 126 g kg−1 of the total nitrogen), and the highest amount of denaturation of the whey protein (262 versus 200 g kg−1 of the total nitrogen). Copyright © 2004 Society of Chemical Industry

Effect of thickeners and sweeteners on the release of blended aroma compounds in fat-free stirred yoghurt during shear conditions

Abstract The present study was conducted on aromatised fat-free stirred yoghurts and dealt with the influence of some thickeners and sweeteners on aroma compounds release and rheology. Thickeners (starch, pectin, locust bean gum and guar) and sweeteners (fructose, fructo-oligosaccahrides, aspartame and acesulfame) were added and mixed together in fruit preparations, which were then introduced in yoghurt. Different concentrations of thickeners and sweeteners were used in the fruit preparations and were obtained with an experimental design. The headspace composition was assessed by solid-phase microextraction during shear conditions in a closed vessel. Rheological properties of yoghurts were determined to obtain the flow behaviour index n and the consistency index K with the Ostwald law. The presence of pectin in yoghurts, tended to reduce the concentration of aroma compounds in the headspace of the samples. We observed also a significant decrease of aroma compounds in the headspace of yoghurt in the presence of starch. In the case of locust bean gum, a significant increase of flavour release was observed. Sweeteners and guar appeared to have no effect. We concluded that rheological parameters did not explain the difference on aroma release and it seemed that during shear conditions, the composition of fruit preparations showed a major role on aroma release. The influence of aroma compounds characteristics on the flavour release was observed, but experiments must be performed to highlight this observation.

Effects of lactoperoxidase and hydrogen peroxide on rheological properties of yoghurt

The effects of activation of the lactoperoxidase (LPO) system by H2O2-NaSCN and hydrogen peroxide (H2O2) on the accessibility of sulphydryl groups (SH) in skimmed milk, and on the dynamic rheological properties of the resulting yoghurt were investigated. Four different concentrations of each reagent (20-80 mg H2O2-NaSCN/kg milk and 100-400 mg H2O2/kg milk) were compared. Clear negative correlations were noted between the accessibility of SH groups and both LPO activation rate and H2O2 concentration. Also the native PAGE pattern of the heat-treated samples showed that with increase in the H2O2-NaSCN and H2O2 concentrations, the level of interaction between beta-lactoglobulin (beta-lg) and kappa-casein (kappa-CN) decreased. The complex modulus (G*) of skimmed milk yoghurts declined gradually with the decrease in the concentration of accessible SH groups accordingly. Tan delta values of yoghurt samples were found to be different from the control, but close to each other, indicating that protein interaction forces taking place in the formation of gel networks of treated yoghurts were different from the control.

Effect of milk source on the rheological properties of yogurt during the gelation process

This paper presents research on the effect of milk source on the rheological properties of curd during the gelation process of yogurt. The highest value for viscosity was exhibited by ovine milk, followed by caprine, bovine and camel milks. For bovine, ovine and caprine milk, three different transient viscosity stages were identified and described by mathematical expressions, whereas camel milk showed no significant variation in viscosity during gelation. The chemical composition of milk, namely total solids and protein content, has a major effect on the rheological properties of curd. A power law model allows the determination of the flow behaviour index and the consistency coefficient of curd made from different milk sources.

Lactoperoxidase Effects on Rheological Properties of Yogurt

ABSTRACTA mixed culture (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus) was inoculated into reconstituted skim milk and incubated. When 0.45 Unit/g or more of lactoperoxidase (LPO) was added, the yogurts had softer and smoother texture than untreated yogurt. The apparent viscosity of yogurt was also reduced by addition of LPO. The storage modulus (G') of the gel of LPO‐treated yogurt decreased but the frequency‐dependency remained unchanged. The amount of exopolysaccharide or degree of proteolysis in yogurt was not changed by addition of LPO.

Effet des agents humectants sur 1'Aw et les propriétés rhéologiques du yogourt

The effects of humectants (NaCl, sucrose and sorbitol) on Aw and rheological properties of yogurt have been studied. 27 experimental samples were prepared by adding humectants to plain yogurt according to a complete 33 factorial design with 3 factors at 3 levels : NaCl (0,4, 8%), sucrose (0, 7.5, 15%) and sorbitol (0, 7.5,15%). Aw, viscosity and syneresis were measured after various cold storage times. All humectants studied significantly influenced Aw, NaCl showing the greatest depressive effect. Aw ranged from 0.974 (no addition) to 0.908 (8% NaCl, 15% sucrose and 15% sorbitol added). Rheological characteristics (consistency and flow index) were affected by humectants and pH. Consistency decreased while flow index increased with increasing levels of humectants in the order: NaCl < sucrose ≃ sorbitol. The evolution of these parameters during storage was influenced by humectants and pH of the sample. In particular, samples with 4% salt added showed an increase in consistency while the effect was opposite with 8% salt added. Syneresis increased with humectant level and time, salt and saccharose being the main factors. Regressions have been calculated from the results on Aw and rheological characteristics of the samples for the initial time. These equations will allow the prediction of these parameters according to humectant level in yogurt.