Spontaneous Whey Separation Research Articles

Effect of adding curcumin and betacyclodextrin nanocomplex on some of the physicochemical, microbiological and sensory traits of yogurt

The aim of this research is to study the effect of adding free curcumin and curcumin loaded with betacyclodextrin to laboratory-made yogurt by conducting a set of tests on the final product. Yogurt was prepared using three treatments. The first treatment was a standard yogurt without any additions, while the second treatment involved adding free curcumin (CU) at concentrations similar to the third treatment, which contained curcumin loaded with betacyclodextrin (B-CU) at concentrations of 100 and 200 μg/ml. which involved measuring the protein percentage, moisture percentage, fat percentage, and ash percentage of the samples studied from day one of production until day 28 of refrigerated storage, which were (4.61, 86.62, 5.64, 0.86) respectively. Various physical and rheological tests were conducted, including total acidity, pH, spontaneous whey separation, viscosity value, and water-holding capacity, which were (0.97, 4.61, 3.1, 2680, 72.36) respectively, up to day 28 of refrigerated storage. Microbiological tests were also conducted on the manufactured yogurt under the same conditions, which showed the efficiency of the active compound, whether in its free form or loaded with β-cyclodextrin, as a microbial inhibitor and preventing contamination of the manufactured product. The tests demonstrated the superiority of the B-CU treatment containing 200 mg/ ml of curcumin-loaded β-cyclodextrin over the other treatments. The sensory evaluation was well-received, ranging from (95-77)% from day one of production until day 28 of refrigerated storage.

Effect of adding sodium caseinate, whey protein concentrateand milk protein concentrate on the physical, rheological and sensory properties of yogurt produced from goat milk

Yogurt is a widespread fermented dairy product widely consumed worldwide. The present study aimed to examine the impact on the yogurt's made from goat milk's physicochemical, rheological, and sensory aspects by incorporating varying percentages of Sodium Caseinate (SC), whey protein concentrate (WPC), and cow milk protein concentrate (MPC) during production to produce good quality yogurt. Four different treatments weremade and assigned as control treatment C, treatment C1 (SC was added at a percentage of 1% and 2%), treatment C2 (WPC added at a percentage of 1% and 2%), and treatment C3 (MPC added at a percentage of 1% and 2%). Estimates of pH and total acidity were among the physiochemical tests performed. Rheological evaluations includedviscosity, the ability to separate whey spontaneously, and moisture retention. Texture tests included hardness, springiness, and cohesiveness. A sensory evaluation of the produced yogurt was also done. The results showed an approximation in the additional treatments' total acidity and pH ​​values immediately after manufacturing. Compared to the control treatment, the additional treatments decreased the values of spontaneous whey separation and increased the water holding capacity and viscosity. However, the product's texture was enhanced due to the addition of the three protein components, which contributed to its hardness, springiness, and cohesion. There was an improvement in the sensory properties of the addition treatments compared to the control treatment and directly related to the increment in the addition percentage.The yogurt resulting from the addition of (SC, WPC, and MPC) led to the production of good-quality yogurt.

Characterizing acidified and renneted gels with different soy milk and skim milk proportions

To understand the properties of differently acidified and renneted gels with different soy milk and skim milk proportions, mixed milks were prepared and treated using sequenced renneting then acidification at 4 °C, and the formation, spontaneous whey separation, and microstructure of the gels were monitored. The results showed that both acidification and renneting promoted gel formation in mixed milk and increased gel strength. In gels, the pH range was narrow (pH ≤ 5.2) at low soy milk proportion (25%) and low renneting degrees (0%, 25% and 50%), and the pH range was extended such that gels were formed with increasing soy milk proportion and renneting degree. When compared with pH, soy milk proportion and renneting degree influenced whey separation percentages more strongly. Soy milk significantly reduced whey separation percentages in gels, and gel strength was highest at 50% soy milk. Protein aggregates sizes increased with increasing soy milk proportion at the 75% renneting degree, and the pore sizes of skim milk-dominated gels was strongly influenced by acidification. These results help us understand mixed gel properties and provide valuable information for the manufacture of mixed fermented dairy products.

Evaluation of milk source on physicochemical, texture, rheological and sensory properties of yogurts

Yogurt is one of the world's most widely consumed dairy products and can be produced from different types of milk. The present research aimed to compare the effects of utilizing whole raw milk from cattle viz. cows, buffaloes, sheep, and goats in four different milk-type yogurts, T1- cow’s milk (CM), T2- buffalo’s milk (BM), T3- sheep’s milk (SM), and T4- goat’s milk (GM) on their physicochemical, texture, rheological, and sensory qualities. The physiochemical testing included estimating pH, the percentages of moisture, fat, protein, ash, and total acidity, and the sensory assessment of the yogurt. The rheological tests also included the viscosity test, spontaneous whey separation, water-holding capacity, firmness, cohesiveness, and springiness. The results showed that T2- BM yogurt and T3- SM yogurt excelled in the percentage of fat, protein, lactose, and ash, where the ratio (4.35, 8.4, 4.70, 0.8) (5.55, 7.45, 4.80, 1.1) was for each of T2- BM yogurt and T3- SM yogurt respectively. The rheological properties were the spontaneous whey separation for each of T1-CM, T2-BM, T3-SM, T4-GM yogurt were 4.1, 2.2, 2.1 and 5.3, respectively, while the water holding capacity was 55, 60, 71 and 53 and the viscosity was 7000, 8900, 6700 and 1510 respectively. The results of the texture properties were 126.2, 199.5, 176.3, and 38.9 for firmness, while the cohesiveness values were 0.41, 0.63, 0.65, and 0.4, respectively. Finally, T2-BM and T3-SM yogurt excelled in the value obtained for sensory evaluation. The importance of the present study lies in the fact that milk with a high percentage of total solids gives good-quality yogurt.

Study of rheological characteristics of fermented milk paste with spices

Research object: rheological characteristics of fermented milk pastes with spices.
 Investigated problem: according to the results of the degree of syneresis and rheological properties, the consistency of the product is characterized. This allows to improve and optimize the technological process for the production of fermented milk products.
 Main scientific results: the results of the dispersion analysis of spices show that their particle size is in the range of 250 … 400 microns. This size is optimal for the extraction of biologically active substances of spices, which will contribute to greater assimilation by the human body. And also optimally to avoid caking of raw materials and the manifestation of moisture-absorbing properties.
 It was found that the degree of syneresis of fermented milk pastes with spices is within 10 %. With the addition of cinnamon and fenugreek spices, the degree of syneresis is lower compared to other examples. This is due to the fact that spices contain a solution of fiber, which allows them to retain free moisture. The spice cinnamon is a dried bark that naturally contains polysaccharides, while fenugreek contains up to 30 % mucus, which also has a high moisture-absorbing capacity. With such a low serum separation, the stabilizing effect is sufficient to prevent spontaneous whey separation.
 Based on the results of the rheological properties of the product, it was established that fermented milk pastes belong to viscoplastic systems with a normal consistency. There was a slight difference in the shear stress index for pastes based on fermented milk low-fat soft dietary cheese and sour cream with a fat mass fraction of 20 % – by an average of 50 Pa. This characteristic is due to the hydrophilic characteristics of the protein.
 The area of practical use of the research results: scientific research concerns the technology of dairy products in the food industry.
 Innovative technological product: the obtained scientific result is an opportunity to improve the technological process of fermented milk products.
 The scope of implementation of an innovative technological product: the results obtained give grounds to assert the ability to translate into a real industrial creation.

Development of a Rapid and Accurate Prediction Model for Whey Separation in Pasteurized Drinking Yogurt Caused by Long-term Ambient Storage

Long-term ambient storage of pasteurized drinking yogurt (PDY) may result in excessive whey separation. This study aimed to develop a rapid and accurate prediction method of PDY whey separation when maintained at 25 °C for 45 and 90 days. Predictive models of whey separation were created by regression analysis based on the results of physical property measurements including the 48 hour Turbiscan Stability Index (TSI). The predictive model based on TSI showed a high coefficient of determination and high suitability not only when the pectin concentration in PDY was changed but also when the manufacturing conditions of PDY were changed. In addition, this model was superior in prediction accuracy and versatility as compared to models based on other physical property measurements. The advantage of the TSI model is that it allows for more rapid, accurate and versatile evaluation of spontaneous whey separation.

Effects of polymerized whey protein on survivability of Lactobacillus acidophilus LA-5 during freeze-drying.

Probiotic cultures are commonly freeze‐dried for storage and distribution. However, freeze‐drying and subsequent storage are accompanied by a decline in cell viability. Whey protein (WP) or polymerized whey protein (PWP) was used to protect Lactobacillus acidophilus LA‐5 against damage during freeze‐drying process and the subsequent storage. The protection capacity and effects of polymerized whey protein protected freeze‐dried L. acidophilus LA‐5 on physiochemical properties of cow and goat milk yoghurts were evaluated in comparison with maltodextrin (MD). The survival rate of L. acidophilus LA‐5 after freeze‐drying decreased in the order of MD (80.91%) > PWP (69.86%) > WP (64.89%). The particles of WP‐ and PWP‐based freeze‐dried samples showed an average diameter of about 10 μm, which was significantly higher than that of MD‐based particles (1.5 μm). Both whey protein preparations showed higher protecting effect than MD at high humidity condition during the 180‐day storage. Addition of freeze‐dried L. acidophilus LA‐5 with the presence of WP or PWP improved the protein content and decreased spontaneous whey separation and syneresis significantly for both yoghurts. PWP‐protected L. acidophilus LA‐5 addition significantly improved the firmness and adhesiveness of the yoghurt. Freeze‐dried L. acidophilus LA‐5 mixed with PWP had higher survivability in yoghurts compared with the culture alone at the end of storage. Data indicated that whey protein can be used to protect probiotics during freeze‐drying and may also improve the physiochemical properties of the yoghurt.

Effects of polymerised whey protein-based microencapsulation on survivability of Lactobacillus acidophilus LA-5 and physiochemical properties of yoghurt

Lactobacillus acidophilus LA-5 has poor survivability in yoghurts. This study aims to microencapsulate L. acidophilus LA-5 using polymerised whey protein as wall material and evaluate its survivability and effects on physiochemical properties of yoghurts. The microencapsulation yield was 92.90% and average beads size was 744 μm. Microencapsulated L. acidophilus LA-5 showed higher survival at simulated gastrointestinal conditions. Microencapsulated L. acidophilus LA-5 had improved survivability (∼106 cfu/mL) than free cells (∼104 cfu/mL) after digesting by simulated intestinal juice (6 h) in yoghurts during 10-week storage. Microencapsulated L. acidophilus LA-5 improved the firmness and protein content and decreased the spontaneous whey separation and synaeresis of yoghurts significantly (p < 0.01). Changes in pH and free microorganism population of yoghurts during storage were not affected by adding microencapsulated L. acidophilus LA-5. Microencapsulation of L. acidophilus LA-5 using polymerised whey protein as wall material improved its survivability and the physiochemical properties of the yoghurt.

Effect of milk proteins aggregation using Transglutaminase and Maillard reaction on Ca2+ milk gel

Effect of transglutaminase, Maillard reaction induced crosslinking and the combination of transglutaminase and Maillard reaction induced crosslinking between whey proteins and caseins in milk on calcium milk gel properties were investigated. Treatment of milk with transglutaminase, Maillard reaction, and transglutaminase + Maillard reaction cause to the appearance of new high MW protein bands. Water holding capacity, gel strength and sensory scores of gel samples increased and spontaneous whey separation decreased in calcium-induced milk gel made from transglutaminase and combination of transglutaminase and Maillard reaction treated milk compare with calcium-induced milk gel made from untreated milk alone.

Utilization of konjac glucomannan as a fat replacer in low-fat and skimmed yogurt

Konjac glucomannan (KGM) has been reported to be beneficial to human health, as well as having potential functional properties as a fat replacer in dairy products. In this study, 0.5% KGM solution was added to prepare low-fat (LFKGM) and skimmed (SKKGM) yogurts, and their physicochemical properties were compared with those of full-fat yogurt control (FFC), low-fat yogurt control (LFC), and skimmed yogurt control (SKC). Properties and composition were determined and the microscopic structures of all yogurts were observed during storage at 4°C for 21d. Generally, addition of KGM to yogurts had no significant effect on composition, pH, and titratable acidity at each storage day. The LFKGM and SKKGM had higher whiteness, greenness, and yellowness hues compared with those of the LFC and SKC. The proteolysis of LFKGM and SKKGM was similar to that of FFC, whereas it was lower than in LFC and SKC after 14d of storage. Addition of KGM had no positive effects on the water-holding capacity, but led to a decrease in syneresis and spontaneous whey separation in LFKGM and SKKGM compared with those of LFC and SKC. The spontaneous whey separation of LFKGM was similar to that of FFC. Presence of KGM in skimmed yogurt affected textural characteristics, while having little effect on texture of low-fat yogurt. Additionally, LFKGM and SKKGM showed stronger and more stable gel structures than those of FFC, LFC, and SKC. Overall, no substantial changes were found in the characteristics for each yogurt during storage, except for pH and gel structures. Results indicated that KGM may be a good fat replacer to develop reduced-fat yogurts with desired characteristics.

Effects of Pleurotus eryngii polysaccharides on bacterial growth, texture properties, proteolytic capacity, and angiotensin-I-converting enzyme–inhibitory activities of fermented milk

Pleurotus eryngii is one of the most favored oyster mushrooms and contains various beneficial bioactive compounds. Polysaccharide extracted from P. eryngii (PEPS) was added as a natural-source ingredient to milk before fermentation, and the effects of additional PEPS on fermented milk were investigated in this study. The PEPS were extracted and added to reconstituted skim milk (12%, wt/vol) at 0.5, 0.25, and 0.125% (wt/vol) and fermented by a non-exopolysaccharide-producing strain, Streptococcus thermophilus Australian Starter Culture Collection (ASCC) 1303 (ST 1303), or an exopolysaccharide-producing Strep. thermophilus ASCC 1275 (ST 1275). Bacterial growth, texture properties, microstructure, proteolytic capacity, and angiotensin-I-converting enzyme–inhibitory activities of fermented milk (FM) were determined during refrigerated storage at 4°C for 21d. Viable counts of starter bacteria in FM with 0.5% PEPS added were the highest. Changes in pH were consistent with changes in titratable acidities for all samples. The FM samples with added PEPS showed denser protein aggregates containing larger serum pores in confocal micrographs compared with those without PEPS at d 0 and 21during refrigerated storage. The values for spontaneous whey separation of FM with added PEPS were significantly higher than those of FM fermented by ST 1303 or ST 1275 without PEPS. The proteolytic activities of ST 1303 of FM with added PEPS were higher than those of FM fermented by ST 1303 without PEPS. The FM with added 0.125% PEPS had similar angiotensin-I-converting enzyme–inhibitory activity to that fermented by ST 1303 without PEPS; both were higher than those of other samples during refrigerated storage. Firmness and gumminess values of FM with added PEPS were higher than those of FM fermented by ST 1303 or ST 1275 without PEPS.

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.

Rheology, spontaneous whey separation, microstructure and sensorial characteristics of probiotic yoghurts enriched with passion fruit fiber

Beyond demonstrated beneficial health attributes, passion fruit rinds are a by-product of the fruit pulp industry, rich in total dietary fiber, particularly pectin. The aim of this study was to evaluate the influence of the addition of passion fruit fiber on the whey formation, rheological parameters, microstructure and sensorial characteristics of probiotic yoghurts. Skim milk bases enriched with 1% of passion fruit fiber or not were heat treated and inoculated with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, and divided into four groups according to the probiotic strain added — Lactobacillus acidophilus strains L10 and NCFM and Bifidobacterium animalis subsp. lactis strains Bl04 and B94. Fermentations were performed until the pH reached 4.5. Rheological characteristics of yoghurts were determined by a rotational rheometer in two cycles of shear rate ranging from 0 to 15s−1 in both upward and downward curves. Sensorial analysis of passion fruit fiber yoghurts, either without any probiotic or co-fermented by L. acidophilus L10 or B. animalis subsp. lactis Bl04, was evaluated against a control yoghurt without fiber. Photomicrographs of freeze-dried yoghurts were made by field-emission scanning electron microscope (SEM). Thixotropy of enriched yoghurts was higher than that of their respective controls in the two cycles of shear rate. Apparent viscosity was significantly higher in fiber yoghurts co-fermented by the lactobacilli than in their controls at the end of cold storage. Photomicrographs demonstrated that in passion fruit fiber yoghurts the casein gel was more compact and overlaid the fiber, while filaments of exopolysaccharides were more frequent in control yoghurts. Appearance, odor and color of the passion fruit fiber yoghurts received scores as ‘good’, and the intensity of the passion fruit flavor was considered weak by the sensory assessors. Results indicate that the passion fruit fiber is an almost neutral ingredient for the design of new high value-added yoghurt.

Effects of stabilizers and exopolysaccharides on physiochemical properties of fermented skim milk by Streptococcus thermophilus ST1

In this study, the effects of exopolysaccharide (EPS) produced by Streptococcus thermophilus ST1 and stabilizers on the textural and microstructural properties attributes of fermented skim milk were investigated. ST1 had a high capacity to produce acids and EPS. Within 4.5 h, the pH dropped to 4.5, the acidity reached 0.8%, and EPS reached 65.27 mg/L. ST1 was more effective at increasing viscosity as well as water holding capacity and reducing spontaneous whey separation compared with control sample (ST0). ST1 combined with carrageenan had the highest viscosity and the best water holding capacity as well as the lowest spontaneous whey separation. ST1 combined with whey protein concentrates showed better results with xanthan, but worse with carrageenan. The microstructure of the fermented milk was examined using scanning electron microscopy. ST1 combined with carrageenan resulted in compact and uniform structure, which is agreed with quality improvement of fermented skim milk that used ST1 combine carrageenan as fat substitute. Key words: Streptococcus thermophilus ST1, exopolysaccharide, fermented skim milk, texture.

Effect of exopolysaccharides and inulin on the proteolytic, angiotensin-I-converting enzyme- andα-glucosidase-inhibitory activities as well as on textural and rheological properties of low-fat yogurt during refrigerated storage

Whey separation during storage continues to be one of the major problems in low-fat yogurts. Exopolysaccharides (EPS) produced by lactic acid bacteria have been recognized as a solution to this problem. Inulin is accepted as a fat replacer in products such as low-fat yogurts, in addition to providing physiological benefits. A combination of EPS and inulin could give both health- and texture-promoting properties to low-fat yogurt. Therefore, the aim of this study was to comprehensively study the influence of using an EPS-producing strain of Streptococcus thermophilus along with inulin (3%, wt/vol) on the viability of S. thermophilus and Lactobacillus delbrueckii ssp. bulgaricus, their proteolytic, ACE- and α-glucosidase-inhibitory activities, as well on the textural and rheological properties of low-fat yogurt during storage at 4 °C for 28 days. The time to reach a pH of 4.5 was less in the presence of EPS-producing S. thermophilus. However, during storage, EPS and inulin together did not influence the pH and lactic acid, and the effect on ACE-inhibition activity varied with the period of storage. The presence of EPS showed a protective effect on the survival of Lb. delbrueckii ssp. bulgaricus and partially on the extent of proteolysis. The α-glucosidase-inhibitory activity was more apparent in EPS-containing yogurt. The yield of EPS varied with the period of storage, being maximal (110.77 mg·100 g−1)at d 14. EPS-containing yogurts showed lower firmness, spontaneous whey separation, storage modulus, yield stress, consistency index and hysteresis area than non-EPS producing yogurts. It was concluded that low-fat yogurt with a stable and compact texture having anti-hypertensive and anti-diabetic potential could be obtained using an EPS-producing strain of S. thermophilus.

Effect of exopolysaccharides on the proteolytic and angiotensin-I converting enzyme-inhibitory activities and textural and rheological properties of low-fat yogurt during refrigerated storage

The aim of this study was to examine the influence of using exopolysaccharide (EPS) producing strain of Streptococcus thermophilus on the viability of yogurt starters, their proteolytic and angiotensin-I converting enzyme-inhibitory activities, and on the textural and rheological properties of the low-fat yogurt during storage at 4°C for 28d. The use of an EPS-producing strain of S. thermophilus did not have influence on pH, lactic acid content, or the angiotensin-I converting enzyme-inhibition activity of low-fat yogurt. However, EPS showed a protective effect on the survival of Lactobacillus delbrueckii ssp. bulgaricus. Presence of EPS reduced the firmness, spontaneous whey separation, yield stress, and hysteresis loop area but not the consistency and flow behavior index of low-fat yogurt.

Effect of Addition of Versagel® on Microbial, Chemical, and Physical Properties of Low‐Fat Yogurt

The objective of this study was to examine the effect of Versagel on the growth and proteolytic activity of Streptococcus thermophilus 1275 and Lactobacillus delbrueckii ssp. bulgaricus 1368 and angiotensin-I converting enzyme inhibitory activity of the peptides generated thereby as well as on the physical properties of low-fat yogurt during a storage period of 28 d at 4 degrees C. Three different types of low-fat yogurts, YV0, YV1, and YV2, were prepared using Versagel as a fat replacer. The fermentation time of the low-fat yogurts containing Versagel was less than that of the control yogurt (YV0). The starter cultures maintained their viability (8.68 to 8.81 log CFU/g of S. thermophilus and 8.51 to 8.81 log CFU/g of L. delbrueckii ssp. bulgaricus) in all the yogurts throughout the storage period. There was some decrease in the pH of the yogurts during storage and an increase in the concentration of lactic acid. However, the proteolytic and ACE-inhibitory potential of the starter cultures was suppressed in the presence of Versagel. On the other hand, the addition of Versagel had a positive impact on the physical properties of the low-fat yogurt, namely, spontaneous whey separation, firmness, and pseudoplastic properties.

Growth, Proteolytic, and ACE‐I Activities of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus and Rheological Properties of Low‐Fat Yogurt as Influenced by the Addition of Raftiline HP®

This study investigated the influence of incorporating Raftiline HP on the pH, growth, proteolytic, and angiotensin-I converting enzyme inhibitory activities and on spontaneous whey separation, firmness, and rheological properties of low-fat yogurts during storage for 28 d at 4 degrees C. Three types of yogurts were prepared from skim milk containing 0% (YI0, control), 2% (YI2), and 3% (YI3) Raftiline HP, respectively. The incorporation of Raftiline HP improved the growth of starter organisms, particularly that of Lactobacillus delbrueckii ssp. bulgaricus, resulting in shorter fermentation time. There was a significant improvement in total proteolysis, which was highest in yogurt containing 3% Raftiline HP. The ACE inhibitory activity was maximal in YI3 compared to YI2 and YI0. Incorporation of Raftiline HP did not affect whey separation and firmness of the low-fat yogurts. All these products were more fluid like with distinct pseudoplastic properties and lesser ability to resist deformation upon applied shear.

RHEOLOGICAL PROPERTIES, WHEY SEPARATION, AND MICROSTRUCTURE IN SET‐STYLE YOGURT: EFFECTS OF HEATING TEMPERATURE AND INCUBATION TEMPERATURE

The effects of heat treatment and incubation temperature on the rheological and microstructural properties of yogurt were studied. A central composite experimental design and response surface methodology were used for data analysis. The rheological properties were determined by dynamic low amplitude oscillation and the amount of spontaneous whey separation was quantified by the volumetric flask test. Confocal scanning laser microscopy was used to examine the gel structure. The storage moduli of yogurts increased with an increase in heating temperature and a decrease in incubation temperature. The maximum loss tangent value during gelation, permeability, and amount of spontaneous whey separation of yogurts increased with a decrease in heating temperature and with an increase in incubation temperature. These parameters indicated an increased possibility for rearrangements, which was confirmed by presence of large pores in the gel network. Second order polynomial models successfully predicted the effects of heating temperature and incubation temperature on the rheological properties, permeability, and whey separation of yogurts. Whey separation was negatively correlated with storage modulus (r= ‐0.66), and was positively correlated with the maximum loss tangent (r= 0.63) and permeability (r= 0.78). This study demonstrates that weak yogurt gels, which have high loss tangent values, favor rearrangements in the network and the resulting network has larger pores (high permeability) and exhibits greater whey separation.

Effect of heat treatment on the physical properties of milk gels made with both rennet and acid

The effects of heat treatment of milk, and a range of rennet and glucono- δ- lactone (GDL) concentrations on the physical properties of combined rennet/acid gels, made from reconstituted skim milk at 30°C, were investigated. Microstructure was determined using confocal scanning laser microscopy and permeability measurement. Whey separation was determined using two differently shaped containers. Confocal scanning laser micrographs of combined gels made from unheated milk showed that these gels had much larger pores compared with combined gels made from heated milk. This trend was confirmed by permeability measurements. Combination gels made from unheated milk were very prone to spontaneous whey separation, possibly due to considerable rearrangements of aggregated particles at an early stage of the gelation process. Gels made with both rennet and acid are a useful model system for studying the behaviour of natural and fresh cheese products.