Progress 10/01/02 to 09/30/06
Outputs
This study investigated the mechanism(s) responsible for the occurrence of whey separation, which is the key defect in yogurts. We have identified key rheological parameters (including low storage modulus, high loss tangent, low yield stress, low yield strain) that can be used to indicate gelation conditions where this defect occurs. In general weak gels with large pores and where the bonds are highly mobile are prone to this defect. Whey separation involves the shrinkage of the matrix and the expulsion of whey (through pores in the matrix). High incubation temperatures and low inoculation rates were two factors that increased whey separation. We have also studied the flow properties of yogurts where gels were made in situ in the rheometer and disrupted by shearing while the rheological properties were monitored. Initially the apparent viscosity of yogurt increases as the intact gel resists shearing due to its yield stress, eventually the shearing overcomes the yield
stress (i.e. bond breakdown exceeds bond formation) and the gel flows. The initial behavior is not accurately predicted by models that are only designed for the breakdown part of this process. Quantitative descriptive sensory analysis was used to describe the sensory properties of stirred yogurt gels. Significant correlations were observed between rheological properties of the initial intact gel, sensory attributes, physical properties and the rheological properties of stirred products. This indicates the importance of the structural characteristics of the initial gel in dictating the properties of the final stirred yogurt product even after the considerable shearing that yogurt is subjected to in the manufacturing process.
Impacts
Commonly used models used to describe the viscosity of yogurt were found to have limitations and not valid to describe all the flow behavior of yogurt. Clear linkages were found between the initial gel and stirred yogurt emphasizing the importance of the initial gel properties in controlling the texture of stirred yogurt (which is the main type on the US market place). We have described the impact of the main processing variables on yogurts which will assist manufacturers in optimizing process conditions to improve quality and reduce defects, such as whey separation.
Publications
- Castillo, M., F. A. Payne, T. Wang and J. A. Lucey. 2006b. Effect of temperature and inoculum concentration on prediction of both gelation time and cutting time. Cottage cheese-type gel. International Dairy Journal 16: 147-152
- Castillo, M., F. A. Payne, T. Wang and J. A. Lucey. 2006c. Effect of temperature and inoculum concentration on gel microstructure, permeability and syneresis kinetics. Cottage cheese-type gels International Dairy Journal 16: 153-163
- Lee, W. and Lucey, J. A. 2006. Impact of gelation conditions and structural breakdown on the physical and sensory properties of stirred yogurts. Journal of Dairy Science 89: 2374-2385
- Castillo, M., J. A. Lucey, and F. A. Payne. 2006a. The effect of temperature and inoculum concentration on rheological and light scatter properties of milk coagulated by a combination of bacterial fermentation and chymosin. Cottage cheese-type gels. International Dairy Journal 16: 131-146
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Progress 01/01/05 to 12/31/05
Outputs
In this reporting period our research objectives were to understand structure-function relationships between the initial yogurt gels and stirred yogurts made from these gels. Yogurt gels were made from milk heated at 75 or 85C for 30 min and incubated at 32, 38, or 44C. Yogurt gels were sheared at 5 s-1 for 1 min to make stirred yogurts. Small amplitude oscillatory rheology and shear rate sweeps (flow curves) were performed to determine the dynamic moduli and apparent viscosity of yogurts, respectively. Confocal scanning laser microscopy was used to examine the microstructure. The sensory properties of stirred yogurts were determined using quantitative descriptive analysis. Yogurt gels made from high heating temperatures and low incubation temperatures exhibited an increase in storage modulus, apparent viscosity values at structural breakdown, yield stress and a decrease in the maximum in loss tangent values and permeability. During the shearing of intact gels there
was an initial increase in apparent viscosity in the low shear rate region, because of the resistance derived from the intact network; commonly used models for stirred yogurts could not adequately describe this type of flow behavior. Increasing milk heating temperature and decreasing incubation temperature resulted in stirred yogurts with increased apparent viscosity, oral viscosity, sensory mouth coating attribute and decreased particle size. Storage modulus values of the initial gels were positively correlated with the apparent viscosity at 10 s-1 (r = 0.61), oral viscosity (r = 0.77), and mouth coating sensory attributes (r = 0.72) of stirred yogurts. A significant negative correlation was observed between maximum in loss tangent values of initial gels and shear stress values (r = -0.68) of gels at structure breakdown, and in stirred yogurts, the oral viscosity (r = -0.85), and apparent viscosity at 10 s-1 (r = -0.78). Permeability was negatively correlated with the oral viscosity
(r = -0.84) and apparent viscosity at 10 s-1 (r = -0.82) in stirred yogurts. These highly significant correlations demonstrated that the structure of the initial gel network and structural breakdown process had a major impact on the physical and sensory attributes of stirred yogurts. The presence of a yield stress in intact yogurts gels resulted in commonly used models that could not adequately predict flow behavior in this region, although they were able to predict the remainder of the flow profile.
Impacts
Commonly used models used to describe the viscosity of yogurt were found to have limitations and not valid to describe all the flow behavior of yogurt. Clear linkages were found between the initial gel and stirred yogurt emphasizing the importance of the initial gel properties in controlling the texture of stirred yogurt (which is the main type on the US market place).
Publications
- No publications reported this period
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Progress 01/01/04 to 12/31/04
Outputs
In the past year we have studied the flow properties of yogurts where gels were made in situ in the rheometer and disrupted by shearing while the rheological properties were monitored. Unusual features were noticed in these flow curves where the apparent viscosity appeared to initially increase until obvious structural breakdown resulted in the steady decrease in apparent viscosity. The initial increase in apparent viscosity was found to be caused by the build up of stress during the very slow initial application of shear; similar to the stress overshoot type of rheological test. Once the structure began to breakdown the viscosity started to decrease, as expected. Simple previous models (e.g. power law) for the flow behavior of yogurt have not taken into account this initial phase of the shearing process or have used stirred products where structural breakdown has already occurred. Quantitative descriptive sensory analysis was used to describe the sensory properties
of stirred yogurt gels. Relationships between the flow properties of intact gels, the flow behavior of stirred gels and sensory attributes were compared. The use of high milk heat temperatures and low incubation temperatures resulted in yogurts with high initial apparent viscosity in the low shear rate region, and increased oral viscosity and decreased particle size in the stirred yogurt samples. Significant correlations were observed between rheological properties of the initial intact gel, sensory attributes, physical properties (e.g. permeability) and the rheological properties of stirred products. A better understanding of these relationships will help in controlling the texture of stirred yogurt.
Impacts
Better control of the viscosity and texture properties of stirred yogurt. Modifications of the manufacturing process to control texture and reduce the number of physical defects. Improved models to help us understand the impact of processing on the flow characteristics of stirred yogurt.
Publications
- Lucey, J. A. 2004. Cultured dairy products: an overview of their gelation and texture properties. International Journal of Dairy Technology 57: 77-84.
- Wang, T. and J. A. Lucey. 2004. Predicting cottage cheese cutting time using a light backscatter sensor. Milchwissenschaft (in press)
- Robinson, R. K. and J. A. Lucey. 2004. Production of yogurt. In Fermented Milks (Tamime, AY. Ed.) (in press) Blackwell Publ.
- Lee. W. J. and J. A. Lucey. 2004. Effect of starter inoculation rates and incubation temperatures on physical properties of yogurt. Presented at 2004 ADSA annual meeting. Journal of Dairy Science, 87: suppl. 1, page 93.
- Castillo, M., J. A. Lucey, and F. A. Payne. 2004. Rheological and light scattering properties of cottage cheese-type gels made under different gelation rates. Presented at 2004 ADSA annual meeting. Journal of Dairy Science 87: suppl. 1, page 233.
- Castillo, M., J. A. Lucey, T. Wang, and F. A. Payne. 2004. Gel microstructure, permeability and syneresis kinetics of cottage cheese-type gels under different gelation rates. Presented at 2004 ADSA annual meeting. Journal of Dairy Science, 87: suppl. 1, page 286.
- Castillo, M., F. A. Payne, T. Wang, and J. A. Lucey. 2004. Gelation time and curd setting predictions on cottage cheese by light backscatter. Presented at 2004 ASAE International meeting held in Ottawa, August 1-4, 2004, poster #612-9.
- Lee, W. and Lucey, J. A. 2003. Rheological properties, whey separation, and microstructure in set-style yogurt: effects of heating temperature and gelation temperature. Journal of Texture Studies 34: 515-536
- Lee, W. and Lucey, J. A. 2004. Structure and physical properties of yogurt gels: combined effects of different inoculation rates and incubation temperatures. Journal of Dairy Science 87:3153-3164 .
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Progress 01/01/03 to 12/31/03
Outputs
In the past year we studied the combined impact of using different rates of starter culture inoculation (0.5 to 4%) in gels made at two different incubation temperatures (40 and 46C). These two variables are critical in the control of the texture properties of yogurt and are easily manipulated by a manufacturer. The properties of yogurt were studied using dynamic low amplitude oscillatory rheology, a specially developed yield stress test, gel permeability (porosity), amount of serum separation, and confocal laser scanning microscopy. Gel stiffness (storage modulus) was increased while permeability and serum separation decreased in gels made with higher inoculation rate and lower incubation temperature. Positive significant correlation was observed between serum separation and permeability (r = 0.94) suggesting the large pores contribute to serum separation; this was supported by the confocal micrographs of the gels. We have a draft manuscript in preparation from this
work. We are currently comparing the use of two different types of measuring systems, profiled couette and a vane, as a method to shear the gels that are made in situ in the rheometer. Optimizing of the test conditions is underway to develop flow curves and compare the rheological parameters derived from the two measuring systems. It was considered that there was the possibility of slip occurring during the shearing test (would introduce an artifact), which is why we investigated these two methods. We will investigate the impact of various shearing conditions on gel viscosity and the relationship between gelation conditions, stirred gel viscosity and sensory textural attributes.
Impacts
Better control of the viscosity and texture properties of stirred yogurt. Modifications of the manufacturing process to control texture and reduce the number of physical defects.
Publications
- Lucey, J. A. 2003. Improving yogurt texture. Proceedings of the 40th Annual Marshall Cheese Seminar & Dairy Expo in Visalia, CA, Sept 17-18, pp. 75-83.
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Progress 01/01/02 to 12/31/02
Outputs
This project just started in October 2002 and our initial work has focused on setting up methods for studying the rheological properties of stirred yogurt by making the gels in situ in a rheometer. By performing a flow curve on a set yogurt gel we are able to obtain information about the initial structure and detailed rheological data for the stirring process. We are currently testing the applicability of various rheological models for this flow curve data. We will then use this approach for the rest of our study where we investigate the impact of conditions such as incubation temperature, heat treatment, and ingredients on the rheological properties of stirred yogurt. We are currently investigating if the rate of acidification is a major factor influencing the rheological properties, as it would help explain some of differences obtained when using different starter strains even when all other experimental conditions are similar. Another key objective of our study is
to understand how exopolysaccharides (EPS) that are produced by the starter culture, influence the rheology and texture of yogurt. From our initial research we believe that there are three possible methods by which EPS could impact yogurt texture: (1) the concentration produced by the culture, (2) structure of the EPS and whether it becomes attached to the protein matrix or exhibits incompatibility and (3) its size or molecular weight as a broad range has been reported for EPS produced by yogurt starter culture. Our initial work is using a model acid gel system (made using glucono-delta-lactone) and investigating how the addition of a neutral polysaccharide (dextran) influences the rheology of the gel. We will investigate the impact of the molecular weight of the polysaccharide (15,000 to 2,000,000 Da) on these physical properties, as we believe that the size of the polymer could be a major factor influencing the performance of EPS in yogurt.
Impacts
Increased knowledge of the rheological properties of stirred yogurt is important to manufacturers as they decide on processing conditions, such as pumping and stirring speeds, and how they impact yogurt properties. This project will result in manufacturers being able to better design fermentation conditions that produce stirred yogurts with desired textural properties.
Publications
- No publications reported this period
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