IMPACT OF PROTEIN-POLYMER INTERACTIONS ON YOGURT GELATION

• Sponsoring Institution: National Institute of Food and Agriculture
• Project Status: COMPLETE
• Funding Source: HATCH
• Reporting Frequency: Annual
• Accession No.: 0229473
• Project Start Date: Oct 1, 2012
• Project End Date: Sep 30, 2016
• Program Code: [(N/A)]- (N/A)

Recipient Organization
UNIV OF WISCONSIN
21 N PARK ST STE 6401
MADISON,WI 53715-1218

Performing Department
Food Science

Non Technical Summary
Yogurt is the main method by which the US public consumes healthy probiotic bacteria and consumption is rapidly increasing. Polymers like pectin are added as stabilizers although the exact stabilization mechanism is unclear. We propose to use model systems to study the impact of key physicochemical properties of pectin (molecular weight, polymer content, calcium concentration), on yogurt gelation. We want to understand the nature of pectin-pectin and pectin-protein interactions during acidification process. Some bacteria produce exopolysaccharides (EPS) during fermentation as an alternative approach to yogurt stabilization. Conflicting results have been reported for the impact of EPS on yogurt gelation; a confounding factor is different rates of acidification by different strains of EPS-producers (as rate of acidification influences gelation). We will exploit the fact that the chemical/molecular characteristics of different types of EPS have been reported for many yogurt strains. We plan to study a range of single strains of yogurt cultures with various different types of known EPS characteristics and adjust all their growth profiles (acidification rates) to be similar. This novel approach allows us to understand how the precise nature of EPS impacts yogurt gelation. This information will provide fundamental information on controlling yogurt texture which will aid manufacturers.

Animal Health Component

(N/A)

Research Effort Categories

Basic

(N/A)

Applied

(N/A)

Developmental

(N/A)

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
502	3470	1000	100%

Knowledge Area
502 - New and Improved Food Products;

Subject Of Investigation
3470 - Other dairy cattle products;

Field Of Science
1000 - Biochemistry and biophysics;

Keywords

yogurt, texture, stabilizers, probiotics, functionality,

exopolysaccharide, pectin

Goals / Objectives
The overall goal of this research will be to determine how polymers like pectin and exopolysaccharides (EPS) interact with milk proteins and how those interactions impact yogurt gelation. Objective 1: Modeling Pectin Interactions in Acid Gel Systems. To investigate how factors like the concentration and molecular weight of added pectin and calcium content impact pectin-protein interactions as well as acid-induced casein gelation. Objective 2: How the Nature/Characteristics of EPS Impact Yogurt Gelation. To investigate how the chemical composition/structure of EPS impacts yogurt gel properties when the acidification/growth profiles for the single strain EPS-producers are kept the same. Expected outputs includes presentations at meetings/symposia, short course training, publications, visits to companies involved in the manufacture of these products.

Project Methods
We propose to use sodium caseinate as the milk protein source since this product has minimal calcium content. We propose to selectively add different concentrations of calcium to this model system to elucidate the gelation mechanism of pectin in milk gels. Pectins are produced from a number of different plant sources including apple pomace and the peels of various citrus fruits. Industrially, it is known that a number of factors modify the gelling performance of pectin and one of the most important is the molecular weight, which can vary due to degradation of pectin during the extraction and processing stages (e.g., residual enzymatic activity, breakdown by heat/shear, etc). We propose to fractionate commercial pectin into various molecular weight fractions and use these fractions in acid gel systems to examine the importance of molecular weight on the gelation mechanism and yogurt properties. We will use a multilevel factorial design to explore how the variables (and their interactions) pectin concentration and molecular weight, in addition to calcium content, impact gelation properties. For the Objective 2, we aim to understand the role of EPS on rheological and physical properties of acid milk gels. EPS from different yogurt bacterial strains have different properties depending on molecular weight, chemical structure and type of linkages. Therefore, use of single strain instead of mixed cultures to produce acid gels would allow us to better understand how specific molecular weights and chemical structures of EPS affect acid milk gels. We will select EPS producing cultures that has been well characterized in terms of their EPS structure and composition. We propose to use these single strains to form acid gels and compare their detailed physical and rheological properties when their acidification profiles are adjusted to be similar.

Progress 10/01/12 to 09/30/16

Outputs
Target Audience:The food industry, specifically yogurt suppliers and food scientists are our target audiences. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?The project allowed for training of one undergrad and one graduate student. How have the results been disseminated to communities of interest?Results were dissemnated through presentations at national meetings like ADSA and a publication of the results. We also had visits with and gave presentations to yogurt culture suppliers. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? In keeping with our overall goal, we successfully characterized the detailed physical and chemical properties of EPS produced from 7 strains of Streptococcus thermophilus, one of the key cultures used for yogurt manufacture. We also determined the yields of EPS produced under standardized growth condition in milk.

Publications

• Type: Journal Articles Status: Accepted Year Published: 2017 Citation: Pachekrepapol, U., J. A. Lucey, Y. Gong, R. Naran, and P. Azadi. 2016. Characterization of the chemical structures and physical properties of exopolysaccharides produced by various Streptococcus thermophilus strains. Journal of Dairy Science (in press).

Progress 10/01/14 to 09/30/15

Outputs
Target Audience:food scientists, industry Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?On this project, a PhD student did most of the research work, which is towards the completion of a thesis. There was also an undergradaute student that assisted in some parts of the study. How have the results been disseminated to communities of interest?We have published multiple scientific papers on the results of this study, with several more in preparation. We have presented the results of this work at scientific meetings, like the annual meeting of the American Dairy Science Association. We have also presented the results at industry training and short courses. What do you plan to do during the next reporting period to accomplish the goals?We are completing the remaining work on several project objectives, including when exopolysaccharides are produced during yogurt fermentation and the nature of these materials. We also plan to submit several more papers for publication.

Impacts
What was accomplished under these goals? We used a model yogurt system and tested the impact of adding uncharged dextran (DX; MW ~ 2.0 × 106 Da) and negatively charged dextran sulfate (DS; MW ~ 1.4 × 106 Da) to this system. Two concentrations (0.075 and 0.5%, w/w) of DX or DS were added to cold milk (~ 0°C) that had been acidified to pH values 4.4, 4.6, 4.8 or 4.9. Acidified milks containing DX or DS were then quiescently heated at the rate of 0.5°C/min to 30°C, which induced gelation, and gels were then held at 30°C for 17 h to facilitate gel development. Dynamic small amplitude oscillation rheology, and large deformation (shear) tests were performed. Microstructure of gels was examined by fluorescence microscopy. Gels made with high concentration of DX gelled at lower temperature, but after 17 h at 30°C these gels exhibited lower storage moduli and lower yield stress values. At pH 4.8 or 4.9 (pH values > the isoelectric point of caseins), addition of 0.5% DS to acidified milk resulted in lower gelation temperature. At pH 4.4 (pH values < the isoelectric point of caseins) addition of 0.5% DS to acidified milk resulted in gels with very high stiffness values. Gels made at pH 4.8 or 4.9 with both concentrations of DS had much lower stiffness and yield stress values than control gels. Microstructural analysis indicated that gels made at pH 4.4 with the addition of 0.5% DX exhibited large protein strands and pores, while gels made with 0.075% DX or the control gels had finer protein matrix. At higher pH values (> 4.4), gels made with 0.5% DX had a finer structure. At all pH values, gels made with 0.5% DS exhibited larger pores than the control gels. This study demonstrated that low concentrations of uncharged DX did not significantly impact the rheological properties of model acid milk gels; high concentrations of DX resulted in earlier gelation, possibly caused by depletion induced attractions between casein particles, which altered the microstructure and created weaker gels. At pH values < 4.6, negatively charged DS produced stiff casein gels, which might be due to attractive crosslinking by electrostatic interactions between DS and caseins at pH values below the isoelectric pH of casein (i.e., positively charged casein regions interacted with negatively charged DX molecules).

Publications

• Type: Journal Articles Status: Published Year Published: 2015 Citation: Pachekrepapol, U., D. S. Horne, and J. A. Lucey. 2015. Impact of dextran and dextran sulfate on the structural and rheological properties of model acid milk gels. Journal of Dairy Science 98:2843-2852.
• Type: Journal Articles Status: Published Year Published: 2015 Citation: Ozcan, T., D. S. Horne, and J. A. Lucey. 2015. Yogurt made from milk heated at different pH values. Journal of Dairy Science. 98:6749-6758.

Progress 10/01/13 to 09/30/14

Outputs
Target Audience: The target audience consists of scientists andfood manufacturers. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? The graduate student has mentored two part-time undergraduates assisting on this project. How have the results been disseminated to communities of interest? We presented results at theannual meeting of American Dairy Science Association. We also published two peer-reviewed papers and one book chapter. What do you plan to do during the next reporting period to accomplish the goals? We have several draft papers that are close to submission, so we plan on having multiple papers published in the next reporting period. We have ongoing work investigating EPS structure and we are isolating different types of EPS and adding them back into yogurt gels to better understand how the type of EPS impacts gels.

Impacts
What was accomplished under these goals? Under objective 1, we successfully studied how different types of pectin impact milk properties and then studied how they impact milk gel systems, work is being written up for publication. Under objective 1, we completed model studies to explore the mechanisms by which EPS impact yogurt gels.

Publications

• Type: Journal Articles Status: Accepted Year Published: 2015 Citation: Pachekrepapol, U., D. S. Horne, and J. A. Lucey. 2014. Impact of dextran and dextran sulfate on the structural and rheological properties of model acid milk gels. Journal of Dairy Science (in press)
• Type: Book Chapters Status: Accepted Year Published: 2015 Citation: Lucey, J. A. 2014. Formation, Structural Properties and Rheology of Acid-Coagulated Milk Gels, In Cheese: Chemistry, Physics and Microbiology, 4th edition, (P.F. Fox, P.L.H. McSweeney, T.M. D. Everett and P. Cotter, eds), Academic Press, Oxford (in press)
• Type: Journal Articles Status: Published Year Published: 2014 Citation: Pachekrepapol, U., D. S. Horne, and J. A. Lucey. 2014. Interactions between acidified dispersions of milk proteins with dextran or dextran sulfate. Journal of Dairy Science 97:53715382

Progress 01/01/13 to 09/30/13

Outputs
Target Audience: scientists, food manufacturers Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? Graduate student has mentored two undergraduates assisting on this project. How have the results been disseminated to communities of interest? Presented at American Dairy Science Association (ADSA) annual meeting. Presented at dairy industry research forum in November. What do you plan to do during the next reporting period to accomplish the goals? We have several draft papers that are close to submission so we plan on having multiple papers published in the next reporting period. One PhD thesis should be completed.

Impacts
What was accomplished under these goals? Under objective 1, we have conducted a range of experiments on impact of different types of pectin on milk proteins and yogurt gels. We have also conducted extensive studies on the impact of EPS and model EPS polymers on milk protein systems. We have developed a rapid test to improve the quantification of EPS, we have determined when EPS is produced during yogurt fermentation, we have determined the structure of several different types of EPS that are used commercially.

Publications

• Type: Book Chapters Status: Awaiting Publication Year Published: 2014 Citation: Lucey, J. A. 2013. Acid coagulation of milk. In Advanced Dairy Chemistry, Volume 2, Proteins. 4rd edn. pp. P. L. H. McSweeney, J.A. OMahony, eds. Kluwer Publishers (in press)
• Type: Book Chapters Status: Awaiting Publication Year Published: 2014 Citation: Lucey, J. A. 2013. Milk Protein Gels. In Milk Proteins: From Expression to Food (Boland, M., Singh, H. and Thompson, A., eds), 2nd edition. Elsevier (in press)

Progress 01/01/12 to 12/31/12

Outputs
OUTPUTS: Experiments were conducted using model biopolymers and model yogurt systems. Results were analyzed based on likely interactions between biopolymers and milk proteins, as influenced by pH and biopolymer concentration. Dissemination included presentation of findings to R&D staff at one of the largest yogurt manufacturers in the US. PARTICIPANTS: PI: Lucey, John; Graduate Students: Regina Horak and Ulisa Pachekrepapol TARGET AUDIENCES: Target audiences are the dairy and food industries and the scientific community of food researchers. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
New fundamental knowledge was gained on how different types of natural biopolymers impact the yogurt gelation process. We demonstrated that the influence of charged biopolymers was greater than uncharged biopolymers, that uncharged biopolymers could induce earlier gelation due to a depletion flocculation mechanism. This knowledge will assist in reducing the amounts of biopolymers needed for the stabilization of yogurt and well as the selection of starter cultures with the "best" exopolysaccharide type for yogurt improvement.

Publications

• No publications reported this period