Browsing by Author "Dr. E. Allen Foegeding, Committee Chair"

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Aggregation of alpha-Lactalbumin at PH 3.5-6.0
(2006-12-28) Schneider, Paula Anne; Dr. John Cavanagh, Committee Member; Dr. Brian Farkas, Committee Member; Dr. E. Allen Foegeding, Committee Chair
Heat-induced protein aggregation is an important reaction in food processing because it causes undesirable appearance changes, which may lead the beverage to phase separate. Recently, there has been an increased demand for and growth in the area of sports beverages. Sports beverages can be categorized several different ways; although the category of current interest is that of beverages having a low pH (from 2.8 to 3.5) and representing a good source of protein. Generally these beverages derive a high proportion of their protein from bovine whey, since it is a source for branched chain amino acids which can benefit muscle building and maintenance. These low pH beverages can be astringent. Astringency is positively related to low pH where the degree⁄intensity of perceived astringency increases as the pH of the beverage decreases. One way to minimize astringency is to increase the pH; although aggregation readily increases the nearer the pH is to the protein's isoelectric point. The selection of a less heat sensitive protein source (ex. α-lactalbumin) and/or the use of molecular aggregation blockers can provide ways to control aggregation. The first objective of this research was to study the aggregation of α-lactalbumin over a pH range 3.5 to 6.0. Protein solubility and turbidity development were used to monitor aggregation. Turbidity was evaluated using a spectrophotometer set to monitor at 400 nanometers, and a turbiditimeter that measures scattering at a 90° angle from the incident light. Change in protein solubility in response to pH adjustment with 1M phosphoric acid and thermal (120 seconds in a 85°C water bath) treatment was determined by measuring dispersed protein after centrifugation at 11950 x g (18-24°C) for 60 to 90 minutes. As expected, the lowest protein solubility and highest degree of turbidity resulted near the isoelectric point of α-lactalbumin. The affect of calcium content was also evaluated. In the holo-form, α-lactalbumin was more stable to native aggregation; although no difference was seen in stability after heat treatment. Indirect determination of the mechanism for α-lactalbumin aggregation was facilitated through the use of various compounds that had previously demonstrated aggregation-blocking or reduction abilities in different systems (ex. protein, pH, heating conditions, etc.). Blocking agents included amyloid/β-sheet blockers (ex. thioflavin-T and quercetin), hydrophobic amino acid-interacting molecules (ex. Hydroxyl-propyl-β-cyclodextrin), and the use of proteins as blockers (ex. αs-casein) were evaluated. The amyloid/β-sheet blockers were not effective in suppressing turbidity development or maintaining/increasing protein solubility. Further investigations were made to determine if a specific type of aggregate was possible. Under the conditions tested, α-lactalbumin did not form the specific aggregates (spherulites), which helps explain why these blockers were ineffective in an α-lactalbumin protein system. The use of sodium dodecly sulfate (SDS) was used to control aggregation by binding to exposed hydrophobic patches on the protein's surface. SDS was not effective in controlling aggregation in β-lactalbumin systems, as monitored through turbidity development and protein solubility, but was effective in systems that contained β-lactoglobulin. The difference in response may be attributed to the structural differences between these proteins. Blocking of hydrophobic amino acid residues with hydroxyproply- β-cyclodextrin had the most potential for success in suppressing aggregation of α-lactalbumin. However, further work is needed to determine where they are binding, strength of binding, and the effect of polar components on the core cyclodextrin molecule.
Cheese Texture
(2002-06-17) Brown, Jennifer Amber; Dr. Marcia Gumpertz, Committee Member; Dr. Mary Anne Drake, Committee Member; Dr. E. Allen Foegeding, Committee Chair; Dr. Christopher R. Daubert, Committee Member
Cheese is a popular food due to its diversity in application, nutritional value, convenience, and good taste. Producing high quality cheeses that meet consumer expectations is crucial in order for cheese makers to remain competitive. These expectations include proper end-use functionality (shred, melt, stretch, etc.) and appropriate texture. Currently, there is not a complete understanding of what characteristics govern these aspects. This study seeks to determine what transitions occur during the early stages of maturation of certain cheeses, specifically, how the changes in physicochemical properties in young cheeses affect textural changes perceived when consumed. Mozzarella and Pizza cheeses were tested at 4, 10, 17, and 38 d of age; Process cheese was tested at 4 d of age. Rheological methods were employed to determine the linear, non-linear, and fracture properties of the cheeses. A trained sensory panel developed appropriate descriptive language and product-specific reference scales to evaluate cheese texture. Both sensorial and rheological methods differentiated the cheese varieties, and patterns were observed as the cheese aged. Rheological analysis showed the cheeses were viscoelastic gels with greater storage (G´, elastic) than loss (G'', viscous) moduli. The overall magnitude of G´ decreased as the cheeses aged; creep recovery analysis confirmed the loss of overall firmness with time. Five sensory terms differentiated the ages of the cheeses within varieties. Correlations between the sensory and rheological methods were observed. Principal component analysis revealed that combinations of both sensory and rheological parameters could distinguish the cheeses based upon variety and age. Comparison of certain large strain rheological methods was also done. Fracture stresses and fracture strains (or apparent strain) at three different strain rates (0.0047, 0.047, and 0.47 -1) were determined using both torsion and vane methods to see how the large strain properties compared in these cheeses. Overall, vane fracture stresses were lower than torsion fracture stresses. As the strain rate increased, the fracture stresses increased. Simple linear regression of the torsion and vane fracture stresses revealed that the torsion fracture stresses were 2.0 times higher than the vane fracture stresses (R²=0.66). Mozzarella is an anisotropic material since the body of this cheese has fibers that are oriented in a specific way. Methodology to appropriately evaluate the sensory perception of such materials was explored. No differences in any of the sensory terms were found between samples tested having the fibers oriented parallel to the force applied and samples tested having fibers perpendicular to the force applied. Finally, the thermal behavior of these cheeses was considered through use of differential scanning calorimetry. Two different heating schemes were used to determine if glass transitions occur in these cheeses and to characterize melting behavior. Glass transition temperatures were determined in the Process cheese. The heating profiles at elevated temperatures (i.e. during melting) were similar in all cheeses at all ages. It is likely that the transitions observed during melting are due to phase changes in certain lipids within the cheese. These results have significant implications in the cheese industry. An understanding of the transitions in both physical and chemical properties in young cheeses can help to explain what causes change in the perceptual texture, which may help in producing customized cheeses. Future testing should focus on how such parameters affect end-use functionality in order develop similar models which will help cheese makers to meet consumer demands.
Determining the Roel of Astringency Mechanism of Whey Protein at Acidic pHs
(2009-09-29) Kelly, Mallory Anne; Dr. E. Allen Foegeding, Committee Chair; Dr. MaryAnne Drake, Committee Member; Dr. Clyde Sorenson, Committee Member
Whey proteins are used in beverages for a wide range of nutritional benefits. Low pH conditions (< 4.5) enhance clarity and stability; however, they are also associated with increased astringency. The goal of this investigation was to determine if interactions between positively charged whey proteins and negatively charged salivary proteins contribute to the astringency of low pH whey protein beverages. In the first experiment, the effect of protein concentration (0.25-13%) on the astringency and basic tastes of beverages containing Ã¯Â Â¢-lactoglobulin at pH 3.5 was investigated by a trained sensory panel (Spectrum method). Controls consisted of pH 3.5 phosphate buffers at concentrations equivalent to the amount of acid required to adjust the protein pH to 3.5. Astringency significantly increased with increasing protein concentration (0.25-4% wt/wt). Astringency of controls was not significantly different; indicating that increased astringency was not due to phosphate concentration or phosphate buffering ability but was contributed from increasing protein concentration. At higher concentration (>4% wt/wt), maximum astringency was not significantly different, sourness significantly increased, showing the effect of increased acid concentration. Time intensity studies showed the maximum astringency and duration were different among protein concentrations, while the time to maximum astringency was similar for all protein concentrations. Saliva and ÃŽÂ²-lactoglobulin interactions, as indicated by turbidity, showed maximum turbidity was between pH 4.0 and 5.5. In the second experiment, astringency of three commercial whey protein isolates (WPI, pI ~ 5.2) and lactoferrin (pI ~ 8.8) at pH 3.5, 4.5, and 7.0 were evaluated At pH 7.0, positively charged lactoferrin was astringent, while all three negatively charged WPIs were not; suggesting that charge interactions play a role in the astringency mechanism. Interactions among whey and saliva proteins were determined. Ã¯Â Â¢-lactoglobulin or lactoferrin (at pH 2.6, 3.5 and 7.0) and saliva were mixed, centrifuged, and the supernatant and pellet compositions were analyzed by polyacrylamide gel electrophoresis. Results showed the presence of Ã¯Â Â¢-lactoglobulin or lactoferrin and saliva proteins in the pellet, indicating that saliva decreased the solubility of Ã¯Â Â¢-lactoglobulin or lactoferrin, presumably through direct interactions. In the third experiment, the effects of increasing sweetness (sucralose) and reducing sugar-base (ribose) modification were investigated. As sucralose concentration was increased from 0.011-0.023% (wt/wt), the astringency was not significantly different at any of the protein concentrations (1, 4, or 14% wt/wt). The addition of ribose to the protein beverages significantly reduced the astringency rating compared to the addition of sucralose to the protein beverages. The addition of ribose was shown to cause a reduction in available amino groups (positive charge). These results support the hypothesis that direct interactions between positively charged whey and negatively charged salivary proteins play a role in the astringency mechanism and modifying the protein may reduce the astringency.
Factors Regulating Astringency of Whey Protein-fortified Beverages
(2006-11-07) Beecher, Jason; Dr. E. Allen Foegeding, Committee Chair; Dr. MaryAnne Drake, Committee Co-Chair; Dr. Clyde Sorenson, Committee Member
Whey proteins are added to a variety of foods and beverages for functionality and added nutrition. A rapidly growing area of whey protein use in foods and beverages is the sports drink category. There are two categories of whey protein-fortified drinks: those at neutral pH and those at low pH. The drinks at low pH have a clear and refreshing appearance, compared to the shake-style drinks at neutral pH. Astringency is very pronounced at low pH. Thought to be caused by compounds in foods that bind with precipitate salivary proteins, astringency at high levels is an undesirable characteristic in foods and beverages. The mechanism of astringency of whey proteins is not understood and has not been investigated. Salivary flow rate, viscosity, and pH are a few variables that have been reported to alter perceived astringency of red wine, tannic acid, alum, chitosan, and cranberry juice. In order to investigate factors regulating astringency of whey proteins, a market survey was conducted and a model beverage was formulated. Trained sensory panelists evaluated the viscosity and pH effects on astringency of whey protein-fortified model drinks (n=8). Changes in optical density of saliva and drink mixtures before and after centrifugation were also investigated to see if a relationship existed between aggregation, precipitation, and astringency. Increasing viscosity (1.6 mPa s – 7.7 mPa s) did not alter maximum intensity, time to maximum, duration, or area under the curve of astringency time-intensity profile. Significant changes were observed over the pH range investigated (pH 2.6 – 6.8). Acidic drinks were higher in astringency and sourness compared to the drink at neutral pH. Astringency decreased from pH 3.4 to pH 2.6. Saliva and drink mixtures showed that aggregation and precipitation were taking place, and the degree of precipitation correlated with perceived astringency. Electrostatic interactions between positively charged whey proteins at low pH and saliva proteins with low isoelectric points are thought to be responsible for aggregation and precipitation, resulting in the perception of astringency.