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Regulatory aspects of fatty acid synthesis and incorporation into tricylglycerol will be studied in mammary tissue explants, in isolated cells, in cell culture, or by preparing subcellular components of mammary tissue using standard enzymatic procedures (IL, OH, VA,WA). In certain unique studies, metabolic limits for incorporation of specific fatty acids (varying in unsaturation or chain length) in vivo will be characterized by infusing them intestinally or intravenously (IL, OH). Kinetic characteristics of stearoyl-CoA desaturase will be determined in microsomal fractions of mammary tissue (IL), and factors regulating SCFA synthesis will be determined in homogenates or explants (IL).A. There is a particular need to develop sound biochemical bases to understand metabolic regulation of the synthesis of individual milk fatty acids, in particular 12:0 and 14:0, which are of specific public concern today. Objective 1 addresses these needs.
Influence of dietary trans-18:1 on fatty acid synthesis by an established mouse mammary epithelial cell line, which produces neutral lipids when stimulated by lactogenic hormones during growth on an extracellular matrix, will be studied (VA). This group is studying rates of fatty acid elongation and fatty acid incorporation into triacylglyceride, activities of acetyl-CoA carboxylase and stearoyl-CoA desaturase, and plasma membrane fluidity in response to varying amounts of 18:0, cis-18:1, and trans-18:1 in the incubation medium. These studies will be developed further in a bovine mammary cell (MAC-T) line to determine the extent to which concentrations of lipid precursors regulate rates of de nova fatty acid synthesis and desaturation of exogenous stearic acid. Collaboration (IL and VA) to develop isolated cells (Hansen et al., 1986) as a metabolic model to study regulation of milk synthesis will be developed.
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Several investigators will use dietary fat supplements to manipulate milk fat composition (CA, IL, OH, SC, SD, VA, WA, WI). Experimental designs will utilize multiple levels of fat supplementation to develop dose response curves so that diet fat effects on milk fat composition may be quantitated. Stage of lactation of cows will be defined in order to remove confounding effects of adipose mobilization on milk fat composition. Correlary studies will examine effects of adipose tissue mobilization on milk fat composition.
In addition to level of supplemental fat, effects of source of dietary fat are also an experimental objective, e.g., whole oil seeds (SD), intestinally-infused vegetable oils (IL), fish oils (OH, VA), varying fatty acid chain-length (OH), and ruminally-protected unsaturated fat (OH, SC). Factors modifying biohydrogenating activity of ruminal microorganisms will be characterized (CA, OH, SC, VA). Because digestibility of fat declines at high intake and thereby influences amount of fat available to tissues, studies to define factors limiting fatty acid absorption and to improve fatty acid digestibility will be appropriate contributions to this objective (CA, IL, OH, VA, WI).
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Milk obtained from studies addressing Objective 2 will be utilized to examine effects on quality of ice cream (IL) and yield and flavor of low fat cheese (SD). Influence of increasing milk fat unsaturation on quantification of milk fat by infrared spectroscopy will be quantified (SD). The quality and sensory attributes of low cholesterol dairy products produced from butter oil by steam-stripping will be evaluated (MN). Quantification of flavor characteristics in modified fat products by gas-liquid chromatography will be developed (MN). In model systems, the emulsification of modified milk fat and its subsequent effects on milk gel formation will be studied. Emulsification will be evaluated microscopically, and gel formation properties will be assessed by formagraph and rheological measures. The behavior of fats in powders as a component of food products will be evaluated in terms of reconstitution properties and sensory acceptability (MN). To gain information on functionality of modified milk fats, solid fat content will be determined by nuclear magnetic resonance, and temperature of phase transitions will be determined by differential scanning calorimetry (WI). Positional arrangement of fatty acids on the glycerol molecule will be determined when appropriate (CA, WI). Milk from all studies will be used to update a national data based on seasonal, regional, and feeding effects on composition of the milk supply (NY).B. Milk and fat from the core group of feeding studies (Objective 2) will be sent to several of the cooperating milk processing research groups to evaluate changes in milk fat composition on manufacturing quality and consumer acceptance. These interactions will be developed and strengthened through the regional process.
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Co-operation among investigators will be encouraged in at least three ways:D. Regionality will bring together scientists competent in all aspects of our concern for modifying milk fat:1) Metabolic regulation of synthesis.
2) Predicting milk composition change from feed inputs
3) Evaluating effects of changing milk fat composition on product quality, and applying new knowledge to product development.
1) Feeding studies will be regionalized to provide a sufficiently large data base to develop appropriate response curves to fat supplementation and to further develop existing whole animal metabolic models to include effects of dietary fat on milk composition (CA, WA).
2) Unique methodologies will be shared to obtain the maximum information from each study; e.g., some laboratories lack the capability to separate the cis/trans isomers of unsaturated fatty acids; this can be provided by other labs through sharing of samples.
3) Milk from feeding studies will be provided to co-operating laboratories to determine effects of changed milk fat composition on product quality (CA, IL, MN, NY, SD). Regional cooperation will enhance exchange of milk products for study. Thus, utilization of sophisticated equipment (NMR) and methodologies (triacylglycerol structure, sensory studies) will be enhanced and appled to a wider range of modified milk fats than would be possible in any individual laboratory.
a. Liver biopsy (WI)E. Interdependency of states is illustrated by sharing of information from methodologies unique to certain laboratories:
b. Intestinal cannulae (IL)
c. Separation of cis/trans isomers of milk fatty acids (CA, OH)
d. Rheology (MN) and nuclear magnetic resonance (WI) of milk fat
e. Computer simulation modeling of milk fat composition responses to dietary fat changes (CA, WA)
f. Collaborative development of mammary cell culture systems (IL, VA)
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Feeding trials will be designed to develop dose response curves to diet manipulation. All fat quantification will include total fatty acid analysis. Milk fatty acid analysis will utilize techniques that preserve and quantify short chain fatty acids (from butyric), and will include separation of the cis/trans isomers of C18:1.