Microbiological Safety of Blue and Cheddar Cheeses Containing Naturally Modified Milk Fat.

Milk containing naturally modified fat was obtained by feeding lactating dairy cows a Control diet and two experimental diets containing either extruded soybeans or sunflower seeds. Milk from cows fed the experimental diets contained higher levels of both long chain (C18-C18:2) and unsaturated fatty acids than the milk from cows fed the Control diet. Each milk was pasteurized, standardized to 3.6% milk fat, and inoculated with Listeria monocytogenes (strains Scott A and V7), Salmonella typhimurium and Salmonella senftenberg , before manufacturing into Blue or stirred-curd Cheddar cheeses. Populations of L. monocytogenes and Salmonella spp. were monitored during manufacture and aging using Oxford and Xylose Lysine Desoxycholate agars, respectively. During the manufacture of Blue and Cheddar cheese, and during the aging of Blue cheese, behavior of Salmonella spp. and L. monocytogenes in the experimental cheese was similar to the Control cheese. During aging of Cheddar cheese, the rate and extent of decline of Salmonella spp. and L. monocytogenes varied among the cheeses. Declines correlated with the accumulation of specific fatty acids, namely C12, C14, C18:1 and C18:2. These fatty acids were also found to be inhibitory to S. typhimurium and L. monocytogenes when incorporated into tryptic soy agar plates at 37°C. Therefore, the natural fat modification of Blue and Cheddar cheeses enhanced the safety of these cheeses.

Alteration of the Fatty Acid Content of Milk Fat.

Milk fatty acid composition can be influenced by several factors, many of which are interactive. These include stage of lactation, seasonal variation, low milk fat syndrome, feeding, genetic variation, and changes in the energy status of the cow due to administration of bovine somatotropin. Utilization of feeding, genetic variation, and bovine somatotropin should produce a milk fat lower in saturated and higher in unsaturated fatty acids. This may be beneficial to consumers, as many health professionals are recommending diets lower in saturated fatty acids. Giving consumers the option of purchasing low saturated fatty acid dairy products may also assist in alleviating the current milk fat surplus in the dairy industry.

Influence of Milk Fat Higher in Unsaturated Fatty Acids on the Accuracy of Milk Fat Analyses by the Mid-Infrared Spectroscopic Method.

An experiment was conducted to investigate the reliability of milk fat measurement by the mid-infrared spectroscopic method when analyzing milk fat containing greater than normal amounts of unsaturated fatty acids. Sixteen mid-lactation Holstein cows were divided into four treatments including a control (C), control with bovine somatotropin (C+), bovine somatotropin and added dietary fat from sunflower seeds (Sun+), or bovine somatotropin and added dietary fat from safflower seeds (Saff+). Milks were sampled weekly for 16 weeks (n=256). Unsaturated fatty acid percentages in milk fat were 25.0, 28.4, 39.6, and 37.9 for C, C+, Sun+, and Saff+ treatments, respectively. Milk fat percentages measured by the Mojonnier fat extraction and mid-infrared spectroscopic methods were 2.99, 2.97; 3.06, 3.01; 2.73, 2.56; and 2.86, 2.74 for C, C+, Sun+, and Saff+ treatments, respectively. Results indicate the mid-infrared spectroscopic method underestimates the fat content in milk which is higher in unsaturated fatty acids. Dairy producers feeding diets with added fat from unsaturated fat sources may be underpaid for milk fat content when the milk is analyzed by the mid-infrared spectroscopic method. A possible remedy for this problem may be to have milk plants calibrate the mid-infrared spectroscopic instrument with milk samples containing higher than normal amounts of unsaturated fatty acids in milk fat.

Composition of Grade A and Manufacturing Grade Herd Milks in South Dakota.

Biweekly Grade A and manufacturing grade herd milk samples were collected from April 1, 1985, to March 31, 1986, from 203 herds in the Sioux Falls, SD, area and were analyzed to compare composition. The average herd milk composition was 3.70% fat, 3.24% protein, 4.80% lactose, 0.63% ash, 8.67% solids-not-fat (SNF), and 12.37% total solids (TS). Grade A milk had higher % lactose (4.83 and 4.76), % SNF (8.70 and 8.61), and % TS (12.41 and 12.30) than manufacturing grade milk. These compositional differences between grades were consistent throughout the year. Grade A milk has more value in the production of dried dairy products (nonfat dry milk and whey powders) and ice cream than manufacturing grade milk due to its higher lactose and SNF content. New milk pricing plans for protein and other constituents should consider compositional differences (lactose, SNF and TS) that exist between grade A and manufacturing grade herd milk before implementing them.

Use of the Osmometer for Quality Control of Ice Cream Mix 1.

Fourteen brands of commercial vanilla ice cream, purchased at three different times, were analyzed for freezing point, firmness, fat, solids-not-fat, total solids, and net weight. Mean freezing points ranged from -2.65 ± .04 to -3.24 ± .19°C, which indicated ice cream composition and mix formulations varied. Values for mean penetrometer readings ranged from 120 ± 6.0 to 193 ± 34.9 mm; fat, 9.3 ± .23 to 16.8 ± .20%; solids-not-fat, 25.2 ± .17 to 29.6 ± .64%; total solids, 35.5 ± .25 to 42.0 ± .26%; and net weights, 1.026 ± .045 to 1.756 ± .017 kg/1.89 liter. The freezing point osmometer has potential use as a rapid quality control instrument to measure the freezing point of ice cream mix and detect when improper quantities of soluble mix ingredients have been incorporated into the mix. More testing of mix formulations will improve product uniformity and quality and assure compliance with regulatory standards.