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Canadian Bovine Mastitis Research Network
 

All about Mastitis I Milk Quality

Impact of SCC on Dairy Product Quality Impact on Cheese Yield 

Somatic Cell Count, Mastitis, Dairy Product Quality, and Cheese Yield

In recent years, dairy product manufacturers have become very concerned about the impact of raw milk quality on finished dairy product quality. Milk quality has become a major concern because more time now elapses between milk production and dairy product consumption. At the same time, new milk testing technology has made it more cost effective and easier to test milk for bacteria count, antibiotics, and somatic cell count (SCC) Therefore, many milk processors now offer premium payment programs to improve raw milk quality. These premium payment programs emphasize lowering the SCC of the milk. Part of the NEDFRC's research and extension program focuses on the impact of mastitis on dairy product quality.
 
Impact of SCC on Dairy Product Quality
 
Mastitis causes milk SCC to increase. During mastitis, the types of somatic cells present in the milk change to mostly white blood cells, which add many proteolytic and lipoytic enzymes to milk. In addition, more blood serum leaks into the milk than usual. Dairy product quality defects resulting from mastitis are due to enzymatic breakdown of milk protein and fat.

Protein breakdown in milk produced by cows with clinical or subclinical mastitis is caused primarily by an enzyme called plasmin. Plasmin is found commonly both in milk and in blood plasma and can cause extensive damage to milk casein in the udder prior to milking. When milk is cooled, plasmin breaks down casein (a type of milk protein) much more slowly. Unfortunately, plasmin is extremely heat stable. Therefore, pasteurization cannot inactivate it and plasmin will continue to damage milk protein during dairy product manufacture and storage. Plasmin damages milk casein by breaking the orginal large protein chains into smaller fragments. As a result, the milk casein does not curdle properly during cheesemaking, and some small casein fragments and an increased amount of milk fat are lost into the cheese whey. This causes low cheese yield. Cheeses made form high SCC milk (800,000 to 1,000,000 cells per ml) also have a higher incidence of unclean flavors and pasty textures.

As milk SCC increases gradually, other quality characteristics of dairy products will also change gradually. In fluid milk, the rate of off-flavor development will increase. Rancid off-flavors due to increased lipase activity, bitter flavors due to proteolytic enzyme activity, and salty flavors due to a change in milk mineral balance will all gradually appear as SCC increases. In UHT shelf-stable milk, plasmin may cause milk to change from a liquid to a gel. Milk fat breakdown, on the other hand, tends to have an immediate impact on milk and dairy product flavor. Enzymes called lipases break down milk fat and release free fatty acids that produce off-flavors that are detectable at very low concentrations, especially in high fat products or dairy products with very mild flavors, such as butter or cream cheese.

The impact of the presence of casein breakdown products on flavor and functional characteristics of whey products that are used extensively as ingredients in formulated foods is not known. Protein breakdown could also cause the body of other cultured products such as yogurt to be weak and result in undesirable separation of the yogurt into curd and whey in the package. The functional characteristics (foam stability, gel strength, heat stability, etc.) of the milk proteins in condensed and dried milk products can change as a result of protein breakdown and decrease the value of these products. Condensed and dried milk products and byproducts are used extensively as ingredients in other foods. Changes in protein functionality or the increased levels of heat-resistant protease (plasmin) may cause problems for food processors that use dried milk products as ingredients.

Impact on Cheese Yield
 
It is very difficult to determine exactly how much a change in SCC will cause cheese yield to change. There has been no formula a cheesemaker can use to predict decreasesing cheese yield as the SCC increases in 100,000 cell per ml increments.

A research project was conducted at Cornell University to determine the quantitative relationship between increasing milk SCC and Cheddar cheese yield and reported in the Journal of Dairy Science (JDS 74:369, 1991). Batches (n=22) of milk were collected from groups of 8 to 10 cows with similar milk SCC from less than 100,000 to 1,300,000 cells/ml. Each batch was split into two portions, and Cheddar cheese was manufactured after one and five days of refrigerated storage. Milk casein as a percentage of true protein and cheese yield efficiency were lower when milk SCC was high and when milk was five days old. Fat and protein losses in whey increased with increased milk age and SCC. Cheese moisture increased with milk SCC and decreased with milk age.

For milk from individual cows or small groups of cows all producing milk at the same SCC, cheese yield does not decrease linearly with increasing SCC. At a SCC of about 100,000 cells per ml, there is a sudden 1% decrease in cheese yield. As SCC increases from 100,000 to 1,300,000, cheese yield decreases by an additional 1-2% depending on time and temperature. Since milk handling and cheese making conditions used in this study were optimum (i.e., rapid cooling and storage at 40 degrees F), the observed changes in cheese yield are probably conservative.

It was concluded from this study that any increase in milk SCC above 100,000 cells/ml will have a negative impact on cheese yield efficiency for milk from individual cows. The change in cheese yield efficiency for commingled milk for a full herd would not show this same nonlinear trend. Herd milk will represent a weighted average of the milk characteristics from individual cows. If milk cooling and handling conditions after milking are very good, then the primary differences in cheese yield from bulk milks with different SCC will result from the damage to milk casein that occurred in the udder prior to milking. Thus, the cheese yield performance of the bulk tank milk from an individual farm will reflect the weighted average cheese yield performance of the milks of the individual cows in the herd plus the separate impact of the age of the milk at the time of cheese making. This will produce a linear decrease in cheese yield for herd milk with increasing milk SCC. Other factors that may cause variation in this linear relationship, such as the distribution of milk SCC of individual cows within the herd, are discussed in more detail in the report "Influence of Milk Somatic Cell Count and Milk Age on Cheese Yield" available from the NEDFC. Any negative impact of high psychrotrophic bacteria counts on cheese yield would be additional.

From a practical farm management viewpoint, prevention of mastitis is the key to high milk quality and high productivity per cow. Other preliminary data presented in the report indicate that milk quality may also decrease with stage of lactation and number of lactations.
 
April 2009

Reprinted from the Northeast Dairy Foods Research Center Newsletter "Dairy Center News", Vol 3, No. 4, July 1991. Taken from NMC website: www.nmconline.org

 





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