Colostrum, refrigerators, and hot weather

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In the current issue of the Journal of Dairy Science, my fellow blogger Kim Morrill published a paper reporting the results of a large survey of colostrum quality, using data taken from 67 farms in 12 states, in 2010 [Nationwide evaluation of quality and composition of colostrum on dairy farms in the United States, K.M. Morrill, E. Conrad, A. Lago, J. Campbell, J. Quigley, H.Tyler, JDS (2012) 95 (7), 3997-4005].

There are a few important points in that report that deserve some additional attention, now that hot weather has become firmly established over much of the U.S.

  1. Colostrum quality, as measured by bacterial content, was acceptable in only 55% of the samples examined.
  2. 33% of the samples of colostrum taken from containers that were not cooled had excessive levels of bacterial contamination.
  3. 77% of the samples of colostrum taken from containers that were refrigerated had excessive levels of bacterial contamination.
  4. 38% of the samples of colostrum taken from containers that were frozen had excessive levels of bacterial contamination.

High levels of bacterial contamination in colostrum are known to do two bad things to calves:

  1. Reduce the amount of IgG absorbed by the calf
  2. Increase the chance that the calf will get sick from the disease caused by the bacteria

Clearly, based on the results of the survey, the refrigerator is typically a BAD place to store colostrum.
But there is more to the story than that, and I am going to use the rest of this blog entry to explain why the survey results are what they are.
In case you don’t want to read any further, the real take home message is:

  1. Be extremely careful to milk the cow in such a way that NO dirty water or dirt enters the colostrum, and that all containers used for colostrum have been cleaned with a sterilizing wash procedure.
  2. Cool the colostrum to storage temperature very quickly – meaning within a few minutes!
  3. When you need to warm the colostrum for feeding, warm the colostrum as quickly as possible – again, within minutes.
  4. Feed the colostrum immediately after warming it to feeding temperature.

Now, why were such a large percentage of the refrigerated samples found to have excessive quantities of bacteria?

  1. In the summer time, every time you open the refrigerator door, the temperature goes up inside the refrigerator.

A “typical” dairy has a refrigerator for storing medicines, and that refrigerator also serves as a place to store colostrum, and maybe a worker’s lunch, along with possibly drinks and anything else that should be cooled. Whatever the exact circumstances, the refrigerator door is opened at least several times a day, and sometimes a lot more often. Every time the door is opened, the temperature inside the refrigerator changes to be closer to the temperature of the air around the refrigerator.

  1. Even with the door shut, cooling colostrum from around 90oF to 42oF can take many hours, depending on the particular refrigerator.
  2. At temperatures above approximately 60oF, the bacteria in colostrum will double in 30 minutes (the rate of growth of bacteria depends heavily upon the temperature, and the species of bacteria, so this is a very approximate statement).

I once had the opportunity to check on the temperature and bacterial levels of hundreds of colostrum samples over a one-year period on two different dairies. While I could not obtain the data in a “controlled environment” suitable for scientific publication, I found that on those two (rather large) dairies, colostrum was often at a temperature above 60oF seven hours after it was put in the refrigerator!

Colostrum that is put in the freezer of the same refrigerator will generally cool much more quickly, because the freezer compartment is, on average, much colder than the main refrigerator area. Another factor is that in a typical situation, the freezer is opened less often than the refrigeration section of the appliance. <compare detailed data from paper, deduce how much more quickly the freezer cooled the colostrum below the “critical” temperature

Warming the colostrum to feeding temperature

Every dairy I’ve ever visited warms colostrum by putting the container of colostrum into a bucket of hot water. After some period of time, the water is replaced with a new batch of hot water. This may happen several times over a half-hour to hour or longer, until the person who will feed the calf decides that the colostrum is warm enough, usually just by feel.

Unfortunately, once that colostrum starts to warm, the bacteria in it begin to multiply. Inside the container of colostrum, some of the colostrum melts first, and some remains frozen for most of the time that the colostrum is being warmed. Exactly what happens inside the container of thawing colostrum involves a lot of complicated physics we won’t discuss here. But, depending on a lot of factors, the inside surface of the container may be warm enough to allow bacteria that are in contact with, or very close to, that surface to multiply rapidly – even though much of the colostrum is still frozen! 

The “take home message” from this is that while you are thawing colostrum in a bucket of hot (warm) water, bacteria may be growing in the colostrum.
There is no way to “instantly” warm the entire contents of the container of colostrum. The best available method is to use a microwave oven. For more comments on using a microwave oven to heat colostrum, see the following link:

We can now understand in a general way why the bacterial content of colostrum that had not been refrigerated was less than colostrum that had been frozen, which in turn was less than the bacterial load of colostrum that had been refrigerated. The authors of the paper state: “The number of [colostrum] samples collected from each dairy was based on availability of [colostrum] at the time of site visit. Frozen, refrigerated, or fresh [colostrum] was selected.” Further reading suggests that the investigators would visit a farm, and take samples of whatever colostrum was available; so we can suspect that sometimes they found a bucket milker full of colostrum that had not yet been refrigerated, frozen, or fed.

The average amount of bacteria in the fresh, refrigerated, and frozen samples allows us to make a few more estimations. The bacterial load in the fresh samples was approximately 10,000 units per mL, which is generally considered “safe” (but any amount of bacteria is potentially unsafe). The frozen samples averaged 31,600 units per mL. This concentration of bacteria can be obtained by allowing the average fresh milk sample (10,000 units per mL) to remain warm for another 50 minutes. A gallon of fresh colostrum, at 90oF, will probably not freeze in an hour if you put it in the freezer, but after an hour the freezer should have cooled the colostrum to 60 degrees, greatly slowing down the multiplication of the bacteria.

Following the same line of reasoning, we find that the difference between the bacterial load of the refrigerated samples and the bacterial load of the fresh samples is 10 times as great: the fresh colostrum averaged 10,000 units/mL, while the refrigerated colostrum averaged 100,000 units/mL. A little calculation shows that leaving that fresh colostrum uncooled for 100 minutes will result in the unsafe bacterial load of 100,000 units/mL. Therefore, we may conclude that the average dairy refrigerator requires 100 minutes to cool warm colostrum to a temperature of less than 60 degrees.  This is too long – and the result of the survey supports this conclusion, with approximately 75% of all refrigerated colostrum being “unsafe”.

Colostrum can be chilled in less than 5 minutes by placing 2-quart bottle quantities of colostrum in sealed plastic bags and submerging the bags in a bath of liquid propylene glycol that is held at a temperature of -20 degrees. This bath can be constructed using a small top-loading home freezer, filled with propylene glycol. The operational problem with this is that after some number of bags are cooled or frozen in a short period of time, the temperature of the propylene glycol rises, making the system less effective.(Eventually the freezer machinery catches up, but a typical low-cost home freezer just cannot handle very many bags per day.)

I tried this system, and it would work to freeze the colostrum quickly. However, without a microwave oven to thaw the colostrum, and warm it to feeding temperature, I still would have high levels of bacteria due to the long period of time required to warm the colostrum. Samples of the frozen colostrum that I sent to the lab would be reasonably clean, but samples of the same colostrum that I thawed using hot water would be unsafe.   

One final point about keeping all contamination out of the colostrum when the cow is milked: one time I split a single batch of colostrum, from one cow, into two parts, each being one-half gallon. Into one part I put one drop of dirty water from an alley near the maternity area. Then, I froze both parts, then later thawed them and sent samples from each part to the lab for analysis. The sample that had not been contaminated was really clean, while the batch into which I had put the one drop of dirty water was loaded with a lethal level of bacteria. That is why it is imperative that colostrum be obtained from the cow under super clean conditions!

About the Author

Roy Williams
Roy is a retired calf grower, and is now a graduate student in the Molecular and Cellular Biology Program at Dartmouth. He is currently involved in research in the genetic factors of cancer at the Geisel School of Medicine at Dartmouth. His long-range research interests are in parasitism and infectious diseases, particularly in calves.


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