Spoiling Milk as a Natural Clock

By Scott Pfahler

The spoilage of milk is something that has had a long and important history. Before modern technological advancements, milk spoilage was a massive concern because people knew that milk would not last long before it became unusable. At this point in history it was difficult to sell milk commercially because there was absolutely no consistency in the shelf life of milk. This problem was solved by a French scientist named Louis Pasteur who discovered the effect of microbes on milk spoilage and in 1862 developed the pasteurization process to help lengthen milk's shelf life. Pasteurization reduces the amount of bacteria in milk, which in turn lengthens the amount of time before it spoils. This is done by heating the milk to approximately 72 degrees Celsius for about 15 seconds. One of the beneficial results of this innovation is the ability of dairy manufacturers to sell milk with an expiration date printed on the packaging, which gives the consumer the ability to tell how long they have to use the milk before it goes bad. This idea of an expiration date brings to mind one possible useful quality of the spoilage of milk: to tell time. Can milk spoilage help tell time? How well does it work as a natural clock?

The first thing to explore in order to determine how milk spoilage functions as a natural clock is to see how well it fits into the following criteria for natural clocks:

  1. Irreversible process
  1. Uniform rate
  1. Initial condition
  1. Final condition

The first and final criteria can be said to be fulfilled without too much problem. Milk spoilage is definitely irreversible; once it is spoiled it cannot become usable again, at least not without human intervention. Also, the final condition is known because we can observe the milk, see that it is spoiled, measure the ph of the milk or measure the bacteria content to see how much it has spoiled. It is difficult to determine whether a uniform rate has been fulfilled. Milk spoilage is caused by bacterial growth, which does happen at an exponential rate. Yet there are many factors which could affect the rate at which bacterial reproduction causes milk to spoil. Among these are the amount of bacteria present in the milk initially, temperature at which the milk is kept, and how much (if any) bacteria are allowed to get into the milk after pasteurization. Assuming that these factors can be isolated and controlled, it might be possible to determine a uniform rate at which milk spoils. Initial condition is probably the natural clock criterion that is most difficult to fulfill with milk spoilage. Each batch of milk is different; some have more bacteria than others and may spoil faster. The only way to determine the initial condition of a particular carton of milk (that is, to know the amount of bacteria present after pasteurization) is in a laboratory setting.

Milk spoiling has its definite strengths as well as its limitations as a natural clock. Milk spoilage can only tell time over a relatively short period of time, a matter of days or weeks at most. Not all milk spoils at the same rate, and not all batches of milk are the same, so the uniformity of milk spoilage is a limitation. With the advancement in modern technology and new government standards on milk, these problems may be lessened somewhat. Government agencies usually set standards about the quality of milk and the allowable amounts of bacteria in them. Also, with the advent of more modern refrigeration, there is a much more uniform way in which milk is kept which may limit some of the variability in milk's spoiling time. But milk's strength as a natural clock is perhaps its simplicity. It is easily observable, you can look at a carton of ilk, smell it and taste it to see if it has begun to spoil and to what degree. Using expiration dates or a basic understanding of how long it takes milk to spoil you can get a rough idea of the passage of time. It is also something that people can use and observe in everyday life, since everyone has milk in their refrigerators. It may be a simple remind of how much time has passed since the last shopping trip. So, although milk may not be an incredibly accurate or reliable natural clock, it is interesting to note how something so seemingly ordinary and common can help us keep track of something as abstract as time.