MadSci Network: Biochemistry
Query:

Re: Will EXPIRED fresh milk work as an alternative to water in watering plants?

Date: Wed Oct 8 13:33:05 2003
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology
Area of science: Biochemistry
ID: 1065625750.Bc
Message:

Hi Valerie,

You've brought a new spin to an old question by asking about using expired milk to water plants. Good job! Unfortunately, I don't think that the status of the milk regarding its expiration date is going to make any difference as far as a plant is concerned. We already have a number of answers to questions about watering plants with milk in the MadSci Archives. You can find those answers here, here, here, and here. To summarize, the sugars and salts in milk make it more difficult for plants to absorb water with their roots, and the sugars, fats and proteins in milk make it easier for bacteria and fungi to infect plants.

First off, I should say that the expiration date for milk is not a well defined value, like the volume of a liter, or the 4 degree celcius temperature or a refridgerator. Expiration dates are set by the local government, and it is likley that they differ from country to country. In addition, it isn't always clear if the date on a container of milk is the "sell by" date, which is a guide to the vendor, or a "drink by" date, which is a guide to the consumer, or something else. In general, it does seem to be recognized that milk will go bad after a certain time, and that it should not be consumed after that time. After this time, milk is considered to be spoiled.

That being said, there are a few other answers in our Archives that describe the changes that occur when milk spoils. You can read those here and here. As you know, unspoiled milk is basically salts, sugar (lactose), fat, protein (casein) and water. The difference between unspoiled and spoiled milk is the result of the action of bateria. Lactococcus and lactobacillus bacteria convert the lactose sugar in milk into lactic acid, making the milk sour. This is generally the taste that people associate with spoiled milk. Other bacteria can digest the fats and casein, producing compounds that are even more distasteful than the lactic acid.

So, assuming that expired milk is spoiled (or on it's way to being spoiled), it seems like it would be just as bad for a plant as fresh milk.


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