Today’s installment of Ask a Microbiologist comes from a reader wondering what might be in that old almond milk in the fridge:
“I found an open container of almond milk in my fridge the other day and it was OLD – I’m talking it had been opened for at least a month at this point. I know that as soon as it’s opened, the pasteurized almond milk is immediately primed for bacterial breeding. I was just curious as to which pathogenic (entero) bacteria are most likely to colonize at this point; I’m curious because there were zero indications of growth (ie no swelling of the container and no abnormal smells, colors, or textures).”
-Wondering what’s in there
Well WWIT, I’m glad you asked this, as I was starting to wonder about my soy milk as well.
This is a very open ended question and three main ways to inoculate your drink that could potentially lead to different contaminating microbes, some of which could be pathogenic.
First scenario: You drank from the carton. It is now contaminated with any of the multitude of bacterial species that live in your mouth.
Second scenario: Tiny environmental bacteria from the air got in, or even fungal spores. At this point it is much harder to know what will be in there and I can’t give a good answer as to what they would be.
Third scenario: The container was physically inoculated by contact between unwashed hands and a surface exposed to the almond milk such as the spout. It could now be contaminated with any of the many species that live on the skin and anything else on your hands.
Since the second scenario is less likely than the first, we’ll stick with the “someone drank from the carton” side of this question before tackling the third. There is no disputing the fact that a human mouth is actually a pretty gross place in terms of sheer numbers of bacteria. There are literally billions of microbes1 representing hundreds2 of species of normal bacteria in your mouth, many of which could make your almond milk go bad.
However, the transition from a 97.8 °F mouth to a 38 °F refrigerator can be growth limiting for many species so growth potential is somewhat limited. While the large majority of oral species are not particularly dangerous, there are members of pathogenic genera in the oral cavity which include Streptococcus, Staphylococcus, Neisseria and Campylobacter. Whether or not these also have the potential to colonize almond milk and successfully grow at low temperature to the point of inducing illness when consumed I don’t know.
Of course there is also the possibility of somehow manually contaminating the spout of the container with unwashed hands, which significantly changes the species that may be there. While the normal flora from your skin isn’t cause for undue concern, unwashed hands bring the risk of fecal contamination. Due to the nature of fecal-oral transmission you could expect to see Escherichia coli, which could potentially grow at lower temperatures in the almond milk. Given weeks on end to grow in the container, it could then possibly induce illness when ingested.
I hope that this gives some insight as to what might be in that almond milk in the fridge. Nothing particularly exotic would end up in the almond milk, as all of these species are a regular part of us and our normal flora. Still though, even if it’s unlikely to be pathogenic I wouldn’t drink it the moment something looked or smelled off about the carton.
In the spirit of honesty, my soy milk this morning was embarrassingly old but I still had some in my oatmeal. I just make sure to never touch the spout or drink from the carton and always keep it properly refrigerated.
Works Cited:
1. From: Chapter 6, Normal Flora Medical Microbiology. 4th edition. Baron S, editor. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Copyright © 1996, The University of Texas Medical Branch at Galveston.
2. Aas, J. A., Paster, B. J., Stokes, L. N., Olsen, I. & Dewhirst, F. E. Defining the normal bacterial flora of the oral cavity. Journal of clinical microbiology 43, 5721–32 (2005).
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K.D.Shives 
I was looking around the net about almond milk in general, and found this forum. I actually have some fermentation projects at home for making vinegar. Oddly enough, tonight I decided to try mixing almond milk with dextrose and diluting it with water to ferment it with some wine making yeast. It was fresh from the carton and put into a sanitized jar, then I covered the jar with cheese cloth and left it in a closed kitchen cabinet at around 70 – 80 degrees F. Will this likely curdle or sour and become rancid or will the mixture actually turn into a safe product in which to add a Mother to?
That’s a good question. If you used wine-making yeast you can expect ethanol fermentation, but the fact that you used an open top with cheesecloth means you don’t have an anaerobic environment that the yeast prefers for fermentation. I personally would not drink this as it sounds like there is a high likelihood of spoilage, but with all experiments you won’t know the outcome until it’s finished. Let me know what it turns into and how it smells!
We buy the friendly farms almond milk from Aldi. We noticed about a month ago a red thick liquidy film growing on a pot in our sink we had made mac n cheese with. Then in a cup we had mixed the milk and protein. And then cereal bowls. We checked an empty carton realizing it must be the milk and it was covered inside in the red stuff. We had always bought this brand of milk and we saw something called serratia in our google searching and started freaking out. We still see a pink film on certain dishes after washing them and new dishes even though we have scrubbed and sanitized the sink. Thank you