Does adding protein make it spoil faster?

I add 3 scoops of casein powder to each bottle (one bag) of Soylent. Normally I make it at night and then eat it for breakfast and dinner the following day and breakfast the day after that. This gives me a total time of about 32 hours in the fridge for the last meal. However, I’ve noticed that if that is stretched out to something more like 48 hours, the taste changes and it seems to be more bitter.

I’m wondering if this is a problem (i.e. the fats went rancid or the protein spoiled) or whether it’s ok to drink but just tastes worse. I know that Soylent is ok to be kept for 48 hours, so I’m guessing it must have to do with the protein? It doesn’t smell bad, that’s the only reason I think it may just be fine. And the taste is certainly worse and is bitter, but I can still drink it and have done so a couple times without any consequences that I know of. I don’t really have time to mix individual meals separately, mixing 3 at once is one of the things I enjoy most about Soylent.

Any insight into whether this is a problem?

My first thought is to get a fridge thermometer and make sure your fridge is as cold as you think it is.


I’ve no idea how long mixed casein powder will last, but it couldn’t hurt to make sure you’re cleaning your pitcher completely, including both silicone O-rings. Any residue could harbor bacteria.


Yes…and to the OP…casein protein is not exactly the same as every other protein. So your results may vary with other proteins.

I’ve been adding ~60g of whey protein powder to my batches for the past couple months, both with v1.3 and now v1.4, and haven’t had any problems with spoilage. I typically use up each pitcher in under 36 hours, but I’ve gone over 48 hours a couple times and haven’t noticed a difference.

+1 to @wezaleff’s point to be sure your’e cleaning stuff out well, I lightly wash the pitcher between each batch, and about every third batch (~once per week) I thoroughly clean everything including removing the O-rings. Even after just three batches I’ll sometimes see the beginnings of something growing on the screw-top’s ring.

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It’s really hard to beat 3 days in the fridge for spoiling time. That’s what Soylent does out of the pouch. I don’t think you could make liquid soylent more hospitable to bacteria if you tried, although that does sound like a fun experiment. Durability after mixing is not, and probably never will be a priority for soylent, and probably shouldn’t be.