Hi.  My wife keeps telling me that I shouldn't mix the last leftovers, say from a container of hummus, with the new one.  I say there is no reason not to.  What do you think?
- Jake

There is good reason behind your wife's concern, and that is that you are mixing food that has already had a chance for bacterial growth into new food, thus accelerating the time at which it will go bad.

Take as a hypothetical example a fresh 16 fluid ounce container of store-bought hummus.  Let's say, for the sake of discussion, that there is one bacterium per tablespoon of hummus (no food is ever completely bug free), that the bacteria multiply once every 6 hours under refrigeration, and lets just say that once there are more than a billion bacteria per tablespoon you are likely to get sick from eating it.  Remember, these are just made up numbers for the example.  Real numbers would depend on what bacteria, what food, what temperature, and a host of other factors.

So, after 6 hours in our theoretical hummus there will be 2 bacteria per tablespoon, after another 6 there will be 4, and so on.  After a week, there will be 2²⁷ = 134,217,728 bacteria per tablespoon, give or take a few million. You may be OK if you eat it.

Now, let's suppose that you take one tablespoon of leftover hummus from the 7-day old container, and stir it into another new one.  Now you have "diluted" the old hummus, and raised the per tablespoon bacteria count in the fresh stuff from one to a mere 4,194,304.  After a week of sitting around in the fridge, there will be 1,125,899,906,842,620 bacteria per tablespoon, a million times our hypothetical limit. Not good!

Do the same thing again, and you will have 35,184,372,088,832 bacteria per tablespoon at the start of the week, and around 9½ sextillion bacteria per tablespoon at the end of the week.

OK, so you get my drift, whatever the real numbers may be.  Mixing old food into new makes the new food go bad quicker, and doing it repeatedly just accelerates the rate at which your food goes bad.

Your best bet is to either throw away the last little bit of any old container, whether it's hummus or something else, or eat it separately (assuming it hasn't gone bad), before digging into the new stuff.