I am unplugging my refrigerator for the winter months, I live in the northern United States so the temperature is sub freezing most of the time during winter. I have a semi-insulated garage that is about 40 degrees F and am placing refrigerator items in a box there. For freezer items I have started with a cooler placed in the snow outside. My concern is, is there a risk of bacterial contamination with food placed outside? I am also worried about animals visiting, any suggestions?
A few things to keep in mind.
Food safety for unrefridgerated food is a very complex thing and the answer very much is "it depends." Factors include temperature, time in the so-called danger zone (40-140F), pH, other bacterial activity, salt content, and much, much more.
In general a few guidelines:
If your food stays between 32F and 40F you can treat it as refridgerated.
Frozen food itself is not in significant danger but you may need to watch for thaws.
Between 40F and about 60F there are a significant number of things you can do to keep food edible and safe. First and foremost, uncooked food can be subject to moderate salting and then lactic fermentation (sauerkraut, salami, yogurt, cheese, traditional pickles, etc). Note that these are all live, cultured foods and the combination of the pH and living culture are important. Additionally many have added salt which aids preservation too. Finally I would note that if you make traditional butter, you ferment the cream first and then salt it, and this way the butter will keep more than a few days. (Edit to clarify: Many, though not all, of these foods become shelf-stable at higher temperatures after fermentation too. Examples include cheese, where well-aged cheese can be left out at room temperature without food borne illness risk.)
Chances are you aren't going to be above those temperatures so that should suffice. If you are above those temperatures, alcoholic fermentation of fruit is possible and lactic fermentation, while possible, requires a great deal more salt.
Other concerns: Yes, you do need to protect from animals and some animals are pretty crafty. Ravens and racoons, for example, can easily get into all kinds of things. Both these have been known to get into closed coolers.
In general you should assume low-grade contamination for food to be reheated. The question is preventing it from being a problem (cold storage and refridgeration). I don't see additional risk in being in a closed container well below freezing, but when it thaws, that could change fast. You need to keep an eye on things.
When you use a refrigerator, very nearly all the energy used by your refrigerator ends up as heat in your home. During winter, that means that the real cost of using your refrigerator is the difference between heating through electricity and heating through your normal heating system for that amount of energy.
If you store food outside, you'll lose a lot heat from your home every time you open the door to retrieve it, which all has to be replaced. Minimaly, you'll also lose the heat stored inside the food. More importantly, you are at a much greater risk of losing food due to predation and thawing. These are in addition to any resources spent on preserving food.
My suggestion is to not do it. If you want to reduce your energy costs or your footprint, reduce heating in your home and/or improve insulation.
I have an answer to operating a refrigerator with a freezer in an unheated, uninsulated garage in winter. Originally my freezer would at times get near thawing or 32 degrees because the refrigerator compressor was not running (refrigerator at or below 34 degrees). I am currently testing a second generation two stage controller that adds the precise heat needed to maintain safe and efficient operation. There is always some concern about the impact of the cold on the closed refrigeration system but I have not seen or heard of many problems.
By next Fall I hope to have someone producing and marketing the two stage system. It works here in MD. I check temperatures of refrigerator and freezer several times each day - no problem. Temperatures are where you would expect them to be.
There is one poor solution provided for models of several manufactures refrigerators that seem to work by heating (tricking) the thermostat, but no one talks about how it messes up the cooling the rest of the year. Some people have added a summer/winter switch to improve operation - still not a great solution. I suspect that is why most manufactures do not have a modification for using refrigerators/freezers in an unheated garage.