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.