True chocolate, made with cocoa butter, especially of the non-milk variety has a very long shelf life when stored in dry, cool conditions. The fact that the shape is a chip rather than a bar or disc or callet is not really relevant, except for the total surface area on which blooming can occur.
Chocolate is very, very dry, which discourages mold, bacteria, or other micro-flora or micro-fauna from growing, not even counting the preserving effect of theobromine and other alkali in the chocolate.
It also resists rancidity very well. Stored properly, it can last for many years safely.
Many brands of chocolate chips are not true chocolate, because the manufacturer may have used less expensive fats than cocoa butter, which do not have such excellent storage properties. Assuming you have a quality chocolate chip made from real chocolate, the issues you may find include:
- Sugar bloom, where the sugar comes to the surface due to moisture dissolving it and then leaving it on the surface when it evaporates
- Fat bloom, where the cocoa butter separates out onto the surface for reasons not thoroughly understood
- The chocolate can lose its temper from warming and cooling cycles, if it gets too warm; this will change its texture to softer, more gritty, and less pleasant
- The cocoa butter could go rancid (which I have never experienced)
The first three are aesthetic and cosmetic issues, but the chocolate can still be used in recipes or melted down and re-tempered.
Having it go rancid would be cause to throw it out--but chocolate is very hardy, and resists rancidity--so if it tastes fine, it is still usable.
After two years, you are quite likely to have experienced bloom. The chips will look like they have a white coating, and may feel gritty. While this makes it less pleasant to eat out of hand or in applications where it won't be melted down, it will still perform well in recipes where it is melted.
They are certainly safe to eat (again, assuming real chocolate, and no rancidity), but they may not have the same pleasant texture and crispness that you would expect. This is why manufacturer's give them a best by date.
Note: you can tell whether they are real chocolate by the ingredient list (at least in the US, and other places with comparable labeling laws). Real chocolate will consist of:
- Cocoa solids, cocoa, cocoa mass, cocoa butter, cocoa nib, or chocolate liqueur (all words indicating products of the cocoa bean)
- Sugar
- Flavoring (such as vanilla or salt)
- Perhaps lecithin as an emulsifier
Signs that the product is not real chocolate include other ingredients, especially other fats in lieu of cocoa butter (which is comparatively expensive, and marketable to the cosmetics industry).
Milk chocolate, which also contains milk solids and milk fat will not last as long, but still probably has a shelf life measured in years when stored under proper (cool, dry) conditions.
White chocolate chips do not have the additional preservative effects of the cocoa solids and their alkali, and also have dairy solids and possibly milk fat, so they have the shortest shelf life of all. It also tends to pick up off flavors if not in a perfectly sealed, air tight container. White chocolate, I would not keep more than year or so.
Of course, chips that are made from other ingredients than true chocolate are going to have a shelf life based on their ingredients, but I cannot speak to that, and anyway, in my mind, they are not worth storing.
See also:
Why does dark chocolate turn white after being in cold for some time?
Yes, it differs by cheese.
As a very general and vague rule, the softer the cheese, the less time it will last. Large chunks may last longer than small or grated pieces.
A ripe Camembert is at its peak for hours. A nice chunk of Parmesan may last for weeks.
Dairy Googness summarizes as follows:
- Fresh cheese: a few days to 2 weeks
- Soft cheese: 1 week
- Semi-soft cheese: 2 to 3 weeks
- Firm cheese: 5 weeks
- Hard cheese: 10 months
Note: my opinion on the above is that the firm and hard times are a little long for opened cheese.
Eat By Date is more conservative, suggesting only up to a couple of months for opened hard cheeses.
You can also freeze cheese with minimal loss of quality, which will substantially increase its shelf life.
Best Answer
I regularly make passata (tomato puree) home, and use a simple hot bath sterilisation (instructions below). I can usually keep it for about a year, but the result is not consistent and not 100% reliable: every 20 bottles I make, one leaks and gets spoiled. There are more sophisticated sterilisation methods that use a pressure pan (similar to an autoclave in the lab). Both of these methods rely on killing the bacteria naturally present in the preparation as well as in the residual air inside the can / jar.
However, as pointed by @Jefromi none of these techniques are 100% reliable for home preparation.
To be certain, you should use an acidity regulator. Lowering the pH of your sauce will prevent a big class of bacterias to develop. A cheap option which is commonly used in the industry is citric acid, which you might find in supermarkets and surely in brewing shops. If not available, you could also use vinagre or lemmon juice (less ideal). In any case, note that acidity alone will not do the job. Always use together with sterilization.
For a reference, see this link for tomatoes in general, and this one for tomato paste specifically (thanks @Jefromi for these references).
Simpler water sterilisation process:
For this to work well, you need to be sure the lids are tight and no air can enter/scape. Be attentive to any leakage and use good jars.