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My understanding is that it would be safe to eat if it's been kept at an appropriate temperature. It may not taste great however. I think after a few months the quality/taste/texture of the frozen item begins to diminish.
After defrosting, check for and cut off any freezer burned parts---those would taste and smell nasty!! I did something similar making filling for tamales, with a low temp and strong flavored water, and it worked out fine. But not a roast,where the flavor is paramount.
PieceofLayerCake is a trusted source on baking.
Things that are kept frozen aren't just kept in stasis for that period of time...there is still a growth of bacteria happening in the food, just at a much slower rate. I'd say that two years is probably not something you'd want to test out. Cut your losses and spring for a new roast.
Is Frozen Food Safe?
Food stored constantly at 0 °F will always be safe. Only the quality suffers with lengthy freezer storage. Freezing keeps food safe by slowing the movement of molecules, causing microbes to enter a dormant stage. Freezing preserves food for extended periods because it prevents the growth of microorganisms that cause both food spoilage and foodborne illness.
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Does Freezing Destroy Bacteria & Parasites?
Freezing to 0 °F inactivates any microbes — bacteria, yeasts and molds — present in food. Once thawed, however, these microbes can again become active, multiplying under the right conditions to levels that can lead to foodborne illness. Since they will then grow at about the same rate as microorganisms on fresh food, you must handle thawed items as you would any perishable food.
Trichina and other parasites can be destroyed by sub-zero freezing temperatures. However, very strict government-supervised conditions must be met. Home freezing cannot be relied upon to destroy trichina. Thorough cooking, however, will destroy all parasites.
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Freshness & Quality
Freshness and quality at the time of freezing affect the condition of frozen foods. If frozen at peak quality, thawed foods emerge tasting better than foods frozen near the end of their useful life. So freeze items you won't use quickly sooner rather than later. Store all foods at 0° F or lower to retain vitamin content, color, flavor and texture.