What is Frozen Food?
Freezing food preserves it from the time it is prepared to the time it is eaten. Since early times, farmers, fishermen, and trappers have preserved their grains and produce in unheated buildings during the winter season. 
Freezing food slows down decomposition by turning residual moisture into ice, inhibiting the growth of most bacterial species. In the food commodity industry, there are two processes: mechanical and cryogenic (or flash freezing). The freezing kinetics is important to preserve the food quality and texture.
Are they Healthy or Unhealthy?
The health properties of frozen foods like parathas, fries, meats, and desserts are questionable. This is because their ingredients will not be the same as freshly made meals. A major concern is the use of partially hydrogenated palm oil which might have hidden, dangerous trans-fats.
Secondly, the sodium content is also an issue. Many frozen meals contain between 700 to 1800 mg of sodium. With the daily recommended maximum dosage of 2300 mg of sodium, it is hard to stay in this limit! This puts people with high blood pressure at further risk.
Not all frozen food are bad
Fruits and vegetables – these two food groups are just as good purchased frozen as they are fresh. In some cases, they may actually be better, because if you keep vegetables and fruits in your fridge for a long time, they lose some of their nutritional value. Whereas, buying them frozen and then defrosting when you want the fruit/vegetable can actually retain more nutrients.
Peas, green beans lose a lot of vitamin C within hours of being picked. When foods are canned or frozen, they are usually processed and packaged within hours of being harvested. This retains their nutrients until they are defrosted which should be just prior their consumption.
Some tips you should keep in mind while purchasing these frozen food items:
Vegetables
Look For
Plain, 100 percent vegetables (they should be the only things on the ingredient list). There’s no nutritional difference among the boxed, bagged, or steam-in-bag plain varieties, so go with whatever you like. Make sure the vegetables haven’t frozen into a lump―a sign that they have been thawed and refrozen, which may have fewer vitamins.
Avoid
Anything mixed with cheese, topped with sauce (such as garlic), or accompanied by pasta or rice. A cup of plain frozen broccoli, for example, has 30 calories, no fat, and 20 milligrams of sodium. Package that same vegetable in a cheese sauce and the calories more than double and the sodium soars to about 600 milligrams.
Fruits
Look For
Again, pure and simple fruit is the best option. The bags will stay good for about two months after they’ve been opened. Look for resealable bags to help prevent ice crystals, which can signal the fruit has become dehydrated and possibly lost some of its flavor or nutrients.
Avoid
Fruits packaged with sugar. Frozen strawberries in syrup, for instance, can contain up to 11 times as much sugar (about 17 teaspoons) as natural, unsweetened ones.
Frozen meals
Look For
The word healthy on the box. It’s a reliable indicator of good nutrition. Select entrées with no more than 3.5 grams of fat per 100 calories, no more than 600 milligrams total sodium, and no MSG or trans-fats.
Avoid
Dishes that pack half a day’s worth of calories into one tray. Some can be 700 calories per serving. While a vegetarian frozen meal may seem as if it’s automatically healthy, that’s not always the case. Some meatless dishes can contain excessive amounts of cheese, which is high in saturated fat and sodium.
