Frozen food is more nutritious than you think.

Quick-frozen food adopts a relatively unique freezing process. Compared with the general freezing process, it can quickly reduce food temperature to minus 18°C. Due to the rapid cooling rate, the water in the food will enter a supercooled state, and the ice crystals formed at this time are very small, so small that they will not seriously damage the food cells and preserve the original taste of the food to the greatest extent.
Moreover, due to the low temperature, cell activity is stopped, and the activity of microorganisms is also greatly restricted, so quick-frozen foods can usually be stored for a long time.
Meat mainly provides protein and minerals. In extremely cold conditions, protein and mineral machinery do not change.
When fruits and vegetables are picked, they are still undergoing biochemical activities such as respiration. Before entering supermarkets and farmers' markets, they have to go through transportation, storage, and other processes and reach consumers' homes, often after a long journey. Nutrient loss is inevitable.
Frozen fruits and vegetables are quickly frozen at -18°C as soon as they are picked. At this temperature, the respiration of fruits and vegetables is almost stagnant, and microorganisms cannot grow and reproduce, which in theory is even more conducive to the retention of nutrients.
Even many studies have found that quick-frozen vegetables and fruits have their advantages in nutrition: The USDA survey results show no significant difference in the nutrient content of frozen fruits and vegetables compared with fresh ones. In some cases, the preservation rate of vitamin C, B, and carotenoids in frozen fruits and vegetables is even higher. 

Frozen food is more nutritious than you think. PDF