Heat treatment may reduce the amount of available nutrients in foods, especially the Vitamins. Some of them are destroyed during processing. Nutrients can be added back through fortification. The popular fortified products are rice, noodles, fruit juice and canned goods.
On the other hand, fermentation can do a reverse effect. It may increase the level of nutrients in final product. I read this information from Fermented Fruit and Vegetables, A Global Perspective, by Mike Battcock and Sue Azam-Ali
Fermentation processes can result in increased levels of vitamins in the final product. Saccharomyces cerevisiae is able to concentrate large quantities of thiamin, nicotinic acid and biotin and thus form enriched products.
Sorghum beer in Southern Africa contains relatively high levels of riboflavin and nicotinic acid, which are important for people consuming a high maize diet. Pellagra (a vitamin deficiency disease associated with high maize diets) is unusual in communities in which sorghum beer is consumed. Even children benefit from consuming the dregs which contain relatively little alcohol but are rich in vitamins.
Palm wine in West Africa is high in vitamin B12, which is very important for people with low meat intake, and who subsist primarily on a vegetarian diet.
Pulque (a fermented plant sap) is an important source of vitamins for the economically deprived in Mexico. The fermentation process involved in Pulque production increases its vitamin content. For instance the vitamin content (milligrams of vitamins per 100g of product) of pulque increases from 5 to 29 for thiamine, 54 to 515 for niacin and 18 to 33 for riboflavin (Steinkraus, 1992) during fermentation.
Idli (a lactic acid bacteria fermented product consumed in India) is high in thiamine and riboflavin.