From Wiki:
Quorn is made from the soil mould Fusarium venenatum strain PTA-2684 (previously misidentified as the parasitic mould Fusarium graminearum[35]). The fungus is grown in continually oxygenated water in large, otherwise sterile fermentation tanks. Glucose and fixed nitrogen are added as a food for the fungus, as are vitamins and minerals to improve the food value of the product. The resulting mycoprotein is then extracted and heat-treated to remove excess levels of RNA. Previous attempts to produce such fermented protein foodstuffs were thwarted by excessive levels of DNA or RNA; without the heat treatment, purines, found in nucleic acids, are metabolised by humans to produce uric acid, which can lead to gout.
The product is dried and mixed with egg albumen, which acts as a binder. It is then textured, giving it some of the grained character of meat, and pressed into a mince resembling ground beef; forms resembling chicken breasts, meatballs, and turkey roasts; or chunks resembling diced chicken breast. In these forms, Quorn has a varying colour and a mild flavour resembling the imitated meat product, and is suitable for use as a replacement for meat in many dishes, such as stews and casseroles.
The final Quorn product is high in protein and dietary fibre and is low in saturated fat. It contains less dietary iron than most meats and the manufacturers have not released much information about additives they use to make Quorn resemble meat. Quorn is considered acceptable in small amounts for babies over nine months old but should be introduced gradually. The high fibre and low calorie content is better for adults than babies and too much fibre can cause flatulence. The salt content should be checked before giving Quorn to babies since the salt content varies among products.