Nutrient losses are induced by a number of factors, including cooking method, temperature, and cooking time. It’s also dependant on the type of food, variety of vegetable, and even influenced by the way ingredients are cut and prepared.
Historically, research has focused heavily on vitamin C losses in cooking because it seems to be the most fragile micronutrient, being influenced by oxidation and leaching. Using vitamin C and other water soluble and heat sensitive vitamins as indicators, we see that boiling and poaching (in that order) result in the most significant nutrient losses. Boiling leads to a massive 35-60% loss in nutrients. In comparison, pressure cooking and frying account for only 5-10% nutrient loss. Steaming accounts for 14%. The average nutrient loss across all cooking methods is 10-25% in most cases.
Certain vegetables retain their nutrients more readily than others. Broccoli, for example, loses 38% of its nutrients when boiled and only 5% when steamed. Compared these numbers to those of spinach, which loses 60% when boiled and 11% when steamed.
However, cooking food is also associated with certain benefits. The first of these is that cooking destroys anti-nutrients that ordinarily prevent absorption of nutrients within our bodies. For example avidin is an anti-nutrient in raw egg which prevents the absorption of biotin (a B-vitamin used in energy production and in supporting healthy hair and skin). Cooking also increases fibre content in some starches as the starch becomes resistant. It also seems that bioavailability (how a nutrient is absorbed) may increase for certain nutrients, such as beta-carotene, due to cooking.
Seymour always aims to use the most appropriate preparation and cooking methods to retain nutrient quality and quantity.
We use a software program called Apicbase that calculates the food composition of each of our recipes. This program reports nutient composition on uncooked food items. However, in light of the fact that we use best practice to retain nutrients we would assume and apply a 5% - 10% nutrient loss in our reporting. In Apicbase we account for water losses in methods such as boiling and fat changes in methods such as frying.
Additionally, we have set up a process whereby individual menu items are sent to certified laboratories for spot-check analysis to confirm our Apicbase reports. In this way we can be confident in the information that we send you and in the claims that we make about our food.
( Note: we have a related position statement on the use of microwaves.)
Selected references:
Comparison of vitamin losses in vegetables due to various cooking methods
https://pubmed.ncbi.nlm.nih.gov/2081989/
Nutrient losses and gains during frying: a review
https://www.tandfonline.com/doi/abs/10.3109/09637489809089395
https://www.beyondveg.com/tu-j-l/raw-cooked/raw-cooked-2e.shtml
Vitamin C: optimal dosages, supplementation and use in disease prevention, July 2015, Functional Foods in Health and Disease volume 5(issue 3):89-107, DOI:10.31989/ffhd.v5i3.174