Disadvantages
Some vitamins and other trace nutrients are lost, particularly from vegetables, partially by enzyme action during cooking and partially due to heat degradation. When vegetables are cooked at higher temperatures these enzymes are rapidly denatured and have less time in which to act during cooking. Since slow cookers work at temperatures well below boiling point and do not rapidly denature enzymes, vegetables tend to lose trace nutrients. Blanched vegetables, having been exposed to very hot water, have already had these enzymes rendered largely ineffective, so a blanching or sauteing pre-cook stage will leave more vitamins intact. This is often a smaller nutrient loss than over-boiling and can be lessened to an extent by not removing the lid until the food is done.
Raw kidney beans, and to a lesser extent some other beans (such as broad/fava beans), contain the toxin phytohaemagglutinin, which is destroyed by boiling for at least ten minutes, but not by the lower temperatures of a slow cooker, so dry beans should be either boiled prior to slow cooking to avoid poisoning, or alternatively, leaving the beans to soak in water overnight, sprouting them, will neutralize the toxin and increase nutritional values. Even a few beans can be toxic, and beans can be as much as five times more toxic if cooked at 80 °C (175 °F) than if eaten raw, so adequate pre-boiling is vital. Cases of poisoning by slow-cooked beans have been published in the UK; poisoning has occurred in the USA but has not been formally reported. This risk can be avoided entirely by using canned cooked beans, adding them towards the end of the recipe's cooking time.
Slow cookers do not provide sufficient heat to compensate for loss of moisture and heat due to frequent removal of the lid, e.g., to add and remove food in perpetual stews, (pot au feu, olla podrida). Added ingredients must be given time to cook before the food can be eaten. If food is allowed to cool below about 70 °C (158 °F) and not reheated, harmful bacterial growth is possible; some bacteria produce toxins or spores which are not destroyed even by reheating.
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