Factors that affect nutrient loss in cooking include cook time, temperature and amount of liquid used. As each of these factors increases, nutrient loss increases. Because microwaving generally includes a short cooking time and uses less heat, it retains more nutrients than conventional methods. Cooking spinach in the microwave is a fast and simple preparation method that retains most of the nutrients.
Nutrients in Spinach
The USDA Nutrient Database for Standard Reference lists the following vitamins and minerals in significant amounts in raw spinach: calcium, iron, magnesium, potassium, folate, vitamin A, ascorbic acid (vitamin C), beta-carotene, lutein, zeaxanthin and vitamin K. Cooking spinach does slightly reduce the nutrient content, so try to keep cooking times short to preserve the most nutrients.
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Nutrients Lost During Processing
The macronutrient composition of spinach -- carbohydrates, fat and protein -- is not changed during processing, but vitamins and minerals can be lost. The most sensitive of these vitamins and minerals include water-soluble vitamins A and K and heat-labile vitamins. A study published in the "International Journal of Food Properties" by A. Kala and Jamuna Prakash showed that cooking spinach in the microwave caused a significant difference only in the ascorbic acid and beta-carotene content.
Microwave vs. Conventional
Because microwave cook times are short, microwaving spinach is a good cooking method to preserve the most nutrients. According to the study in the "International Journal of Food Properties," there was no difference in iron, phosphorus or calcium content of spinach in the three different preparation methods: conventional boiling, pressure cooking and microwaving. A Cornell University study cited in "The New York Times" showed that spinach retained nearly all of its folate when cooked in a microwave, compared to a 77 percent loss when cooked on the stove.
Benefits of Microwaving
Microwaving spinach requires no added water, which preserves more of the water-soluble vitamins A and K found in spinach. Additionally, microwaving requires very short cook times, retaining many of the nutrients that are lost over time in conventional methods such as boiling. While cooking spinach may leak some vitamins and minerals, it is a safe preparation method that eliminates harmful bacteria and prevents food poisoning.