Nutritional implications of preparing and cooking grains
Germination
Grains are germinated by soaking the seeds in water for at least 24-72 hours.
Larger sprouts are achieved when seeds are soaked for 7-10 days. The water enters
the seed and activates different enzymes which bring about biochemical changes.
Germination improves the nutritional quality and digestibility of grains:
- Phytic acid and phytate are degraded, making minerals such as zinc, calcium,
phosphorus and magnesium (which are bound to phytate) available for absorption
into the body. Some iron leaches into the water.
- Germination also causes degradation of antinutrients, such as trypsin inhibitors.
- The fat content decreases by 17-60%, whilst the fibre content increases
by 50%.
- Vitamin C and riboflavin content increase substantially in germinating
seeds.
Cooking/Baking
Cooking can affect the nutritional value of grains in several ways:
i. Nutrient content and bioavailability:
- Boiling and extrusion cooking (in the manufacture of breakfast cereals)
can result in losses of around 40% for most B vitamins and 50% of folate.
Boiling milled rice results in significant loss of B vitamins, especially
vitamins B1 and B6 and also of minerals.
- Losses from baking are generally low for riboflavin and niacin, around
5-10% for pyridoxine, 20-30% for thiamin and higher for folate.
- In the production of leavened bread, phytases in the yeast breaks down
phytate and in so doing, increases the availability of minerals. In some parts
of the Middle East, where unleavened bread is a dietary staple, phytate has
been reported to cause zinc deficiency possibly by inhibiting its absorption
into the body.
- During the manufacture of ready-to-eat breakfast cereals, 70% of phytate
is destroyed in puffing and 33% in flaking.
- Toasting destroys thiamin - up to 25% of the thiamin in bread may be destroyed
during toasting, depending on the length of toasting and thickness of the
slice.
- The preparation of tortillas improves niacin availability but results in
losses of thiamin, riboflavin and niacin (60%, 52% and 32% respectively).
Maize contains little niacin or the amino acid trypotophan (precursor of niacin).
The traditional alkali cooking method for tortillas (using a lime cooking
step) increases the availability of niacin and may have prevented pellagra
in populations relying on maize tortillas as a staple food.
- Maillard reactions or nonenzymatic browning takes place when high levels
of heat are applied to sugars such as glucose, fructose and lysine making
the lysine unavailable and hence reducing the quality of the protein (eg.
in the formation of crusts in bread-making).
ii. Increase amount of resistant starch
Depending on the chemical and physical composition of the grain, cooking can
decrease the availability of starch, making it harder to digest and hence increase
the resistant starch content.
iii. Lower the GI of food
Similarly, cooking can decrease the availability of starch, decreasing the
glycaemic index of the grain.
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