Fruits and vegetables begin to lose their nutrients as soon as they are harvested. Any change in pH and temperature, exposure to light and air, and food preparation techniques such as cutting and blending will affect the nutritional value of a food. Cutting or blending a fruit or vegetable increases its surface area. This means more of the nutrient contents are exposed to air and light.
Time also plays a role in the loss of valuable vitamins. The longer the time between when a food is prepared and when it is consumed the more nutrients it will lose. Your best bet is to prepare your smoothie as close to the time you plan to drink it as possible. But if that simply is not an option, store it in an airtight container and prepare it as late as possible to reduce the amount of time between when you blend it at night and have it in the morning.
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