On a hot summer day, enjoying a glass of fresh fruit and vegetable juice can be both fashionable and healthy. Fruits and vegetables come in various natural colors, and when made into juice, they should be vibrant and beautiful. It is even more appealing when served in a beautiful glass. However, one frustrating thing when making fruit and vegetable juice is that the pulp or juice turns dark brown within minutes to tens of minutes, which can be quite unappetizing.
Why does fresh fruit and vegetable juice turn brown?
This is because many fruits and vegetables contain "polyphenols" and "phenol oxidases". Normally, these two substances are separated in different compartments of the cells and cannot react with each other. However, when the cells are disrupted during juicing, these two substances come into contact with each other. Combined with the abundant oxygen in the air, the phenol oxidases quickly catalyze the oxidation reaction of the colorless polyphenols, resulting in the formation of colored "quinone compounds". These quinone compounds can aggregate with each other, causing the color to become darker and darker. It starts as pink, then turns into light brown, brown, and even black. The discoloration of potatoes, sweet potatoes, eggplants, apples, and other fruits and vegetables all stem from this reason.
Why don't commercially sold fruit juices, vegetable juices, and frozen french fries appear brown?
After understanding this principle, some people may wonder: why don't commercially sold fruit juices, vegetable juices, and frozen french fries appear brown? This is because special protective measures are taken during the juice-making process. There are a few methods:
First, a short "blanching" in boiling water is performed to deactivate the phenol oxidases, preventing them from catalyzing the oxidation reaction.
Second, a small amount of harmless substances such as sulfites, citric acid, and vitamin C is added during juicing. Some of these substances can inhibit the activity of phenol oxidases, some can bind with metal ions that promote oxidation, and some can reduce the oxidized colored compounds back to colorless substances.
Third, some of the brown-colored substances are removed through adsorption. Through these efforts, the product can maintain its beautiful color.
Although it is not feasible to use such complex methods for homemade fruit and vegetable juice, the same principle can be applied by adding some vitamin C (by crushing vitamin C tablets purchased from a pharmacy) or blanching pears or apples in boiling water until partially cooked before juicing. However, this method should not be used for juices that tend to develop off flavors after heating.
Although browned juice may not look appealing, the substances that cause the discoloration are actually antioxidant health components. So, don't discriminate against these fruits and vegetables just because their color may turn brown. In addition to color changes, these components may also cause juice to become turbid and form sediment over time. However, these are natural phenomena and do not produce harmful substances that affect health.
Of course, the simplest way is to choose fruits and vegetables that are less prone to browning for juicing. These include citrus fruits, strawberries, tomatoes, carrots, mangoes, watermelons, and so on. They also undergo browning, but at a slower rate compared to apples, taking several hours or even longer to show noticeable changes.
