Sometimes a whole tree produces either striped or blushed fruit, but sometimes both types are found on the same tree and even within a single cluster. There has been quite a bit of research with 'Honeycrisp' to learn why some apples are striped whereas others are blushed. Apple has 17 sets of chromosomes and so far genes for skin color have been identified on chromosomes 2 and 9. For three of the first six steps in the process the genes are controlled by light and for three steps the genes are controlled by both light and temperature, but the gene involved in the final step is controlled primarily by temperature. Each step in the process is catalyzed by a different enzyme and each of these enzymes is controlled by one or two of the three genes. Recent research has shown that anthocyanin synthesis in apple skin and flesh is controlled by a family of genes. The precursor for anthocyanin is the amino acid phenylalanine and there are seven biochemical steps required to synthesize anthocyanin from phenylalanine. The biosynthetic pathway of anthocyanin is fairly well understood. When peaches are exposed to water with a high pH, streaking or inking of the skin often develops as a dark blue or nearly black color. The color of these glycosides can change if the pH of the cellular solution is modified in high pH the color is blue and at low pH the color is red. These glycosides can range in color from orange to blue to purple. Anthocyanins are glycosides, which are organic molecules containing sugar molecules and of the 6 common glycosides the most common is cyanidin-3-galactoside. Anthocyanins are antioxidants and may protect plant tissues from UV light and high temperatures. Anthocyanins belong to a class of odorless molecules called flavonoids and taste moderately astringent. In contrast, the red pigments in peppers and tomatoes are carotenoids and remain red when cooked because they are not water soluble. More about anthocyaninsĪnthocyanins are water soluble and that is why they become bleached when cooked. The redness of the skin depends on the proportion of these cells containing anthocyanin. The apple skin has two types of cells: the epidermis is 2 or 3 cells deep and under the epidermis there are 6 to 10 hypodermis cells. At the same time anthocyanins may increase up to 5 fold. As apples mature, chlorophyll is degraded and carotenoids increase as chloroplasts transition to chromoplasts. Anthocyanins are red, blue or purple and are located in the vacuoles in the cell. Carotenoids are yellow, orange or red and are located in chromoplasts within the cells. Chlorophylls are the green pigments responsible for photosynthesis and are located in chloroplasts within the cell. There are three groups of pigments in apple skin and the concentrations of all three pigments change during the season. ![]() To maximize red color, we can influence and manipulate some, but not all of these factors.
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