Oxalates are an antinutritive component of food. They are salts of oxalic acid that are formed and accumulate in the vacuoles of special plant cells, idioblasts. Oxalates occur in plants in various forms. They can appear as soluble salts, formed with Na+, K+ and NH4+, and as insoluble salts formed with Ca2+, Fe2+ and Mg2+. In plant they are most commonly found in the form of calcium oxalates. Calcium oxalates are visible as crystals of various shapes. They have several different functions in the plant. If the plant grows in a calcium-rich environment, oxalates may have the function of regulating the calcium levels in the plant. Oxalates, however, can also serve as a defense mechanism, as crystals can represent static or active defense structures that deter an animal from grazing. Some oxalate-rich plants are important in detoxifying the environment with aluminum and other heavy metals. The content of oxalates in plants can be between 3% and up to 80% of the plant's dry weight. Plants rich in oxalates are usually consumed in small portions in human consumption, but there are also crop plants that are consumed in larger quantities, despite the fact that they contain relatively large amounts of oxalates. These are mainly spinach (Spinacia oleracea) and beetroot (Beta vulgaris var. vulgaris). The oxalate content of food can be reduced by some methods of preparation. This paper presents the forms of crystals formed by calcium oxalates, the role of oxalates in plants and the method of formation, agricultural and ornamental plants rich in oxalates, and the impact of oxalates on human and animal health.
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