Pectin is a hydrophilic vegetable gum. Widely present in the fruits, roots, stems and leaves of higher plants, it is a component of the cell wall. The pectin content of different plants or different parts of the same plant varies greatly. So far, only a few plants with high pectin content have been found and used as raw materials for industrial production, mainly citrus peels, sunflower trays, and sugar beets.
Pectin is the general term for protopectin, pectin and pectic acid. Protopectin is insoluble in water and only exists in the cell wall; pectin is soluble in water and exists in the cell juice; pectin acid is slightly soluble in water and exists in both the cell wall and the cell fluid.
Chemical Structure of Pectin
Pectin is a macromolecular polysaccharide formed by the polymerization of D-galacturonic acid residues through α-1,4 bonds, with a molecular weight between 50,000 and 300,000. Among them, the carboxyl group of galacturonic acid may be methylated to varying degrees (0-85%) and partially or fully salted. The fully demethylated pectin is called pectin acid; the water-insoluble pectin substance combined with cellulose and hemicellulose in plants before extraction is called protopectin. Protopectin is degraded into water-soluble pectin by protopectinase in plants, and finally decomposed into galacturonic acid under the action of polylacturonase, also known as pectinase and pectinase. The parameter to measure the degree of esterification of pectin is the DE value (Degree of Esterfication), which refers to the percentage of methyl esterification at the C6 position of every 100 galacturonic acid residues in the pectin molecule. Generally, pectin with a DE value higher than 50% is called high methoxyl pectin, whereas pectin with DE value lower than 50% is called low methoxyl pectin. The naturally occurring pectins in natural fruits are all high methoxyl pectins, and low methoxyl pectin can be obtained after acid or alkali treatment of high methoxyl pectin to reduce the degree of esterification. The molecular structure of pectin determines its many physical and chemical properties.
Properties of Pectin
Solubility Properties of Pectin
Pure pectin is white or light yellow powder with a slightly peculiar smell. It is almost completely dissolved in 20 times of water to form a negatively charged viscous colloidal solution, but it is insoluble in organic solvents such as ether and acetone. It is more soluble in water if sucrose syrup is used or mixed with more than 3 times of granulated sugar. Generally speaking, the solubility of pectin in water is related to its own molecular structure. The longer the polygalacturonic acid chain is, the smaller its solubility in water is.
Acid-base Properties of Pectin
Under the condition of not adding any reagent, the aqueous solution of pectin substances is acidic, mainly pectinic acid and galacturonic acid. Therefore, under moderately acidic conditions, pectin is stable. However, under strong acid and alkali conditions, pectin molecules will degrade.
Gel Properties of Pectin
Gelation is the most important property of pectin, and the main use of pectin is as a gelling agent under acidic conditions. Due to the difference in structure between high-methoxyl pectin and low-methoxyl pectin, the gelation conditions of the two are completely different.
High methoxyl pectin solution is required to be in the range of pH 2.0-3.8 and the system must contain more than 55% soluble solids (mostly sucrose), and it can form an irreversible gel after cooling. The principle is that first of all, only when pectin molecules are close to each other to form many binding regions, can the three-dimensional space network of gel be formed, and if the charge of pectin molecules is more, the mutual repulsion between them will be more serious, and the formation of gel will be more difficult. In addition, the degree of dehydration between pectin molecules is also an important factor affecting gel formation. There are a large number of hydrophilic groups on the pectin molecule, which can be fully hydrated in water, and there is a layer of water molecules around the formed single pectin molecule, which also hinders the approach between pectin molecules and cannot form a binding zone. At this time, adding a highly hydrophilic substance such as sucrose to the system will compete with pectin molecules for water molecules, resulting in dehydration between pectin molecules to form a bonding area, which is conducive to gel formation. Therefore, the pH value of the system, pectin DE value, pectin content, soluble solid content and type will all affect the gel formation of the high methoxyl pectin system.
The mechanism of gel formation of low-methoxyl pectin is quite different from that of high-methoxyl pectin. Due to its low DE value, there are relatively more -COO– in pectin molecules, and it is difficult for pectin molecules to form a binding zone only by adjusting the pH value of the solution. At this time, the participation of calcium ions is required. Forms an “egg-box” model binding region. When the gel is formed, it is greatly affected by the concentration of calcium ions, but less affected by the content of sugar and acid. Therefore, the gel condition has a wide range of pH value from 2.6 to 6.8, and there is no requirement for the amount of soluble substances. The general range is between 10% and 80%, and the formed gel is soft, elastic and thermoreversible. The main factors affecting the gel formation of low-methoxy pectin system were pH value, content and type of soluble solid matter, pectin quality, pectin DE value, pectin content and calcium ion concentration, etc.
Application of Pectin in Pharmaceutical Industry
In recent years, the application of pectin in the field of medicine has attracted much attention. Pectin is an excellent matrix for pharmaceutical preparations. As a hydrophilic emulsifier, gel and thickener, pectin can be used alone or in combination with other excipients to prepare pharmaceutical preparations such as ointments, suppositories, and microcapsules. Especially the application in sustained and controlled release preparations is paid more attention. People take advantage of the fact that high methoxy pectin is insoluble in water, acid, alkali and other solvents, and can only be degraded by pectinase in the colon. It is used in the colon-localized drug delivery system to protect the drug from passing through the stomach and small intestine, and localized release in the colon to exert local or systemic effects. This method can be used for targeted treatment of diseases in the colon such as colitis and colon cancer, avoiding the absorption of drugs in the stomach and small intestine and reducing the toxic and side effects of drugs. The residence time of food in the colon of the human body is generally 13 days, so the study of colon-localized drug delivery is helpful to the development and utilization of some sustained-release preparations that can be absorbed in the colon, and the drug system has a requirement for the pH value of the body Therefore, pectin, as the carrier and matrix of pharmaceutical preparations, overcomes the shortcomings of previous sustained and controlled release preparations affected by factors such as individual differences in diet, disease, etc., and has accurate positioning and high utilization rate.
