Pectins: Structure And Uses

Pectins are complex plant heteropolysaccharides in the plant cell wall which are vital in cell structure and functioning. They are found in all the higher plants and occur amongst other componentry in the intercellular or middle lamellar region between cells, and provide channels for the passage of nutrients and water. In cell walls, pectic substances serve as one of the main agents cementing the cellulose fibrils together and they may be linked covalently to other carbohydrates and even proteins. Pectins influence growth, development and senescence, and affect the textural properties of plant tissues and fruits, especially their sturdiness (Inari et al., 2002).

Pectin makes up between 15 and 45 percent of the dry weight of fruits (Selvendran, 1983).

Pectic substances are classified as protopectins, whereas commercially, the pectins are divided into either high-methoxyl (HM) pectins or low-methoxyl pectins.  The high-methoxyl types are usually industrially defined as either rapid set or slow set pectins depending on their degree of methoxylation (DM) and where the soluble solids or sugar content needs to be above 66% w/w with an acidic pH below 4, for any gelling to occur. The other commercial types are low-methoxyl (LM) pectins which need a 33%w/w solids content and calcium cross-linking to encourage setting and are often found in low sugar jams and the pectic acids.

Chemical Structures Of Pectin

Pectin is a complex heteropolysaccharide with homogalacturonan, rhamnogalacturonan I and rhamnogalacturonan II as the predominant structures (Willats et al., 2001). A number of interesting neutral sugar glycans are also involved (Ovodov, 2009).

Homogalacturonan is composed of (1-4)-linked α-D-galacturonic acid residues. It is the basic pectin structure. Some carboxyl groups are esterified to methyl groups whilst various hydroxyl groups are partially acetylated at the O-2 and/or O-3 groups. There is also some amidation of these carboxyl groups, introduced chemically to alter the gel behaviour. All these groups modify the gelling behaviours of pectins and so have a profound effect on their functional properties.

There are a number of variants:-

Rhamnogalacturonan I is made up of (1-4)-linked α -D-galacturonic acid and (1-2)-linked α -L-rhamnose, which are alternatively combined with each other in the backbone. Some of the rhamnose residues contain side chains, such as arabinan, galactan and arabinogalactan at the 4-O-rhamnose (Vincken et al., 2003).

Rhamnogalacturonan II is considered a highly complex structure. It is composed of a (1-4)-linked α -D-galacturonic acid residue backbone and unique sugars such as D-apiose, 2-keto-3-deoxy-D-manno-2-octulosonic acid (Kdo), 3- deoxy-D-lyxo-2-heptulosaric acid (Dha), aceric acid and others, which are present in side chains attached at O-2 or O-3 of some of the galacturonic acid residues in the backbone (Ridley et al., 2001; Willats et al., 2006).

Degree Of Methoxylation (DM)

Pectins are HM or LM depending on the extent of methoxylation of the galacturonic acid (GalA) residues. A methoxylation level greater than 50% puts a pectin into the HM category.

Pectin Extraction

Good sources of pectin are extracted from apple pomace, citrus peel especially orange and lemon, and sugar beet pulp (Kar & Arslan, 1999). Each source offers a slightly different setting capability and is exploited by product developers looking for slight variations. 

Pectin is commercially extracted from apple and citrus sources using strong acid or alkali. Some of the other potential commercial sources include waste fruit and vegetable peels such as carrot pomace (Jafar et al., 2017).

The quality of the pectin varies in each case.

Apple pomace contains 1.2% w/w pectin (Virk & Sogi, 2007).

Fruits are divided into four groups based on their pectin and acid contents:

(1) Rich in pectin and acid: sour and crab apple, sour plum, jamun, sour orange, lemon, grape, sour guava.

(2) Rich in pectin but low in acid: low acid apples, unripe banana, sour cherry, unripe fig, pear, orange and its peel, grapefruit, ripe guava.

(3) Low in pectin but rich in acid: sour apricot, sweet cherry, sour peach, strawberry and pineapple.

(4) Low in pectin and acid; apricot, ripe peach, raspberry, mainly over-ripe fruit..

Pectin Applications

Pectins are widely used in the food industry because of their gel-forming properties and were recognised for this when different fruit types were used. They are generally used as gelling agents and stabilisers in jams, jellies and acid milk products. Raspberry jam relies on its set for pectins released during the boiling process. Strawberry jam on the other hand does not contain enough pectin in its cell structure and so must rely on added pectin to generate the set. 

Pectins can be blended with other materials such as starch. When they are blended with this material and plasticized with glycerol, they form edible and biodegradeable films that have a range of mechanical properties. Plenty of research papers have been written over the years on pectin films from the Eastern Regional research center of the USDA in Wyndmoor, Pennsylvania USA (Coffin & Fishman, 1993; Fishman & Coffin, 1995; Fishman et al., 1996, 1999).

Nutritional Benefits

Pectins have several biological and physiological functions:-

1. reduction of serum cholesterol (Brown et al., 1999),
2. stimulation of phagocytes and macrophages (Iacomini et al., 2005; Popov et al., 1999),
3. spleen cells proliferation (Wang et al., 2005; Zhao et al., 2006),
4. anticomplementary activity (Samuelsen et al., 1995; Yamada, 1994)
5. inhibition of hyaluronidase and histamine release (Sawabe et al., 1992).

It is also reported that citrus pectin and its modified derivatives have inhibitory effects on fibroblast growth factor signal transduction (Liu et al., 2001; 2002), suppression of LPS-induced inflammatory responses (Chen et al., 2006) and preventive effect on cancer growth and metastasis (Platt and Raz, 1992; Nangia-Makker et al., 2002a & b).

Pectins In Gut Health

Pectins are resistant to digestion in the gut which is why they are very useful as soluble dietary fibre. They are however broken down by enzymes produced by bacteria in the gut and so these carbohydrates make good prebiotic ingredients along with insulin. The enzymes produced are bacterial beta-glucosidases and the substrates are galacturonic acid molecules which are then fermented by the bacteria. In our favour, the bacteria then produce short-chain fatty acids (SCFA), carbon dioxide, water and methane (Cummings et al., 1979). All these products are metabolized further by the bacteria, or excreted, or absorbed in the mucosa of the colon (Pomare et al., 1985).

It is reckoned that about 55% of the energy content of pectin becomes available to its host having eaten it through colonic absorption of SCFAs alone. It is well known that SCFAs entering the portal blood circulation are removed by the liver to serve as an important carbon source (Demigne and Remesy, 1985; Soergel, 1982). That just shows how useful pectin is in the diet.

Pectins And Lipid Metabolism

Pectins have a profoundly beneficial impact on lipid metabolism. It has been known for many years that pectin lowers serum cholesterol levels (Miettinen & Tarpila, 1977;  Demigné et al., 1986;  Brown et al., 1999). The mechanisms by which it does so are many fold. It includes reducing cholesterol absorption (Kiriyama et al., 1969), binding bile salts and increasing faecal sterol losses.

References

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