Pectin

 -Chemistry of Carbohydrates and its Application in Food System

Introduction

Pectin is a carbohydrate polymer, with a relatively high molecular weight and is found naturally in most plants and particularly in fruits (Flutto, 2003). Fruits such as berries, citrus and apples contain naturally occurring pectin. Pectin comes from the Greek word pektos which means firm and hard, reflecting its ability to form gels and stabilize products (Flutto, 2003). For centuries pectin has been known for its gelling properties, but it was not until the beginning of the twentieth century that it started to be used commercially (Flutto, 2003). 

Pectin is classified as a complex carbohydrate that is relatively soluble in water (Kent, 2016). It is a polysaccharide material and found in fiber within the cell walls and lamella of most plants (King et. al, 2013). Pectin also contributes to the cell structure of plants.

Structure/Chemistry of Pectin

Pectin is known as a pectic polysaccharide that is rich in galacturonic acid (GalA). Different types of plants produce a number of different polymers of pectin and have different functional properties. The different polymers of pectin vary depending on the molecular weight, amount of natural sugar and chemical configuration. Pectin contains a homogalacturonic acid backbone that is made up of a galacturonic acid chain (Figure 1) that is partly esterified with methyl esters and linked by -1,4 glyosidic bonds (Thibault et al., 1993; Zhan et al., 1998). The distribution of esters are critical as it affects the charge density of the polymer. When the molecules interact with other molecules such as calcium, protein or other pectins it will reduce the repulsion between molecules. Acetyl groups are also important in the classification of specific pectin types.

Figure 1

Figure 1: pectin structure consisting of long sequence of anhydrous galacturonic acid and esterified methanol

The amount of galacturonic acid (GA) present in the entire molecule is known as %GA. To be qualified as a food additive it must have a minimum of 65% GA (Flutto, 2003).  The amount of galacturonic units that are esterified are known as degree of esterification (DE), for high-ester pectins (HE) should be great than 50% and for low-ester pectins (LE) should be less than 50%. The total units amidated in the entire molecules is known as degree of amidation (DA) and is regulated to a maximum of 25%.  

There are two methods of producing pectin, precipitation and amidation. Precipitation is normally used for HE and low-ester conventional pectin and amidation is used to prepare low ester amidated pectin. Using the amidation process pectin mixture is amidated with ammonia and this forms galacturonamide units in the chain and is an important aspect in commercial pectin.     

Another component of the backbone are neutral sugars. Areas in the backbone where neutral sugars are present are known as hairy regions and areas without are called smooth regions (Figure 2). The sugars form short segment side chains. Examples are d-galactose, L-rhamnose, L-arabinose and D-xylose. Some neutral sugars are a part of the side chains, whereas some are incorporated into the backbone and some such as L-rhamnose which causes kinks in the chain. The D-xylose is found in apple peels and can be attached in to the backbone. 

Figure 2

Figure 2: Pectin: primary and secondary structure

Although there are a lot of important components of the backbone, the position of each of these components are also of significance.  For example, in apple pectin the distribution is found to be random but in citrus it is block wise. These distributions can affect the charge density and how molecules will repel each other. The structure of pectin is very resistant to heat, even at a lower pH (~3.5) the backbone is only slightly depolymerized at a higher temperature. In order to improve the heat-stability, the water-activity of the system must be lowered. By adding sugar this can lower the water-activity, this is why sugar is added in jam to improve the gel-forming capabilities. Pectins are known for forming gels with sugar and acid (May, 1990)   

Based on the degree of polymerization, number and location of methyl-ester groups these will affect the solubilization rate of pectin. Pectin is very soluble in water but is insoluble in most organic solvents ( Flutto, 2003). The solvent used to dissolute pectin is also important as pH, temperature and ionic strength can affect the rate of dissolution.

Production of Pectin

Pectin is most commonly found in the form of pectic or protopectin substance and is very important within in the cell wall structure. In this structure these substances are not soluble in water, therefore they act as a hydrating mechanism and cement for the cellular network.

Although there is not a complete comprehension of pectic substance, it is known that it is a convoluted structure that pectin attaches to other parts of the cell wall through covalent, hydrogen and/or ionic bonds.

The commercial production of pectin is a complex process where different fruits, most commonly apple pomace and citrus peels are mixed with water and hot dilute mineral acid (pH~2) and an extraction process is performed (Figure 3).  Pectin is separated from the peels and this allows it to be in a soluble form (Silva Team, 2017). Using a filtration system, the liquid is drained off from the peels that are suspended in the liquid and the protopectin is extracted by a hydrolysis in aqueous solution (SilvaTeam, 2017).  The concentrated liquid is either mixed with alcohol (usually isopropanol) this is known as precipitation or mixed with ammonia called amidation. In order to create high-ester and low ester conventional pectin the precipitation method is used. For low-ester amidated pectins the amidation process is used. The gelatinous mass is then pressed and washed to remove the alcohol or ammonia and is dried and ground up. The pectin then goes through a standardization process to ensure consistency within products.

Figure 3

Figure 3: process of producing pectin

Functionality of Pectin

Pectin has a variety of uses in the food and pharmaceutical industries. It appears as a white to light brown powder. The role of pectin in plants is to ensure the plant walls of adjacent cells stay joined together. Protopectin is a precursor substance found in immature fruits, as fruits start to ripen the protopectin is converted to pectin and increases its water-soluble capacity ( Britannica, 2018). Pectin then aids in maintaining firmness and shape of fruits. As the pectin begins to break down to simple sugars and completely water-soluble the fruit overripens, loses shape and firmness (Britannica, 2018). As the use of pectin is still a newer technology and methods are still developing, it is likely that with more knowledge pectin will contribute new and better functional properties in the near future.

Table 1

Classification

High-Ester Pectin

pH and Ionic Strength

-above 4.5 beta-elimination can occur which causes depolymerization of the galacturonic side chain and the esterified carboxyl will be cleaved

Pectin chains carries a negative charge and often repel each other

Depending on charge density it will affect the amount of repulsion

The higher the pH and charge density results in a lower degree of esterification and stronger repulsion  and more difficulty forming gels

Under these conditions hydrogen bonding is impossible between ionized pectin chains

–A lower pH is necessary for high-ester pectins to gel

The low pH (~3.6) lowers repulsions between molecules enough that the distance between chains is adequate to allow hydrogen bonding to occur

Under a certain pH (critical level) the gel strength is reduced as the gelling occurs to quickly and an unorganized polymer network and precipitation wil occur

Gelling Mechanism

Gel with sugar and acid by forming cross-linking polymers in junction zones (Flutto, 2003)

–to stabilize the molecular network the water activity of the system must be reduced

–sugar is added to achieve sufficient hydrophobic interactions

Although more common in low-ester pectins, calcium can alter gelling properties

Hydrogen bonding and hydrophobic attractions between methyl-ester groups play a significant role

Calcium bridges are also form if there are free acids from esters positioned in blocks

–The distribution of esters on the backbone that are marked in a block distribution will contribute to calcium gelling and will greatly increase gelling temperature (Flutto, 2003)

Pectin Concentration

With higher concentration of high-ester pectin will increase the number of junction zones and side chains with elastic activity, which in turn will increase over-all gel strength

An increase in molecular weight will also have same effect (Flutto, 2003)

Degree of Esterification

The galacturonic acid degree of esterification affects the charge density and how many sites are available for hydrophobic interaction (Flutto, 2003)

The higher degree of esterification the less charged and can form gels at a higher pH and temperature

The degree of esterification will determine the optimal pH for digestion (Flutto, 2003)

Acetylation and Branching

As the side of the acetyl groups decreases pectin chains are not close enough to each other to interact

Neutral sugars present can result in steric hinderance and reduce the molecular interaction between molecules and makes it difficult to form gels

Water Activity

Reducing water activity allows hydrophobic interaction to occur easier and this increases gelling rates and strength of gel

By using sugar, the water activity is reduced, it allows less space between molecules so interaction occur easier

Cooling and Storage

gelling will occur under ideal conditions, where intermolecular interactions are formed, and the molecular movement has ceased to allow closer interactions

as cooling rate is increase so is gelation rate

When cooled gel is stored the texture turns into a stronger final gel

Slowly the network will re-organize and there will be an enlargement of existing junction zones and creation of new junction zones between pectin molecules

Should be cooled slowly to avoid difficulty in forming hydrophobic interaction and hydrogen bonding

Table 1:  Functionality of high-ester pectin’s

Table 2

Classification

Low-Ester Pectins 

pH and Ionic Strength

 Used in food systems high in sugar and low pH due to their specific properties

Low sugar content the pH is decreased, and pectin molecules are neutralized with protons

-reduces amount of junction zones interacting with calcium

At a low pH there is more calcium requirements and creates a looser gel texture

At a lower pH natural calcium will be increased and reduces need for extra added calcium

Gelling Mechanism

When conditions are not met with high-ester pectins, low-ester pectins are used

Properties are dependent on type of pectin used (conventional or amidated)

Occur through ionic linkages through calcium bridges of two pectins carboxylic groups which form hydrogen bonding (Flutto, 2003)

-occurs upon cooling system

-pectin chains bridges by ions (usually calcium, could also be magnesium or potassium) which integrate into their coordination shells two polyanion oxygen atoms from one pectin molecule and three from another chain (Flutto, 2003)

Calcium is ideal for bridging in complex carbohydrates as it ionic radius is big enough (0.1 nm) to interact with multiple oxygen atoms

Based on length of junction zones or number of galacturonic acid involved in bonds with calcium will affect how gel in formed

When at least 7 carboxyl groups from each chain are involved the bonds become stabilized (Flutto, 2003)

If junction zones become too long precipitation may form

Number/distribution of ester & amide groups 

 Was developed to achieve better gelling control by controlling functional properties of low-ester pectin

Amidation increases the gelling properties

–due to potential of hydrogen bonding with amide groups

Gels formed with amidation do not require as much calcium resulting in a much firmer gel and are more thermo-reversible

Degree of Esterification

Amount of calcium required depends, process parameters , rate of cooling and on degree on esterification

-increase in ionic strength, pH or decrease in esterification lowers amount of calcium required

Calcium bonds can only occur in esterification-free zones, so gel strength increases with decreasing esterification

The degree of esterification should be above 30% to control length of junction zones

Molecular weight 

 The length of the polymer affects how many junction zones needed to make a network

The higher the molecular weight will increase gelation rate, lower calcium requirements and create a more cohesive and elastic gel  reducing syneresis (Flutto, 2003)

Water Activity

 The more solid content the decrease in amount of calcium required

–also accelerates gelling, increases setting temperatures and overall gel strength (Flutto, 2003)

A higher degree of esterification should be used for a higher solid level

Ionic Strength 

 At higher ionic strength there is an increase in gel strength

As the polymers are neutralized by ions the chains become closer together and leads to organization of the network and a stronger gel

Table 2: Functionality of Low-ester Pectins

Figure 4: Eggbox model

Figure 4: Egg-box model: overview of low-ester pectin gel mechanism including calcium bridges and possible hydrogen bonding types

Application  in the Food Industry

Pectin has the ability when heated to form a thick gel-like solution and therefore is used as a thickening agent in cooking and baking. The most traditional use for pectin is in jams, jellies and preserves (IPPA, 2001). Higher quality jams are usually made with better quality fruit and requires less pectin and therefore less sugar (May, 1990). Fruits with high pectin and pH levels such as grapefruits and lemons are difficult to make a high-fruit content jam as they tend to create an over-strong gel and must be carefully controlled.

Table 3

Different types of commercial pectin

Application

Rapid Set Pectin

Slow Set Pectin

High sugar products:

Jellies

Some jams

Preserves

bakery and biscuit jams

Stabilizing Pectin

Stabilizing acidic protein products: yogurts

Whey and soya beverages

Low Methyl Ester and Amidated Pectin

Lower sugar products:

Reduced sugar preserves

Dessert gels and toppings

Sauces and marinades

Table 3: Comparison of different types of commercial pectins and application in the food industry

Pectin has the unique ability to reduce low-density lipoprotein (LDL) and in turn, can lower cholesterol levels. Pectin also delays stomach emptying and helps to prevent swings in blood sugar (Flutto, 2003). Pectin also has a strong antibacterial effect on food spoilage microorganisms and therefore is a good method or food preservation techniques (Daoude et. al, 2013).

Pectin Interaction with Proteins

 Food proteins (e.g. casein) in acidic environments tend to form sediment and may dehydrate easily after heat treatment. Protein sources need an effective method to stabilize proteins in a low pH system. In ideal high-ester pectin concentrations it has been discovered to be a stabilizer in this environment. On the galacturonic backbone the presence of free carboxyl blocks of pectin allows protein stability through stearic repulsion (Flutto, 2003). The lower amount of carboxyl groups in high-ester pectins has been shown to be effective due to the weaker electrostatic interactions with protein and in turn can allow for static repulsion. This interaction will depend on where the carboxyl groups are located on the backbone and protein structure and distribution of ionizable groups on the surface (Flutto, 2003). Based on the overall system, pH, ionic strength and if sugars or fat are incorporated into the system. pH is the most important system factor as it affects ionization of protein and pectin and affects protein structure and interactions within a complex system. For an ideal interaction a pH of 3.6-4.5 in necessary. If the pH is too low the block structures will not properly bind to protein as they are not sufficiently ionized. If the pH is too high the protein-polysaccharide complex is not very strong and the protein-protein repulsions  become dominate and will not stabilize proteins.

Conclusion

Overall pectin is a great natural product with a variety of functional uses. Presently, pectin is used as a thickening and textural ingredient. With an increase in research into its alternate properties and uses it has potential to make a large positive impact in the food and pharmaceutical industries. This natural product has many uses and health benefits and, in the future, could be found in more foods.

References

Daoud, Z., Sura, M., & Abdel-Massih, R, M. (2013). Pectin shows antibacterial activity against Heliobacter phylori. Advances in Bioscience and Biotechnology,04(02), 273-277. doi:10.4236/abb.2013.42a037

Flutto, L. (2003). Encyclopedia of Food Sciences and Nutrition (Second Edition). New Century, KS: Danisco. http://doi.org/10.1016/BO-12-227055-X/00901-9

Kent, M. (2016). Food and Fitness: A dictionary of Diet and Exercise. Oxford: Oxford University Press.

King, R., Mulligan, P., & Stansfield, W. (2013). pectin. In  (Ed.), A Dictionary of Genetics. : Oxford University Press,. Retrieved 30 Oct. 2018, from http://www.oxfordreference.com/view/10.1093/acref/9780199766444.001.0001/acref-9780199766444-e-4947.

May, C. D. (1990). Industrial pectins: Sources, production and applications. Carbohydrate Polymers. 12(1), 79-99. doi:10.1016/0144-8617(90)90105-2

Pectin for Making Homemade Jam. (2017). Retrieved from http://pickyourown.org/pectin.htm

Pectin Production. (2017, July 31). Retrieved from http://www.silvateam.com/en/products-and-services/food-ingredients/pectin/pectin-production.html

Thibault, J.F., Renard, C.M.G.C., Axelos, M.A.V., Roger, P. and Crepeau, M.J. 1993. Studies of the length of homogalacturonic regions in pectins by acid-hydrolysis. Carbohydrate Res. 238: 271–286.