Pectin
From Wikipedia, the free encyclopedia
Biology
In plant biology, pectin consists of
a complex set of polysaccharides (see below) that are present in most primary
cell walls and are particularly abundant in the non-woody parts of terrestrial
plants. Pectin is a major component of the middle lamella, where it helps to
bind cells together, but is also found in primary cell walls.
The amount, structure and chemical
composition of pectin differs among plants, within a plant over time, and in
various parts of a plant. Pectin is an important cell wall polysaccharide that
allows primary cell wall extension and plant growth. During fruit ripening,
pectin is broken down by the enzymes pectinase
and pectinesterase, in which process the fruit becomes softer as the middle
lamellae break down and cells become separated from each other. A similar
process of cell separation caused by the breakdown of pectin occurs in the abscission zone
of the petioles of deciduous plants at leaf fall.
Pectin is a natural part of the
human diet,
but does not contribute significantly to nutrition.
The daily intake of pectin from fruits and vegetables can be estimated to be
around 5 g (assuming consumption of approximately 500 g fruits and
vegetables per day).
In human digestion, pectin binds to
cholesterol in the gastrointestinal tract and slows glucose absorption by
trapping carbohydrates. Pectin is thus a soluble dietary fiber.
Consumption of pectin has been shown
to reduce blood cholesterol levels. The mechanism appears to be an increase of
viscosity in the intestinal tract, leading to a reduced absorption of
cholesterol from bile or food.[3]
In the large intestine and colon, microorganisms degrade pectin and liberate
short-chain fatty acids that have positive influence on health (prebiotic
effect).[citation needed]
Chemistry
Pectins, also known as pectic
polysaccharides, are rich in galacturonic acid. Several distinct
polysaccharides have been identified and characterised within the pectic group.
Homogalacturonans are linear chains of α-(1–4)-linked D-galacturonic acid.[4]
Substituted galacturonans are characterized by the presence of saccharide
appendant residues (such as D-xylose or D-apiose in the respective cases of xylogalacturonan and
apiogalacturonan) branching from a backbone of D-galacturonic acid residues.[4][5]
Rhamnogalacturonan I pectins (RG-I) contain a backbone of the repeating
disaccharide: 4)-α-D-galacturonic acid-(1,2)-α-L-rhamnose-(1. From many of the rhamnose residues,
sidechains of various neutral sugars branch off. The neutral sugars are mainly
D-galactose, L-arabinose and D-xylose, with the types and
proportions of neutral sugars varying with the origin of pectin.[4][5][6]
Another structural type of pectin is
rhamnogalacturonan II (RG-II), which is a less frequent complex, highly
branched polysaccharide.[7]
Rhamnogalacturonan II is classified by some authors within the group of
substituted galacturonans since the rhamnogalacturonan II backbone is made
exclusively of D-galacturonic acid units.[5]
Isolated pectin has a molecular weight of typically 60–130,000 g/mol, varying with origin and
extraction conditions.
In nature, around 80 percent of carboxyl groups
of galacturonic acid are esterified with methanol. This proportion is decreased to a varying degree during
pectin extraction. The ratio of esterified to non-esterified galacturonic acid
determines the behavior of pectin in food applications. This is why pectins are
classified as high- vs. low-ester pectins (short HM vs. LM-pectins), with more
or less than half of all the galacturonic acid esterified.
The non-esterified galacturonic acid
units can be either free acids (carboxyl groups) or salts with sodium,
potassium, or calcium. The salts of partially esterified pectins are called
pectinates, if the degree of esterification is below 5 percent the salts are
called pectates, the insoluble acid form, pectic acid.
Some plants such as sugar beet,
potatoes
and pears
contain pectins with acetylated galacturonic acid in addition to methyl esters.
Acetylation prevents gel-formation but increases the stabilising and
emulsifying effects of pectin.
Amidated pectin is a modified form of pectin. Here, some of the
galacturonic acid is converted with ammonia
to carboxylic acid amide. These pectins are more tolerant of varying calcium
concentrations that occur in use.[8]
To prepare a pectin-gel, the
ingredients are heated, dissolving the pectin. Upon cooling below gelling
temperature, a gel starts to form. If gel formation is too strong, syneresis or a granular texture are the result, whilst weak gelling
leads to excessively soft gels. Pectins gel according to specific parameters,
such as sugar, pH and bivalent salts (especially Ca2+).[9]
In high-ester pectins at soluble
solids content above 60% and a pH-value between 2.8 and 3.6, hydrogen bonds
and hydrophobic
interactions bind the individual pectin chains
together. These bonds form as water is bound by sugar and forces pectin strands
to stick together. These form a 3-dimensional molecular net that creates the
macromolecular gel. The gelling-mechanism is called a low-water-activity gel or
sugar-acid-pectin gel.
In low-ester pectins, ionic bridges
are formed between calcium ions and the ionised carboxyl groups of the
galacturonic acid. This is idealised in the so-called “egg box-model”.
Low-ester pectins need calcium to form a gel, but can do so at lower soluble
solids and higher pH-values than high-ester pectins. Normally low-ester pectins
form gels with a range of pH from 2.6 to 7.0 and with a soluble solids content
between 10 and 70%.
Amidated pectins behave like
low-ester pectins but need less calcium and are more tolerant of excess
calcium. Also, gels from amidated pectin are thermo-reversible; they can be
heated and after cooling solidify again, whereas conventional pectin-gels will
afterwards remain liquid.
High-ester pectins set at higher
temperatures than low-ester pectins. However, gelling reactions with calcium
increase as the degree of esterification falls. Similarly, lower pH-values or
higher soluble solids (normally sugars) increase gelling speed. Suitable pectins
can therefore be selected for jams and for jellies, or for higher sugar
confectionery jellies.
Sources
and production
Apples, guavas, quince, plums, gooseberries, oranges
and other citrus
fruits, contain large amounts of pectin, while soft fruits like cherries, grapes and strawberries
contain small amounts of pectin.
Typical levels of pectin in plants
are (fresh weight):
- apples, 1–1.5%
- apricot,
1%
- cherries, 0.4%
- oranges, 0.5–3.5%
- carrots
approx. 1.4%
- citrus peels, 30%
The main raw-materials for pectin
production are dried citrus peel or apple pomace, both by-products of juice
production. Pomace from sugar-beet is also used to a small extent.
From these materials, pectin is
extracted by adding hot dilute acid at pH-values from 1.5 – 3.5. During several
hours of extraction, the protopectin loses some of its branching and
chain-length and goes into solution. After filtering, the extract is
concentrated in vacuum and the pectin then precipitated by adding ethanol or isopropanol. An old technique of
precipitating pectin with aluminium salts is no longer used (apart from alcohols
and polyvalent cations; pectin also precipitates with proteins and detergents).
Alcohol-precipitated pectin is then
separated, washed and dried. Treating the initial pectin with dilute acid leads
to low-esterified pectins. When this process includes ammonium hydroxide,
amidated pectins are obtained. After drying and milling, pectin is usually
standardised with sugar and sometimes calcium-salts or organic acids to have
optimum performance in a particular application.[10]
Uses
The main use for pectin (vegetable
agglutinate) is as a gelling agent, thickening agent and stabilizer in food.
The classical application is giving the jelly-like consistency to jams or marmalades,
which would otherwise be sweet juices. Pectin also reduces syneresis in jams
and marmalades and increases the gel strength of low calorie jams. For
household use, pectin is an ingredient in gelling sugar
(also known as "jam sugar") where it is diluted to the right
concentration with sugar and some citric acid to adjust pH. In some countries,
pectin is also available as a solution or an extract, or as a blended powder,
for home jam making. For conventional jams and marmalades that contain above
60% sugar and soluble fruit solids, high-ester pectins are used. With low-ester
pectins and amidated pectins less sugar is needed, so that diet products can be
made.
Pectin is used in confectionery
jellies to give a good gel structure, a clean bite and it confers a good
flavour release. Pectin can also be used to stabilize acidic protein drinks,
such as drinking yogurt, to improve the mouth-feel and the pulp stability in
juice based drinks and as a fat substitute in baked goods.[11]
Typical levels of pectin used as a food additive are between 0.5 and 1.0% –
this is about the same amount of pectin as in fresh fruit.[12]
In medicine, pectin increases viscosity
and volume of stool
so that it is used against constipation
and diarrhea.
Until 2002, it was one of the main ingredients used in Kaopectate
a drug to combat diarrhea, along with kaolinite.
Pectin is also used in throat lozenges as a demulcent.
In cosmetic products, pectin acts as stabilizer. Pectin is also used in wound healing
preparations and specialty medical adhesives, such as colostomy
devices.
Yablokov et al., writing in Chernobyl: Consequences of the Catastrophe for
People and the Environment, quote
research conducted by the Ukrainian Center of Radiation Medicine and the
Belarussian Institute of Radiation Medicine and Endocrinology with the
conclusion that "adding pectin preparations to the food of inhabitants of
the Chernobyl-contaminated regions promotes an effective excretion of
incorporated radionuclides". The authors report on the positive results of using
pectin food additive preparations in a number of clinical studies conducted on
children in severely polluted areas, with up to 50% improvement over control
groups.[13]
In ruminant
nutrition, depending on the extent of lignification of the cell wall, pectin is
up to 90% digestible by bacterial enzymes. Ruminant nutritionists recommend
that the digestibility and energy concentration in forages can be improved by
increasing pectin concentration in the forage.
In the cigar industry, pectin is considered an
excellent substitute for vegetable glue and many cigar smokers and collectors
will use pectin for repairing damaged tobacco wrapper leaves on their cigars.
Pectin is also used in jellybeans.
Legal
status
At the FAO/WHO joint Expert
Committee on Food Additives and in the EU, no numerical acceptable daily intake
(ADI) has been set, as pectin is considered safe.[14]
In the US, pectin is GRAS – generally
recognized as safe. In most foods it can be used
according to good manufacturing practices in the levels needed for its
application ("quantum satis").
In the International Numbering
System (INS), pectin has the number 440. In Europe, pectins are differentiated
into the E numbers
E440(i) for non-amidated pectins and E440 (ii) for amidated pectins. There are
specifications in all national and international legislation defining its
quality and regulating its use.
History
Pectin was first isolated and
described in 1825 by Henri Braconnot,
though the action of pectin to make jams and marmalades was known long before.
To obtain well set jams from fruits that had little or only poor quality
pectin, pectin-rich fruits or their extracts were mixed into the recipe.
During the industrialization, the
makers of fruit preserves soon turned to producers of apple juice to obtain
dried apple pomace
that was cooked to extract pectin.
Later, in the 1920s and 1930s,
factories were built that commercially extracted pectin from dried apple pomace
and later citrus-peel in regions that produced apple juice in both the USA and
in Europe.
At first, pectin was sold as a
liquid extract, but nowadays pectin is often used as dried powder that is
easier to store and handle than a liquid.[15]
The entire wiki link on the subject can be found at: http://en.wikipedia.org/wiki/Pectin
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