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E- Immunoassays
Immuoassays are finding increasing use in the food industry for
the qualitative and quantitative analysis of food products.
Immunoassays specific for low molecular weight carbohydrates
are developed by attaching the carbohydrate of interest to a
protein, and then injecting it into an animal.
With time the animal develops antibodies specific for the
carbohydrate molecule.
These antibodies can then be extracted from the animal and
used as part of a test kit for determining the concentration of
the specific carbohydrate in foods.
Immuoassays are extremely sensitive, specific, easy to use and
rapid.
Analysis of Polysaccharides
A wide variety of polysaccharides occur in foods.
Polysaccharides can be classified according to:
1- Their molecular characteristics (e.g., type, number,
bonding and sequence of monosaccharides)
2- Physicochemical characteristics (e.g., water solubility,
viscosity, surface activity)
3- Nutritional function (e.g., digestible or non-digestible).
Indigestible polysaccharides form part of a group of
substances known as dietary fiber.
which also includes lignin (which is a polymer of
aromatic molecules).
Consumption of many types of dietary fiber has been
shown to have beneficial physiologically functional
properties for humans, e.g., prevention of cancer,
heart disease and diabetes.
Analysis of Starch
Starch is the most common digestible polysaccharide
found in foods.
In natural foods, such as legumes, cereals or tubers, the
starch granules are usually separated from the other
major components by drying, grinding, steeping in
water, filtration and centrifugation.
The starch granules are water-insoluble and have a
relatively high density (1500 kg/m3) so that they will
tend to move to the bottom of a container during
centrifugation.
Analysis methods
Once the starch has been extracted there are a number
of ways to determine its concentration:
1- Specific enzymes are added to the starch solution to 
breakdown the starch to glucose. The glucose
concentration is then analyzed using methods
described previously (e.g., chromatography or
enzymatic methods).
The starch concentration is calculated from the 
glucose concentration.
2- Iodine can be added to the starch solution to form
an insoluble starch-iodine complex that can be
determined gravimetrically by collecting, drying
and weighing the precipitate formed.
or titrimetrically by determining the amount of
iodine required to precipitate the starch.
3- If there are no other components present in the
solution that would interfere with the analysis, then
the starch concentration could be determined
using physical methods, e.g., density, refractive
index or polarimetry.
Analysis of Fibers
Over the past twenty years or so nutritionists have
become aware of the importance of fiber in the diet.
Consumption of fiber helps protect against colon cancer,
cardiovascular disease and constipation. Adequate
intake of dietary fiber is therefore beneficial to good
health.
Dietary fiber is defined as plant polysaccharides that are
indigestible by humans. Plus lignin, the major
components of dietary fiber are cellulose,
hemicellulose, pectin, and hydrocolloids.
Crude Fiber Method
The crude fiber method gives an estimate of
indigestible fiber in foods.
It is determined by sequential extraction of a
defatted sample with 1.25% H2SO4 and 1.25%
NaOH.
The insoluble residue is collected by filtration,
dried, weighed and ashed to correct for mineral
contamination of the fiber residue.
Filth detection
Examination of whole foods for insect and rodent
contamination:
Insect tunneling and webbing.
Whole larvae and adults.
Rodent contamination:
rodent excreta.
Animal hairs
Filth recovery methods
1- Sedimentation in heavier than water liquids:
This procedure can be used to separate rodent
excreta pellet fragments from cereals.
In a liquid with a specific gravity near 1.49 the
pellet fragments tend to settle out while
much of the cereals float.
2- Sieving: in certain instances, a size
separation offers a rapid means of
separating filth from food.
3- When gasoline is mixed with an aqueous
mixture containing insects or insect
fragments, the insects float up with the
gasoline layer.
Fruits
The water content of fruits varies from 60- 90%,
fruits undergo a series of progressive changes from
the unripe stage, to ripe stage, to the rotten and
fermentative stages.
The ripening point of a fruit is considered to be that
point at which the sugar content is a maximum.
In oranges and grapefruit, maturity is based
on a minimum invert sugar-citric acid ratio
(eight to one).
Apples are considered ripe when the last trace
of starch disappears.
The browning of fruit flesh on exposure to air
is due to the oxidation of the tannin in the
fruit by an oxidase enzyme in the presence
of the oxygen of the air.
Maturity test for oranges and
grapefruit
Since the colour of grapefruit and oranges is
inadequate as a measure of maturity, a more
objective standard is used.
This is based on the ratio of total solids to acid
present in the juice, this is often termed the
sugar: acid ratio as the major part of the
total solids is sugar.
Procedure:
Determine the acidity by titration with 0.1 N sodium
hydroxide solution and express the acidity in terms
of percentage of citric acid.
Determine the total solid content of the fruits.
The maturity ratio is then the percentage of total
solids divided by the percentage of citric acid.
The minimum ratio for orange is 8: 1, and for
grapefruit is 7: 1.
Maturity test for apples
1- The firmness of the fruit.
2- The starch content.
3- The soluble solids.
4- The development of red colour.
5- The colour of the seeds.
Starch test: As apples mature, the amount of
starch decreases and in some varieties it is
negligible.
Procedure:
1- Dip slices of apple into a solution of iodine
in potassium iodide for 1 minute.
2- Use the depth of the blue staining as an
index of the amount of starch present.
Soluble solids: As apples mature, the soluble
solids content increases.
Colour of seeds: As apples mature, the seeds
turn brown to black in colour.
Frozen fruits and fruit products
Frozen fruits are made by freezing a properly prepared
fruit ingredient or mixture of fruit ingredients specified
in fill of container.
May be added an optional sweetening ingredient:
Sugar, any mixture of sugar with dextrose or corn syrup.
An optional liquid packing medium:
Heavy syrup, medium syrup, light syrup, corn syrup.
Vegetable Products
The analysis of these products includes the
estimation of moisture, ash, metallic
constituents, sugars, acids,….
Maturity in frozen vegetables
This method depends upon the
determination of the specific gravity of the
sample by means of the difference in
weight in air and in a liquid of known
specific gravity.
Canned Peas
Prepared from:
A variety of Pisum sativum .
Water
Optional ingredient: salt, sugar, dextrose,
spice, flavoring, adjusting pH to 8.
Quality: the standard quality for canned peas is:
1- Not more than 4% by count of the peas in the container are
spotted or discoloured.
2- Normally coloured, not artificially coloured.
3- Weight of pea pods and other harmless extraneous vegetable
material is not more than 0.5- 1% of the drained weight of
peas in the container.
4- Weight of pieces of peas is not more than 10%.
5- The alcohol insoluble solids is not more than 23.5%.
Spices, Flavors and Condiments
Spices are aromatic vegetable substances used
for the seasoning of food.
Some of the determinations of spices includes;
moisture, ash, nitrogen, heavy metals,
reducing sugars and starch.
Volatile and non-volatile extract:
Extract 2 g of the sample in continuous extractor for 20 hours
with anhydrous ether.
Evaporate the ether to a small volume.
Transfer to a small weighed beaker.
Evaporate at room temperature, allow to stand over sulphuric
acid for 18 hours in a desiccator.
Weigh and calculate the gain in weight as percentage of total
ether extract.
Heat the extract gradually and then at 110o C until successive
weighings show no loss.
The difference in weight equals the volatile ether extract while
the residue is the non-volatile ether extract.
Alcohol extract:
Shake 2 g of the sample in a 100 ml volumetric
flask with 95% alcohol for 8 hrs.
Filter and evaporate 50 ml of the filtrate to
dryness in a dish.
Dry at 100o C, the residue is the alcohol
extract.