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PREPARATION FOR LABORATORY SAMPLES, Screenshot (196), FATIN NUR ANNISYA…
PREPARATION FOR LABORATORY SAMPLES
Making samples homogenous
Grinders, mixers, slices, blenders
Soft drinks
Carbonated drinks - air bubbles are eliminated by performing the process of the making the tarik / stir
Viscous liquids (honey) - stir gently with a spatula or glass rod before sampling
Oils
The oil was crystallized / not clear -heated until dissolved and filtered while still hot
If not clear - the sample is stirred slowly several times.
AVOID OXIDATION
Grinding
Important both for sample preparation prior to analysis and for food ingredient processing
Various mills are available for reducing particle size to achieve sample homogenization
Grinding wet samples may cause significant losses of moisture and chemical changes. Grinding frozen samples reduces undesirable changes.
The grinding process should not heat the sample - the grinder should
not be overloaded because heat will be produced through friction.
HEAT-SENSITIVE SAMPLE : grinders can be cooled with liquid nitrogen
and then ground samples are stored at −80 °C.
Contact of food with bare metal surfaces should be avoided
cryogenic grinding or cryogrinding - ideal for biological samples and materials that are sensitive to oxygen or temperature
Cryogrinding can be performed manually with a mortar and pestle after freezing
the sample with liquid nitrogen.
The mortar and pestle have to be prechilled with liquid nitrogen before adding
the material
Reducing sample size
If the particle size or mass of the sample is too large for analysis, it must be
reduced in bulk or particle size using the quartering technique.
To obtain a smaller quantity for analysis = the sample can be spread on a clean
surface and divided into quarters.
Ground Beef Sampling ( quartering technique )
Determination of Particle Size
Particle size : controlled in certain mills by adjusting the distance between burrs or blades or by screen mesh size/number
MESH NUMBER : the number of square screen openings
per linear inch of mesh.
The final particles of dried foods should be 20 mesh for
moisture, total protein, or mineral determinations.
Particles of 40 mesh size are used for extraction assays such as
lipid and carbohydrate estimation
To reduce particle size for analysis of samples - important to reduce the particle size of many food
ingredients for use in specific food products.
Simplest way to measure particle sizes of dry materials of
less than 50 μm in diameter = passing the sample through a
series of vertically stacked sieves with increasing mesh number
Mesh number ⬆ = the apertures between the mesh are smaller and only finer and finer particles pass through subsequent sieves
Sieve sizes : salt, sugar , wheat flour , cornmeal , semolina and cocoa
The sieve method is inexpensive and fast, but it is not suitable
for emulsions or very fine powders.
Preventing Changes in Sample
Lipid Oxidation Protection
Lipids present particular problems in sample preparation
High-fat foods
are difficult to grind - may need to be ground
while frozen
Unsaturated lipids
are sensitive to oxidative degradation - should be protected by storing under nitrogen or vacuum
Light-initiated photooxidation of unsaturated lipids can be
avoided by
controlling storage conditions
Antioxidants
may stabilize lipids and may be used if they do not
interfere with the analysis.
Low-temperature storage is generally recommended to protect
most foods.
Microbial Growth and Contamination
Microorganisms are present on all but sterilized surfaces, so sample
cross-contamination
can occur if samples are not handled carefully.
Freezing, drying, and chemical preservatives are effective controls
The preservation methods used are determined by
the probability of contamination, the storage conditions, storage time, and the analysis to be performed.
Enzymatic Inactivation
Food materials often contain endogeneous enzymes that may degrade the food
components being analyzed.
Enzyme activity therefore must be eliminated or controlled - depend on the nature of the food
Heat denaturation to inactivate enzymes and freezer storage (−20 to −30 C) for
limiting enzyme activity are common methods.
Enzymes are more effectively controlled = changing the pH or by
salting out
Oxidative enzymes may be
controlled by adding reducing agents.
Physical Changes
Sample Preparation
Labelled carefully
sample description
time sample was taken
location sample was taken form
person who took the sample
method used to select the sample
Sieve Shaker
FATIN NUR ANNISYA BINTI MOHD FAUZI (S55178) (K2)
