5. Food
Industrial Products (Testing)
Exercise 5.1
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(a) What is the difference between unprocessed and processed foods?
unprocessed – no cooking, cleaning/cutting only
raw meats
fresh fruit & vegetables
(b) What should unprocessed foods be tested for?
freshness
contaminants (especially by organisms – moulds, insects)
(c) Is there anything else that processed foods should be tested for?
additive levels
required physical properties
(d) Are all food product types tested before consumption?
yes, but unprocessed foods only get visual inspection
Shelf life
Exercise 5.2
(a) less tasty
• baked products, e.g. bread, biscuits
(b) unsafe
• raw meat
• dairy products
• baked-for date – a date not later than 12 hours after the time the bread was
baked
• baked-on date – the date on which the bread was baked
• best-before date – will retain any specific qualities
• use-by date – not safe afterwards
Water
CLASS EXERCISE 5.3
Why is it necessary to test for water?
• measure of freshness/processing
CLASS EXERCISE 5.4
What are the limitations of the oven drying method?
• not good for samples with:
• high viscosity (eg honey)
• high levels of other volatiles (eg essences)
• low water content
CLASS EXERCISE 5.5
(a) bread
oven
(b) honey KF, D&S
(c) olive oil KF
(d) vanilla essence GC
Triglycerides and other lipids
• major component of fats in food
Exercise 5.6
What is the structure of a typical triglyceride?
Analysis of fats
• total – solvent extraction (Soxhlet extractor)
• individual triglycerides can’t be analysed
• too many different types
• too similar to each other for chrom. separation
• to obtain a "fat profile" by the Fatty Acid Methyl Ester Method (FAME)
1. hydrolysis of the triglycerides in methanolic KOH to produce the fatty
acid salts
2. conversion of the fatty acids to their corresponding methyl esters using
methanol and the catalyst BF3
3. analysis of the esters by GC
CLASS EXERCISE 5.7
(a) Write the equations for steps 1 & 2.
(b) Explain why this gets around the problem described for the triglycerides.
• fatty acid methyl esters differ by FW
• only 10-20 of them
Simpler QC tests
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saponification value => average chain length
hydrolysis of the triglycerides under reflux with a known amount of KOH
excess KOH is back-titrated with standardised HCl
SV is the mass of KOH in mg that reacts with 1 g of food
shorter chains mean more triglyceride molecules per gram, therefore more
reaction with KOH
• iodine value => degree of unsaturation
• reaction of the triglycerides with iodine monochloride (Wij’s reagent) in the
dark for 1 hour
• C=C undergoes an addition reaction with the ICl
• excess ICl is then back-titrated with standardised sodium thiosulfate
• IN is the mass of iodine that reacts with 100 g of sample
• more C=C => more reaction
Proteins
CLASS EXERCISE 5.8
What is the structure of a typical protein?
R2
O
NH
NH
NH
O
R1
O
Kjeldahl N analysis
• oxidation of the sample in hot concentrated sulfuric acid
• converts the bound nitrogen into ammonium ions
• addition of strong base to the solution converts the ammonium ions to
ammonia gas
• collected by distillation
• in a known volume of standard acid, then back titrated or
• in a solution of excess unstandardised boric acid
• borate ion produced is a stronger base than ammonia and is titrated with
standardised acid
• 1 mole of N gives 1 mole NH3
• amount N related to amount of protein by factor which is the proportion of
N in typical protein of given material, eg flour, milk
Exercise 5.9/10
Calculate the %N & %protein in a sample of meat if 1.0589 g of sample, after
the Kjeldahl procedure, gave a titre of 15.8 mL of 0.103 M HCl.
FW of N is 14. Protein factor is 6.25.
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moles HCl = 15.8 x 10-3 x 0.103 = 1.63 x 10-3 = moles NH3 = moles N
mass N = 1.63 x 10-3 x 14 = 0.0228g
%N = 0.0228 x 100 ÷ 1.0589 = 2.15%
%protein = 2.15 x 6.25 = 13.4%
• Read remainder of chapter yourself