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FOOD ANALYSIS LABORATORY PROCEDURE / 1 / 30

PROXIMATE ANALYSIS

EXPERIMENT 1: DETERMINATION OF MOISTURE CONTENT

OBJECTIVE

To determine moisture content in food.

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INTRODUCTION

Water is the major component of most foods and serves a variety of functions. It influences the structure, appearance and taste of food. The moisture content of a food also influences its spoilage process. Each food has its own characteristics water content. The determination of moisture content is one of the most fundamental and important analytical procedures in food analysis. The approximate water content of a food can affect the choice method of analysis.

The moisture (or total solids) content of foods is important for a variety of reasons. Moisture is an important factor in food quality, preservation, and resistance to deterioration. Moisture content of foods can be determined by a variety of methods. In this experiment, several methods to determine the moisture content of foods will be used and the results compared.

PRINCIPLE

The sample is heated (according to the drying method) and the loss of weight is used to calculate the moisture content of the sample.

METHOD A: FORCED DRAFT DRYING OVEN

Sample: Minced Meat

Apparatus/Instrument/Chemicals/Samples

  • Analytical balance
  • Dessicator
  • Crucible with lid
  • Oven, set at 105ºC
  • Minced meat
  • Tongs

Procedure

1)Perform this experiment in duplicates.

2)Dry the crucible and its lid in the oven at 105ºC overnight and transfer to the dessicator to cool (approx 30 min)

3)Weigh the crucible.

4)Weigh about 5 g sample into crucible. Spread the meat.

5)Replace the lid and weigh the crucible and its contents (WI)

6)Place the crucible with its lid slipped to one side. Dry for 16h or overnight at 105ºC

7)After drying, use a pair of tongs to transfer the crucible and lid to the dessicator to cool (approx 45 min). Reweigh the crucible, lid and its dried content.

8)Replace the crucible with its lid partially covered in the oven for 1h. Transfer to the dessicator to cool, then weigh the dish and its content again. If the weight obtained at this step is less than that obtained at Step 7, it means the sample was not sufficiently dried. In this case, repeat this step until constant weight is obtained (W2).

Calculations

Moisture(%)= w1-w2 x 100

W1

Where, w1 = weight (g) of sample before drying

W2= weight (g) of sample after drying

Calculate percentage moisture (wt/wt):

% moisture = (wt of H20 in sample / wt of sample ) x 100

METHOD B : MOISTURE ANALYZER (AND METHOD)

Sample: flour

***Performed sample in triplicate.

Apparatus/Instrument/Chemicals/Sample

EXPERIMENT 2: DETERMINATION OF MINERAL CONTENT

OBJECTIVE

To determine mineral content in tea leaves and coffee powder.

Introduction

Ash is the inorganic residue remaining after the water and organic matter have been removed by heating in the presence of oxidizing agents, which provides a measure of the total amount of minerals within a food. Analytical techniques for providing information about the total mineral content are based on the fact that the minerals (the “analyte”) can be distinguished from all the other components (the “matrix”) within a food in some measurable way. The most widely used methods are based on the fact that minerals are not destroyed by heating, and that they have a low volatility compared to other food components. The three main types of analytical procedure used to determine the ash content of foods are based on this principle: dry ashing, wet ashing and low temperature plasma dry ashing. The method chosen for a particular analysis depends on the reason for carrying out the analysis, the type of food analyzed and the equipment available. Ashing may also be used as the first step in preparing samples for analysis of specific minerals, by atomic spectroscopy or the various traditional methods described below. Ash contents of fresh foods rarely exceed 5%, although some processed foods can have ash contents as high as 12%, e.g., dried beef.

1.Dry ashing for the majority of the samples,

Dry ashing is incineration at high temperature (525°C or higher) accomplished in a muffle furnace. Crucible selection becomes critical in ashing because type depends on the specific use. Quartz crucibles are resistant to acids and halogens, but not alkali. Porcelain crucibles resemble quartz crucibles in their properties but will crack with rapid temperature changes. These crucibles usually used because they are relatively inexpensive. Steel crucibles are resistant to both acids and alkalis and are inexpensive, but they are composed of chromium and nickel, which are possible sources of contamination. Platinum crucibles are inert are the best crucibles but they are currently far too expensive for routine use for large number of samples.

The advantages of conventional dry ashing are that it is a safe method, it requires no added reagents or blank subtraction, and little attention is needed once ignition begins. A large number of crucibles can be handled at once, and the resultant ash can be used for analyses like individual elements, acid-insoluble ash, and water-soluble and insoluble ash. The disadvantages are the length of time required (12 - 18 h, or overnight) and expensive equipment. There will be a loss of the volatile elements and interactions between mineral components and crucibles. Volatile elements at risk of being lost include arsenic, boron, cadmium, chromium, copper, iron, lead, mercury, nickel, phosphorus, vanadium and zinc.

2.Wet ashing for samples with high fat content (meats and meat products) as a preparation of elemental analysis,

Wet ashing is a procedure for oxidizing organic substances by using acids and oxidizing agents or their combinations. Minerals are solubilised without volatilization. Wet ashing is preferable to dry ashing for specific elemental analysis. The oxidation time is short and requires a hood, hot plate, long tongs and safety equipment. Nitric and perchloric acid acids are preferable, but a special perchloric acid hood is necessary. The disadvantages of wet ashing are that it takes almost constant operator attention, use of corrosive reagents and only small numbers can be handled at any one time.

3.Low-temperature plasma dry ashing for preparation of samples when volatile elemental analyses are conducted.

Apparatus

Date: / 30.12.09 / Revision: / 1 (13.01.11) / File: / Food analysis manual
Name: / CWZ / NBH

/ SFA3091 / Page / of
FOOD ANALYSIS LABORATORY PROCEDURE / 1 / 30

Crucible (or similar porcelain or metal dishes)

Muffled furnace

Sample

Hot plate

Date: / 30.12.09 / Revision: / 1 (13.01.11) / File: / Food analysis manual
Name: / CWZ / NBH

/ SFA3091 / Page / of
FOOD ANALYSIS LABORATORY PROCEDURE / 1 / 30

Procedure

  1. Dry a representative sample (weigh accurately to nearest mg ~ 3 – 5 g of samples) in a crucible in an oven at 130°C overnight. Char the sample on an electric hot plate or over a low flame in a fume cupboard until it has ceased smoking.
  2. Place the above crucibles (containing charred sample) in a cold muffle oven and bring the temperature to 550°C.
  3. Ignite the sample 12- 18 h (or overnight) at 550°C.
  4. Turn off the muffle furnace and wait to open until the temperature has dropped to at least 250°C, preferably lower. Open door carefully to avoid losing ash that may be light and fluffy.
  5. Use safety tongs to quickly transfer the crucibles to a dessicator with a porcelain plate and desiccant. Cover crucibles, close the dessicator, and allow crucibles to cool prior to weighing.

References

1. Pearson, D (1976). General Methods. The Chemical Analysis of Foods. Longman Group Limited London.

EXPERIMENT 3 : DETERMINATION OF FAT CONTENT

OBJECTIVE

To determine fat content in meatball and peanut.

INTRODUCTION

The term ‘lipid’ refers to a group of compounds that are sparingly soluble in water, but show variable solubility in a number of organic solvents (e.g., ethyl ether, petroleum ether, acetone, ethanol, methanol, benzene). The lipid content of a food determined by extraction with one solvent may be quite different from the lipid content as determined with another solvent of different polarity.

The solvent extraction methods used for fat analysis are:

a)Soxhlet method – conventional

b)Mojonnier method

c)Soxhtherm - automated

PRINCIPLE

To determine the fat content of a food sample, the protein component is digested with boiling hydrochloric acid to break the lipo-protein bonds. The digestion solution is filtered and the fat remaining in the filter after the drying period is extracted with petroleum ether or n-hexane. After the evaporation of the solvent, the residue is dried and weighed. The solvent is distilled and the dried residue is weighed. The fat content is calculated based on the given formula.

Apparatus/Instrument/Chemicals/Samples

  • Mechanical blender
  • Analytical balance
  • Desiccator, with drying agent, e.g. Blaugel
  • Soxtherm micro/macro and multistat
  • Drying oven
  • Cotton Wool, chemically clean and fat-free

Procedure

1.Preparation of the extraction beakers

3 -5 boiling stones are put into each extraction beaker. The beakers are dried in a drying oven for about an hour at 103°C ± 2°C. After cooling off in the desiccator to room temperature, weigh the beaker with a precision of +/-0.1mg.

2.Sample Preparation

A representative average sample of at least 200 g should be used. The sample is mixed and homogenized in the mixer at least twice. The sample can be stored in an airtight container to prevent decay and any change in content. Prior to the analysis weigh 10g of the sample on a filter paper and fold into a predried extraction thimble. Cover the thimble with cotton wool.

3.Extraction

Put the thimble in the specified beaker. Any remaining fat traces on the watch glass have to be taken up with some cotton wool, damp with extraction agent, and put into the extraction thimble as well. After adding 140-ml extraction agent the sample is extracted in the Soxtherm automatic using the following program:

Solvent: Petrol ether 40/60

Boiling Point: Boiling range 40 - 60 °C

Amount of Solvent: micro100ml/ macro 150 ml

Parameter for the Program:

Program Step / Parameter / Comment
T-Classification / 200°C
Extraction Temperature / 150°C
Reduction Interval / 4 min
Reduction Pulse / 4 s
Hot Extraction / 30 min / Sample must be completely immersed
Evaporation A: / 5 Intervals
Rinsing Time / 70 min / Level of solvent ca. 1-2 cm below the thimble

After the program is finished, the extraction beakers are dried in the drying oven in an upright position for 60 min at 103°C ± 2°C. Then, place them in a desiccator, leave them to cool down to room temperature and weigh with a precision of +/- 1 mg. In order to check the consistency of the weight, the sample is left to dry for another 30 min and weighed again after cooling down. This procedure is repeated as long as two successive weighings show no more than 1 mg difference. Should the weight increase then the previous lower value should be taken. Extraction, drying, and weighing have to be done immediately after each other.

4.Calculation

The total fat content (w) in g/100 g (corresponds to %) of the sample is calculated using the following formula:

(m2 – m1) * 100

w =

m0

m1: Mass of the empty extraction beaker with boiling stones in g

m2: Mass of the extraction beaker with fat after drying in g

m0: Weight at the start of the analysis in g

The result is expressed to two decimal places.

5.References

  1. AOAC. 2005. Official Methods of Analysis. 18th Edition
  2. Gerhardt training manual
  3. Suzanne Nielen, S. 2003. Food Analysis. Third Edition. Springer Science + Business Media, Inc.
  4. Suzanne Nielen, S. 2003. Food Analysis Laboratory Manual. Kluwer Academic/ Plenum Publishers.

EXPERIMENT 4: DETERMINATION OF PROTEIN CONTENT

OBJECTIVE

To determine a protein content in fish and tempe.

INTRODUCTION

The protein content of foods can be determined by numerous methods. The Kjeldahl, nitrogen combustion (Dumas) and infrared spectroscopy methods for protein analysis are based on nitrogen determination. The methods are from the Official Methods of Analysis of AOAC International (1), and are used commonly in research laboratories working on proteins.

PRINCIPLE

The Kjeldahl procedure can be basically divided into three parts: (1) digestion, (2) distillation, (3) titration. In the digestion step, organic nitrogen is converted to an ammonium in the presence of a catalyst at approximately 370ºC. In this experiment, the sample is digested in H2SO4, using Copper-based catalyst, converting N to NH3 which is distilled and titrated.

In the distillation step the digested sample is made alkaline with NaOH and the nitrogen is distilled off as NH3. This NH3 is trapped in a boric acid solution.

The amount of ammonia nitrogen in this solution is quantified by titration with a standard HCl solution. A reagent blank is carried through the analysis and the volume of HCl titrant required for this blank is subtracted from each determination.

This analysis determines total nitrogen and not usable nitrogen and this is the reason it is called a crude protein analysis.

Method Reference

German official collection of methods § 35 LMBG L 06.00 7, L 07.00 7, L 08.00 7

AOAC Official Method 928.08, Nitrogen in Meat, Kjeldahl Method, alternatively II

Apparatus/Instrument/Chemicals/Samples

  • Sulfuric acid 98% min.
  • Catalyst tablets to be used: Kjeltabs CX
  • Caustic soda 32%
  • Boric acid solution 2%
  • Indicator Solution M5 (Merck) or similar
  • Standard acid 0.1N or c = 0.1 mol/, alternatively sulfuric acid 0.1N or c = 0.05 mol/l
  • Mechanical comminuting instrument
  • Analytical balance (0.001 g)
  • Kjeldahl digestion block Kjeldatherm, Turbotherm, flask heater for Kjeldahl flask with wide neck opening
  • Vapodest distillation System
  • Burette, 50 ml nominal capacity, with a scale on 0.05 ml or titration system (not with the Vap 50) or pH meter with combined electrode

Procedure

1.Sample Preparation

  1. Weigh accurately 2.00g comminuted sample as a start on a piece of a filter paper.
  2. Store the sample air tight so that any changes or decay of the composition is avoided. Prior to the analysis the sample should be at room temperature. The examination of the thus prepared sample has to be done within the following 24 h.

2.Digestion chemicals

  1. The chemicals are added. Sulfuric acid is used to wash down any sample residue, which might remain at the glass walls.

Chemicals
Sulfuric acid / 20 ml
Kjeltabs / 2
Indicator solution M5
Standard acid 0.1N or c=0.1mol/L; alternatively sulfuric acid 0.1N or c=0.05mol/L

3.Digestion with Kjeldatherm

  • When working with a Kjeldatherm-System with 250 ml Kjeldatherm-digestion tubes, the following digestion parameters are recommended:

Time in min / Temperature
in ° C / Comments
40 / 400 / Digestion tubes are put into the preheated block and time it takes for the sample to become translucent
30 / 400 / Dehydrate the sample
  • Foaming during the digestion has to be expected, however, the foaming should not go higher then 2/3 of the glass.
  • If excessive reactions should occur, take out the insert rack.
  • During the digestion black particles remaining at the glass wall are washed back with condensing sulfuric acid.
  • The sample glass has to be translucent after the digestion in order to obtain good results.

4.Digestion with Turbotherm

  • When working with a Turbotherm-System with 250 ml Kjeldatherm-digestion tubes, the following program parameter are recommended:

Time in min / Power in % / Comment
10 to 15 / 100 / Heating up of the system to bring digestion solution to boiling
60 / 70 to 80 / Digestion solution is turning translucent after ca 20 to 30 min

5.Digestion with Flask heater

  • For Serial Flask Heater with 500 or 750 ml Kjeldahl flask with wide neck opening the following procedure is recommended:

Time in min / Power / Comments
20 / 3 / Heating up till the digestion solution is boiling
50 / 1,5 / After 20-30 min. the sample should turn translucent. Wash down remaining sample particles with condensing sulfuric acid into the flask.

6.Suction

  • During the entire digestion period the scrubber should be on. About 1200 ml of a 15% caustic soda is recommended for the washing bottle; this amount is sufficient to neutralize digestion gases of about 60 digestions.
  • The cooling off period after the lifting of the insert rack or the cooling off period after turning off the heating is about 30 minutes; during this time the scrubber should be working as well.

7.Distillation

After the digested sample has cooled off the water steam distillation is done according to the following program:

Program parameter / Vap 50
Water addition in s / 9
NaOH addition in s / 8
Reaction time in s / 0
Distillation time in s / 240
Steam output in % / 100
Suction sample in s / 25
Boric acid addition in s / 6 s
Suction receiver in s / 25
Titration / Auto
Calculation / Auto
  1. Titration (is done automatically when using the Vap 50)
  • 3 - 4 drops of an indicator mixture M 5 are added to the receiving solution and it is then titrated with 0.1 N titration acid till the color changes from green to grey/violet.
  • If the determination of the endpoint is done with a pH-meter or a titrator, the addition of the indicator mixture is obsolete.
  1. Blank Value
  • For the determination of the blank value the analysis (digestion and distillation) is run just using the given chemicals.
  • The consumption of those chemicals has then to be taken into account when the calculation is done.

10.Calculation

1.4007 * c * (V - Vb)

% N =

Sample weight (g)

c : Concentration of the standard-acid solution: Hydrochloric acid 0.1N or c = 0.1 mol/l

Alternative: sulfuric acid 0.1N or c = 0.05 mol/l

V: Consumption of the standard acid in ml (Sample)

Vb: Consumption of the standard acid in ml (Blank Sample)

% raw protein= % N * 6.25

Nitrogen to Protein Conversion Factors for Various Foods

Factor
Egg or meat / 6.25
Dairy products / 6.38
Wheat / 5.70
Other cereal grains or oilseeds / 6.25
Almonds / 5.18
Peanut and Brazil nuts / 5.46
Other tree nuts and coconut / 5.30
Soybean products / 6.08

References

1. Gerhadt manual training

2. AOAC. 2005. Official Methods of Analysis. 18th Edition

3. Nielsen, S. S. 2003. Food Analysis. 3rd Edition. Springer Science

EXPERIMENT 5 : DETERMINATION FIBRE CONTENT

OBJECTIVE

To determine fibre content in whole bran bread and spinach.

DETERMINATION OF CRUDE FIBRE USING FIBREBAG (GERHARDT METHOD)

Apparatus/Instrument/Chemicals/Samples

  • Hotplate
  • 1L Beaker glass without spout and glass condenser with riffle
  • FibreBag – Carousel for 6 FibreBags with bayonet coupling
  • Bag with 100 FibreBags
  • Glass spacer
  • Accessory: crucible for incineration
  • Drying Chamber, Temp. 105°C
  • Muffle furnace, Temp. 600°C
  • Water heater
  • Desiccator
  • Timer or alarm clock
  • Analytical balance
  • Fume cabinet
  • Sulfuric acid c (H2SO4)= 0.13 mol/l
  • Potassium hydroxide solution c (KOH) = 0.23 mol/l
  • Petroleum ether, boiling range 40 to 60
  • Water distilled or demineralized
  • Sodium Hydroxide c (NaOH) = 0.313 mol/l
  • Hydrochloride acid c (HCl) = 0.1 mol/l

System Description: