PART III: LABORATORY METHODS OF ANALYSIS
3.3.1 PROXIMATE
40 8. Replace the dish with its lid partially covered in the oven for 1hr. Transfer to the desiccator to cool, and 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).
Calculation
Moisture (%) = W1 – W2 x 100 W1
Where, W1 = weight (g) of sample before drying.
W2 = weight (g) of sample after drying.
Reference
Association of Official Analytical Chemists (1984). Official Methods of Analysis. 14th Edition (Williams, S. ed.), AOAC, Virginia.
41 METHOD B: VACCUM OVEN
Principle
Samples are dried to constant weight under a constant or reduced pressure at a prescribed temperature for a prescribed time. The moisture is the disparity of the weight measured before and after the drying. % Moisture = (M initial - M dried) / M initial X 100.
Chemicals/Reagents
1. Silica sand, 3-5mm or Celite 545 Apparatus/Equipments
1. Vacuum Oven: thermostatically controlled and connected through a drying train to a vacuum pump capable of maintaining the pressure in the oven below 25mm of mercury. The oven should be provided with an air inlet connected to a silica gel drying bottle and a trap for releasing the vacuum.
2. Convection Oven
3. Dishes: glass and aluminum with covers 4. Desiccators: with silica gel as drying agent Procedures
1. Decide drying condition based on type of sample whether it requires convection or vacuum oven; temperature and time for drying, and suitable amount to weigh.(Refer Table 4)
2. Dry sample (moisture less than 20%) should use Aluminum dish while wet sample should use glass dish with sea/silica sand / Celite 545 and glass rod.
3. Dry aluminum dish with cover or glass dish with silica sand / Celite 545 and glass rod to gain constant weight in convection oven at temperature 105oC for more than 4 hours.
4. Transfer the dried aluminum dish or glass dish into desiccator with the lid partially opened for 20 minutes. Then, fully close the desiccator and cool it for at least 40 minutes.
5. Weigh out the dish using an analytical balance with four decimal places. Record the weight as W1 in worksheet.
6. Add suitable amount of homogenized sample and record the final weight W2.
7. Transfer the dish into an oven and dry the sample using prescribed temperature and time as in Table 4.
8. For sample in glass dish, mix sample thoroughly in silica sand / Celite 545 with glass rod. Place the sample on a steam water bath at temperature not more than prescribed in the drying condition (refer Table 4).
9. Sample with high sugar content need to be mixed by glass stick occasionally to avoid the sugar being crystallized and trapped with moisture. Then, dry the sample using prescribed condition (refer Table 4).
10. Operation of oven is referred to instrument operation manual, and for vacuum oven the reference is Equipment Operation Procedure for Vacuum Oven.
11. Take out the dried samples. Close the lid for Al dish.
42 12. Transfer the dish into desiccator with the lid partially opened for 20 minutes. Then,
close the desiccator fully and cool it for at least 40 minutes.
13. Weigh out the final weight, W3.
Calculation
Moisture, g/100g = (W2 - W3)/(W2 - W1)×100 Where, W1 = Weight of dried dish
W2 = Weight of dried dish with sample W3 = Weight of dried dish and dried sample References
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 33.5.02,Method 927.05
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 31.1.02,Method 931.04
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 27.4.03,Method 945.15
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 33.7.04,Method 948.12
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 44.1.03,Method 924.45
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 44.4.04,Method 969.38
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 27.3.06,Method 935.29
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 39.1.02,Method 950.46
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 33.5.02,Method 927.05
Official Methods of Analysis of AOAC International, 17th Edition, Volume II,Section 32.1.02,Method 925.09
Training Material Provided by JICA’s Expert (2004) extracted from Analytical Manual for Standard Tables of Food Composition in Japan, 5th Revised Edition (2000)
43 Table 4: Conditions of drying
Food Matrix Sample
Weight (g)
Temperature, (o C)
Time (hour)
Type of Dish
Cereal 3-5 105 5 Al can
Grains flour 3-5 135 1 Al can
Bread 2-3 135 1 Al can
Bread with others, jam, cream etc
2-3 V 70 5 Glass dish*
Noodle 2-3 135 3 Glass dish*
Noodle with seasonings ingredients
2-3 105 5 Al can
Potatoes 2-3 100 5 Al can
Starch 2-3 135 1 Al can
Sugar powder 5 V 105 3 Glass dish*
Sugar liquid 1.5 V 100 3 Glass dish*
Honey 1-1.5 V 90 3 Glass dish*
Cake, Pie 2-3 V 70 5 Glass dish*
Cookies, Cracker, Biscuit 3-5 100 5 Al can
Candy 1.5 V 100 2 Glass dish*
Chocolate 2-3 V 70 5 Glass dish*
Snack 3-5 105 3 Al can
Nuts 2 130 2 Al can
Beans 5 130 3 Al can
Boiled beans 2 V 100 5 Glass dish*
Soybean 5 130 2 Al can
Defatted soybean 2 130 1 Al can Fish and its processed products 5-10 105 5 Glass dish*
Meat and its processed products 2-3 135 2 Glass dish*
Egg 2-3 V 100 5 Glass dish*
Milk liquid 2-3 100 3 Glass dish*
Milk powder, Condensed milk 2 100 4 Al can
Cream, Ice cream 2-3 100 3 Glass dish*
Lactic beverages, yogurt 2 V 100 4 Glass dish*
Cheese 3 105 4 Glass dish*
Vegetables, Fruits its processed products
3-4 V 70 5 Glass dish*
Mushrooms, Seaweed 5 105 5 Al can Coffee beans, Coffee powder 2-5 105 5 Al can
Salt 3 140 1.5 Al can
Seasoning 3-5 V 70 5 Al can
Soy sauce, Other sauces 5 V 70 5 Glass dish*
44
Food Matrix Sample
Weight (g)
Temperature, (o C)
Time (hour)
Type of Dish Processed food If drying condition is different between ingredients, choose the
lower temperature and/or vacuum condition. An example is canned fish in tomato paste, oil and seasoning shall use vegetable product condition or seasoning drying condition, NOT fish product drying condition.
* : Use sea sand method V: Vacuum condition
METHOD C: INFRA-RED
Principle
Infrared drying involves the penetration of heat into the food sample being dried. It is a rapid method of moisture content analysis as the heat penetration enables water to evaporate fairly quickly.
Apparatus/Instruments 1. Infra-red balance
2. Disposable aluminum pan liner
3. Desiccator with some moisture absorbent – (silica gel, calcium chloride, concentrated sulphuric acid etc.)
Procedures
1. Pre-dry the disposable aluminum pan liner at 100 C overnight and then let it cool in a dessicator.
2. Balance the infra-red meter at zero level.
3. Accurately weigh 10 g flour and evenly spread it onto the pan liner. Do not compress the powder.
4. Place dish with sample on infra-red meter dish holder and switch on the heater in the moisture meter.
5. Note down the percentage of moisture loss every minute as shown on the display panel until no further changes in moisture content is detected.
6. Allow the moisture balance to cool before repeating this experiment.
7. Plot a graph showing “Moisture loss (g) against time (min)”.
8. Results can be read directly from the balance scale display panel.
Reference
Nielsen, S. S. 1994. Introduction to the Chemical Analysis of Foods. Boston: Jones and Bartlett Publishers, Inc.
45 3.3.1.2 DETERMINATION OF PROTEIN CONTENT
Principles
A food is digested with concentrated sulphuric acid to convert organic nitrogen to ammonium ions.
N(food) (NH4)2SO4
During neutralization, alkali is added and the liberated ammonia is distilled into an excess of boric acid solution.
(NH4)2SO4 + 2NaOH 2NH3 + 2H2O + Na2SO4 NH3 + H3BO3 (boric acid) NH4+
+ H2BO3-
(borate ion)
The distillate is titrated with standardized hydrochloric acid to determine the ammonia absorption in the boric acid.
H2BO3-
+ H+ H3BO3
The amount of protein present is then calculated from the nitrogen concentration of the food.
Chemicals/Reagents
1. Catalyst tablet. (copper sulfate: potassium sulfate = 1:9) 2. Concentrated sulphuric acid
3. Hydrogen peroxide (chilled)
4. Hydrochloric acid 0.2N: Measure 200ml of 1N HCl in 1L volumetric flask and mark up to 1L distilled water.
5. Methyl Red Indicator: Weigh out 100mg of Methyl Red (MR) in 100ml volumetric flask. Dilute with 100ml methanol.
6. Bromocresol Green Indicator: Weigh out 100mg of Bromocresol Green (BCG) in 100ml volumetric flask. Dilute with 100ml methanol.
7. Sodium hydroxide (NaOH), 40%: Weigh out 400g of Sodium hydroxide (NaOH) and dissolve in 1L of distilled water.
8. 1% Boric Acid: Weigh out 10g of Boric Acid and dissolve in 1L of distilled water.
Apparatus/Equipments
1. Automated Digestion Unit: attached with scrubber.
2. Automated Protein Analyzer Machine.
3. Balance: Analytical sensitivity ± 0.1 mg 4. Digestion tube
5. Volumetric Flasks: 100 mL & 1L 6. Conical Flasks: 250 mL
7. Measuring Cylinder: 25 mL 8. Pipet: 5 mL
46 Procedures