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Lab Theory Test...

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151.231 Food Chemistry for Nutrition

2018

Laboratory Theory Test Time Allowed: 90 minutes Answer all questions & Total marks = 90 This is an open book test (only lab manual).

Question 1

(5 marks total)

Choose the single best answer. 1a. If we assume that the Kjeldahl factor used to calculate crude protein from the total nitrogen content for soy bean is 4.5. What percent nitrogen is present in soy bean proteins? a) 16

b) 22

c) 28

d) 45

1b. The determination of protein content in foods by Kjeldahl method is based on the measurement of nitrogen. When the protein content of egg yolk is analyzed by Kjeldahl method, which of the following components would be a major source of non-protein nitrogen? a)

Cholesterol

b) Glycoproteins

c) Lipoprotein

d) Phospholipids

1c. The use of colour indicators is not always necessary for the determination of a) b) c) d)

protein by Kjeldahl method fat by Mojonnier method free fatty acids by titrimetric method salt by Mohr method

1d. The calcium content of whole milk powder was determined by a dye binding assay using ocresolphthalein complexone as a chromogen. The intensity of colour developed after dye binding reaction is : a) b) c) d)

directly proportional to the calcium concentration. inversely proportional to the calcium concentration. not related to the calcium concentration. decreased as the concentration of calcium is increased.

1e. A wavelength of 550 nm was chosen for the measurement of absorbance of the coloured complex because a) b) c) d)

it is in the UV light range. it is the highest wavelength that can be achieved by a spectrophotometer. it is the wavelength of maximum absorption. it is near the midpoint in the visible light range between 400 and 700 nm.

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151.231 Food Chemistry for Nutrition

Question 2

2018

(22 marks total)

2a. List the six (6) categories of compounds determined as the composition of foods by the proximate analysis. (3 marks) 2b. Briefly explain the reasons why two groups of components were NOT analysed in your lab experiments when carrying out the proximate analysis of whole milk powder (WMP)? (3 marks) 2c. In the proximate analysis of WMP, which component was found to be in the highest and lowest content, respectively, according to the Certificate of Analysis (CoA) and the Product Bulletin for WMP? (4 marks) 2d. Briefly define the term "proximate analysis". Describe some limitations associated with the proximate analysis that need to be considered when conducting such analysis and give two examples in your explanation. (5 marks) 2e. The protein content of foods determined by Kjeldahl method is not re proein and is referred to as crde proein Briefl explain the reason why. (3 marks) 2f. Briefly explain why and for what purposes the proximate analysis of foods is conducted. (4 marks)

Question 3

(26 marks total)

3a. A food sample was dried in an air oven overnight and the following results were obtained.

Replicate 1 Replicate 2 Replicate 3

Food original weight (grams) 2.3568 2.0568 3.0456

Final dried weight (grams) 1.1368 0.9541 1.4357

What are the average % moisture and total solids content of the food sample?

(5 marks)

3b. The following results were obtained for the determination of the content of ash in a sample.

Replicate 1 Replicate 2 Replicate 3

Original weight of sample (g) 2.5022 2.4000 2.3588

Weight of crucible (g) 9.2451 10.1571 9.3805

What is the average ash content of the sample?

Weight after ashing (g) 9.6890 10.5524 9.7628 (5 marks) Page 2 of 5

151.231 Food Chemistry for Nutrition

2018

3c. As part of food chemistry course, you were asked to analyse the composition of a cream soup mix powder which was purchased from a supermarket. This product contains proteins derived from wheat flour. The nutritional information of the product is shown below. Nutritional Information Pack: 100 g Servings Per Pack: 5 Serving size: 200 ml Amount Per Serving Protein Fat  Total - Saturated Carbohydrate - Sugars Sodium Calcium

An quantity per 200 mL serve 4.0g 3.0g 1.6g 10g 1.5g 700mg 800mg

An quantity per 100 mL 2.0g 1.5g 0.8g 5.0g 0.8g 350mg 400mg

Protein in the sample was determined in triplicate by the Kjeldahl method. The weight of cream soup powder used in the triplicate analysis was 1 gram. After digestion and distillation, the volumes of 1 M HCl required to reach the end point of titration were 22, 23 and 24 mL, respectively, for the triplicates. i)

What is the average protein content of cream soup powder? Show your calculation. (4 marks)

ii)

Evaluate the precision of your results. Show the coefficient of variance (CV %). Comment whether the triplicate results were within an acceptable range or not. Note that the standard deviation of triplicate results for protein is 0.57. (4 marks)

iii)

Compare your results with the nutritional label, show the degree (%) of difference between the two values and comment on the accuracy of your results. (4 marks)

iv)

Based on the precision and accuracy of your results, list any possible sources of errors with regard to the experiments if there was a significant difference between your results and the reference. (4 marks)

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151.231 Food Chemistry for Nutrition

Question 4

2018

(15 marks total)

The Mojonnier fat extraction was developed for the determination of total fat content in milk and dairy products. This method is highly accurate when it is carefully carried out. 4a. Describe and explain the nature (at least 4) of solvents, such as ethyl ether and petroleum ether, used as an extraction medium, in terms of their function, properties, advantages and disadvantages. (4 marks) 4b. In this method, a mixture of sample and reagents in the Mojonnier flask separates into the two phases (upper and lower) after centrifugation. Compare and contrast the chemical composition and properties between the two phases. (5 marks)

4c. Explain the role of ammonium hydroxide used as a reagent in the analysis of fat content in WMP by the Mojonnier method. (3 marks)

4d. In an experiment, the content of fat analysed by the Mojonnier method was identified to be higher than the actual (true) value. Give an example for the type and source of experimental errors. (3 marks)

Question 5

(12 marks total)

The content of free fatty acids (FFA) is often measured to determine the decomposition of triglycerides in oil caused by lipase or other action. This can be analysed by titrimetric method with the following chemicals: ethanol, phenolphthalein and potassium hydroxide The resls is epressed as acid ale or percen free fa acids 5a. What is the unit of acid value?

(2 marks)

5b. What is the role of ethanol in this method? Explain the reason.

(3 marks)

5c. Why is it necessary for potassium hydroxide to be standardised before being used? (3 marks)

5d. If the acid value of olive oil is 1.5, what is the percentage (%) of FFA when expressed as an oleic acid equivalent? Show your calculations. (4 marks)

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151.231 Food Chemistry for Nutrition

Question 6

2018

(10 marks total)

The Nutrition Facts label on food packages normally shows the content of salt (NaCl) as milligrams (mg) of sodium per 100 g product. The salt content is often determined by Mohr method. 6a. In the Mohr method, which reagents are used as an indicator and a titrant, respectively? (2 marks)

6b. In the determination of salt by the Mohr method, a 12 g of a food sample required 30 ml of a 0.01N silver nitrate solution for the titration of chloride ion. The atomic weights of sodium and chlorine are 23 and 35, respectively.

i) What is the percentage of salt (NaCl) in the product?

(4 marks)

ii) What is the sodium (Na) content of the product when expressed as mg per 100 g product? (4 marks)

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