2023 Cambridge IGCSE Food and Nutrition (0648) 模擬試題連答案詳解

Vitamin D is a fat-soluble vitamin. It can be sourced from foods such as oily fish (salmon, mackerel), egg yolks, liver, and fortified foods like margarine and milk, as well as synthesized in the skin from sunlight. Its primary function is to facilitate the absorption of calcium and phosphorus from the digestive tract, which is essential for healthy bone and tooth development. A deficiency of Vitamin D causes rickets in children (characterized by soft, malformed bones) and osteomalacia in adults (characterized by bone pain and muscle weakness).

Award up to 5 marks:
- 1 mark for each correct dietary source of Vitamin D (max 2 marks).
- 1 mark for correctly stating the function of Vitamin D (absorption of calcium/phosphorus or mineralization of bones/teeth).
- 1 mark for naming rickets as the child deficiency disease.
- 1 mark for naming osteomalacia as the adult deficiency disease.

When planning meals for elderly individuals, key factors include: reducing portion sizes/energy intake since metabolic rates decrease with age; ensuring adequate calcium and vitamin D intake to preserve bone density and prevent osteoporosis; increasing dietary fibre to maintain bowel regularity and prevent constipation; modifying food textures (e.g., stewing, mincing) for those with poor dentition; and minimizing sodium to lower the risk of hypertension and cardiovascular strain.

Award 1 mark for each distinct guideline with a valid explanation (up to 5 marks):
- Lower energy/fewer carbohydrates (1 mark) + due to lower activity/metabolism (1 mark)
- Calcium/Vitamin D (1 mark) + to maintain bones/prevent osteoporosis (1 mark)
- Dietary fibre/NSP (1 mark) + to prevent constipation (1 mark)
- Soft texture/easy-to-chew foods (1 mark) + due to tooth loss/dentures (1 mark)
- Reduce salt/sodium (1 mark) + to manage blood pressure/hypertension (1 mark)
- Iron/Vitamin C (1 mark) + to prevent anaemia (1 mark)

(a) Cow's milk is rich in HBV protein (such as casein and lactalbumin), minerals like calcium and phosphorus, and vitamins including riboflavin (B2) and Vitamin A. (b) Pasteurisation is a heat-treatment process (usually heating to 72 degrees Celsius for 15 seconds) which destroys harmful pathogenic bacteria, preventing foodborne illnesses, and reduces the number of spoilage microorganisms, thereby extending the freshness and shelf-life of milk.

(a) Award 1 mark for each correct nutrient identified (max 3 marks): Accept Protein (or HBV protein/casein), Calcium, Vitamin B2 (riboflavin), Fat, Vitamin A, Vitamin D, Phosphorus. Do not accept Carbohydrate/Lactose unless qualified as minor, as it is dissolved sugar.
(b) Award 1 mark for destroying pathogens/making milk safe, and 1 mark for extending shelf life/killing spoilage microorganisms (max 2 marks).

Digestion of protein in the stomach involves both physical and chemical action. Mechanically, the muscular stomach wall churns and squeezes the food (peristalsis), mixing it thoroughly with gastric juices. Chemically, the stomach secretes gastric juice containing hydrochloric acid (HCl) and inactive pepsinogen. The HCl creates an acidic environment (around pH 1.5 - 2.0) which kills bacteria and activates pepsinogen into the active enzyme pepsin. Pepsin hydrolyses proteins into shorter chains called polypeptides. In infants, the enzyme rennin is also present to clot milk proteins, slowing transit time for efficient digestion.

Award up to 5 marks for any of the following points:
- 1 mark for mechanical churning/peristaltic movement mixing food with gastric juices.
- 1 mark for identifying hydrochloric acid (HCl) creating an acidic environment.
- 1 mark for HCl activating pepsinogen into pepsin.
- 1 mark for pepsin converting proteins into polypeptides.
- 1 mark for rennin clotting/coagulating milk protein (caseinogen to casein).

(a) Hard and tough pastry occurs when too much gluten is developed. This can happen if the dough is over-worked or kneaded during preparation. It can also happen if too much cold water was added when binding, or if excess flour was worked into the dough during the rolling-out process. (b) Pastry shrinks during baking because it was stretched when being rolled out or fitted into the tin, causing the gluten strands to pull back when heated. Not resting the pastry in a cool place before baking also contributes to shrinkage.

(a) Award 1 mark for each identified cause of hard/tough pastry, up to 3 marks:
- Over-working / over-kneading ( gluten development)
- Too much water used to bind the fat and flour
- Excess flour used during rolling out
- Oven temperature too low (fat melts out before starch sets)
- Incorrect fat-to-flour ratio (too little fat)

(b) Award 1 mark for each identified cause of shrinkage, up to 2 marks:
- Pastry stretched when rolled or shaped into the tin
- Failure to rest the pastry in a cool place before baking

Bacteria require specific conditions to reproduce: 1. Warmth: Most food-poisoning bacteria grow best in the danger zone of 5°C to 63°C, with optimum growth at body temperature (37°C). 2. Moisture: Water is essential for bacterial cell functions and absorption of nutrients. 3. Food: Bacteria require nutrients, particularly high-protein foods like meat, dairy, and eggs. 4. Time: Under ideal conditions, bacteria divide rapidly via binary fission every 10 to 20 minutes. 5. Oxygen: Many bacteria require oxygen (aerobic) to grow, though some (anaerobic) grow without it. 6. pH: A neutral or slightly alkaline pH is optimal, whereas highly acidic environments inhibit growth.

Award 1 mark for each condition identified and briefly explained (max 5 marks):
- Warmth (1 mark) + mention of danger zone (5°C to 63°C) or body temperature (37°C) (1 mark)
- Moisture/Water (1 mark) + required for growth/cellular activity (1 mark)
- Food/Nutrients (1 mark) + high-protein/high-risk foods (1 mark)
- Time (1 mark) + explanation of rapid multiplication/binary fission (1 mark)
- Oxygen (1 mark) + aerobic/anaerobic respiration requirements (1 mark)
- pH/Acidity (1 mark) + neutral/slightly alkaline preference (1 mark)

Conduction is the process of heat transfer through solid materials. When heat is applied directly to a material (like a metal pan on a hob), the molecules at the source absorb thermal energy, causing them to vibrate rapidly. These vibrating molecules collide with adjacent molecules, passing the energy along. Metals are excellent conductors because they allow this heat to flow easily. Examples of cooking methods using conduction include shallow frying (where heat moves from the pan directly to the food) and boiling (where heat transfers from the burner to the saucepan base, and then to the liquid and food).

Award up to 3 marks for the explanation of conduction:
- 1 mark for stating that heat is transferred through direct contact / from molecule to molecule.
- 1 mark for explaining that molecules vibrate and pass energy to neighboring molecules.
- 1 mark for mentioning that metals are good conductors / heat travels from the heat source through the pan to the food.
Award 1 mark for each correct cooking method/example, up to 2 marks:
- Shallow frying / stir-frying / pan-searing
- Boiling / simmering (heat transfer through the bottom of the pan)
- Griddling

Convenience foods offer several benefits: they greatly reduce the time and energy needed for food preparation; they have a long shelf-life making them excellent for emergencies; they are easy to prepare for individuals with limited cooking skills; and they ensure consistent product quality. However, they also have drawbacks: they are frequently highly processed and contain high amounts of added salt, sugar, and saturated fats; they can contain synthetic chemical preservatives/additives; they are often more expensive per portion than home-cooked alternatives; and heat processing can destroy volatile nutrients like Vitamin C.

Award 1 mark for each correct advantage identified (max 3 marks):
- Saves preparation and cooking time.
- Saves fuel/energy during cooking.
- Long shelf-life/easy to store (e.g., canned or frozen).
- Requires less culinary skill/knowledge.
- Less waste as portions are pre-measured.
- Consistent quality and taste.

Award 1 mark for each correct disadvantage identified (max 2 marks):
- High in sodium/salt, sugar, or saturated/trans fats.
- Low in dietary fibre/NSP due to heavy processing.
- Expensive compared to fresh, raw ingredients.
- Loss of heat-sensitive nutrients (e.g., Vitamin C, Vitamin B1) during manufacturing.
- Contains artificial additives (preservatives, colorings, flavorings).

(a) Maillard reaction is a non-enzymic browning reaction that occurs when the amino acids in proteins react with reducing sugars (such as glucose or fructose) under dry heat. Two conditions required for this reaction are: presence of a reducing sugar and an amino acid, and application of heat (typically above \(140^{\circ}\text{C}\)) with low surface moisture.

(b) Caramelisation is the oxidation of sugar (sucrose) when heated. The heat breaks down the sugar molecules into glucose and fructose, which then dehydrate and polymerise, forming a complex mixture of compounds called caramel. This results in a golden-brown colour, a rich characteristic aroma, and a slightly bitter sweet flavour.

(c) Three examples of culinary processes/dishes where non-enzymic browning is desirable are: roasting meat (searing/Maillard reaction), baking bread (crust formation/Maillard reaction), and making creme caramel or caramel sauce (caramelisation of sugar).

Part (a) [Total: 3 marks]
- 1 mark: Correct definition of Maillard reaction (chemical reaction between amino acids and reducing sugars).
- 2 marks: State two correct conditions (1 mark per condition: presence of reducing sugar, presence of amino acid/protein, dry heat/temperatures above \(140^{\circ}\text{C}\), low surface moisture).

Part (b) [Total: 3 marks]
- 1 mark: Application of dry heat to sugar (heating sucrose above its melting point).
- 1 mark: Dehydration or breakdown of sugar molecules into new complex structures/compounds.
- 1 mark: Sensory description (change of colour to golden-brown, sweet-bitter flavor, caramel aroma).

Part (c) [Total: 3 marks]
- 3 marks: State three correct examples of desirable non-enzymic browning (1 mark each: e.g., toasted bread, roasted meat, baked cookies/biscuits, caramel sauce, toasted marshmallows, roasted coffee beans).

(a) Gelatinisation process:
1. Starch grains are suspended in liquid (milk) and heated.
2. At around \(60^{\circ}\text{C}\), the starch granules begin to absorb the liquid and swell.
3. At around \(80^{\circ}\text{C}\), the starch granules swell to their maximum capacity, burst, and release starch (amylose and amylopectin) into the liquid, forming a gel.
4. At \(100^{\circ}\text{C}\) (boiling point), the gelatinisation process is complete, and the sauce must be boiled for 1-2 minutes to cook out the raw starch flavor.

(b) Dextrinisation process and sensory changes:
- Dextrinisation is the process where starch molecules are broken down into smaller, simpler carbohydrates called dextrins by the application of dry heat.
- Two visual/sensory changes when bread is toasted:
1. The colour changes from white to golden brown.
2. The texture becomes dry, hard, and crispy.
3. The bread develops a sweet, toasted aroma and flavour.

Part (a) [Total: 5 marks]
- 1 mark: Starch grains absorb liquid on heating.
- 1 mark: Mention of swelling at approx. \(60^{\circ}\text{C}\).
- 1 mark: Mention of bursting and releasing starch at approx. \(80^{\circ}\text{C}\).
- 1 mark: Mention of boiling at \(100^{\circ}\text{C}\) to complete the process.
- 1 mark: Explanation of gel formation/thickening of liquid.

Part (b) [Total: 4 marks]
- 2 marks: Clear definition of dextrinisation (starch broken down [1 mark] into dextrins [1 mark] by dry heat).
- 2 marks: Any two visual or sensory changes described (1 mark each: colour changes to brown, becomes crispy/crunchy texture, sweeter taste/aroma).

(a) Denaturation is the process where the physical and chemical structure of protein molecules is altered or disrupted. This change is irreversible, meaning the protein chains unfold from their native coiled state.
Three other physical agents or conditions that cause denaturation are:
1. Mechanical action / agitation (e.g., whisking egg whites)
2. Acids / low pH (e.g., adding lemon juice to milk/marinades)
3. Alkalis / high pH
4. Alcohol
5. Addition of salt.

(b) Coagulation occurs after protein denaturation. The unfolded protein chains collide and form new cross-links, trapping liquid in a solid, three-dimensional network. When cooking an egg:
- Egg white proteins coagulate first at approximately \(60-65^{\circ}\text{C}\) and change from clear to opaque white.
- Egg yolk proteins coagulate at approximately \(65-70^{\circ}\text{C}\) and become firm.
- Overheating: If the egg is overheated, the protein network shrinks too much and squeezes out the trapped water. This is called syneresis, resulting in a rubbery, dry egg white and a watery discharge.

Part (a) [Total: 4 marks]
- 1 mark: Clear definition of denaturation (unfolding/alteration of protein molecule structure, which is irreversible).
- 3 marks: State three correct physical agents/conditions (1 mark each: mechanical action/whisking, acid, alkali, alcohol, or salt).

Part (b) [Total: 5 marks]
- 1 mark: Definition of coagulation (solidifying of denatured proteins into a network trapping liquid).
- 1 mark: Mention of egg white coagulation temperature range (\(60-65^{\circ}\text{C}\)).
- 1 mark: Mention of egg yolk coagulation temperature range (\(65-70^{\circ}\text{C}\)).
- 1 mark: Explanation of overheating causing the protein structure to shrink/tighten.
- 1 mark: Correct identification of syneresis / watery discharge / rubbery texture.

(a) Process of enzymic browning:
1. When the cell walls of fruit/vegetables are cut or damaged, oxygen from the air enters the tissue.
2. The enzyme polyphenol oxidase (phenolase) inside the plant cells is released.
3. The enzyme reacts with oxygen to oxidise the phenolic compounds naturally present in the fruit, forming a dark brown pigment called melanin on the surface.

(b) Three methods to prevent enzymic browning and their scientific explanation:
1. **Acidulation (adding acid, e.g., lemon juice)**: The low pH of the citric acid denatures the polyphenol oxidase enzyme, changing its shape so it can no longer catalyse the oxidation reaction.
2. **Blanching (heating)**: Plunging the sliced apples into boiling water for a short time denatures the enzyme using high temperatures, making it inactive.
3. **Immersion in water / vacuum packing**: Submerging the sliced apples in cold water or removing air prevents oxygen from coming into contact with the enzyme and the phenol substrates, halting the reaction.
4. **Antioxidants (e.g., Vitamin C/ascorbic acid)**: Ascorbic acid acts as a reducing agent, reacting with oxygen before it can react with the phenols.

Part (a) [Total: 3 marks]
- 1 mark: Damage to cells releases polyphenol oxidase / phenolase enzymes.
- 1 mark: Oxygen from the air reacts with phenol substrates in the food.
- 1 mark: Oxidation results in the formation of brown pigments (melanin).

Part (b) [Total: 6 marks]
- 2 marks per method (1 mark for identifying the method, 1 mark for the scientific explanation of how it works. Maximum of three methods):
- Method 1: Acidulation / lemon juice [1 mark] + Low pH denatures the enzyme [1 mark].
- Method 2: Blanching / boiling [1 mark] + Heat denatures/inactivates the enzyme [1 mark].
- Method 3: Immersion in water [1 mark] + Excludes oxygen from contacting the surface [1 mark].
- Method 4: Use of sulfur dioxide / antioxidants [1 mark] + Reacts with oxygen/acts as a reducing agent [1 mark].

(a) Bicarbonate of soda (sodium bicarbonate) decomposes on heating to produce carbon dioxide, water, and sodium carbonate. The sodium carbonate is alkaline and remains in the cake. It causes two major problems:
1. A bitter, soapy chemical taste.
2. A yellow/greenish discolouration in the cake crumb, which makes white or pale sponge cakes look unappealing.

(b) Composition and reaction of baking powder:
- **Composition**: Commercial baking powder contains an alkaline component (sodium bicarbonate), an acidic component (usually cream of tartar / monocalcium phosphate), and an inert starch/filler (such as cornflour/cornstarch) to absorb moisture and prevent premature reaction in the container.
- **Reaction**: When water (from milk/eggs) is added and heat is applied during baking:
1. The acid and alkali dissolve in the liquid and react with each other.
2. The reaction produces carbon dioxide gas (\(\text{CO}_2\)), a neutral salt, and water.
3. The carbon dioxide gas expands within the gluten network and air bubbles of the batter, causing the cake to rise, leaving behind a neutral-tasting salt with no discolouration.

Part (a) [Total: 3 marks]
- 1 mark: Identification of the undesirable end-product (sodium carbonate / washing soda).
- 1 mark: Explanation of the effect on flavour (bitter/soapy taste).
- 1 mark: Explanation of the effect on appearance (yellowing / discolouration of the cake crumb).

Part (b) [Total: 6 marks]
- 3 marks for Composition (1 mark each for identifying: alkaline component / sodium bicarbonate, acidic component / cream of tartar or acid phosphate, starch/filler / cornflour).
- 3 marks for Chemical reaction description:
- 1 mark: Moisture/water dissolves the ingredients, allowing them to react.
- 1 mark: Heat accelerates the reaction between the acid and alkali.
- 1 mark: Carbon dioxide gas (\(\text{CO}_2\)) is produced, which expands to make the mixture rise, leaving a neutral salt.

### (a) Comparison of Nutritional Value of Oily Fish and White Fish
- **Protein:** Both oily and white fish are excellent sources of High Biological Value (HBV) protein, containing all essential amino acids required for body growth, repair, and maintenance.
- **Fat Content:** Oily fish (e.g., salmon, mackerel, sardines) contains fat distributed throughout its flesh (typically 10-20% fat), while white fish (e.g., cod, haddock, plaice) stores fat in the liver, leaving the flesh very lean (less than 2% fat).
- **Type of Fat:** The fat in oily fish is highly unsaturated and rich in Omega-3 essential fatty acids, which help reduce the risk of cardiovascular disease and support brain development. White fish contains minimal fat and is excellent for low-calorie or low-fat diets.
- **Vitamins:** Oily fish is an excellent source of fat-soluble vitamins A (for healthy vision and skin) and D (for calcium absorption). White fish lacks significant amounts of fat-soluble vitamins in its flesh but is a source of water-soluble B vitamins (such as cobalamin/B12 for red blood cell formation and energy release).
- **Minerals:** Both types provide minerals like iodine (for thyroid function) and phosphorus/selenium. Oily fish consumed with bones (e.g., canned sardines) provides a rich source of calcium for strong bones and teeth.

### (b) Guidelines for Buying and Storing Fresh Fish
- **Buying Guidelines:**
- **Eyes:** Should be bright, clear, and bulging (not sunken, dull, or cloudy).
- **Gills:** Should be bright red or pink (not brown or grey).
- **Flesh:** Firm and elastic to the touch; when pressed, it should spring back immediately.
- **Skin/Scales:** Scales should be intact, shiny, and cling firmly to the skin; skin should have a natural sheen and not be covered in thick slime.
- **Smell:** Should have a pleasant, fresh sea breeze smell, not a strong, unpleasant 'fishy' or ammoniacal odor.
- **Storage Guidelines:**
- **Temperature Control:** Store immediately in the coldest part of the refrigerator (0°C to 4°C) or on a bed of crushed ice to slow bacterial growth.
- **Hygiene/Cross-contamination:** Keep raw fish wrapped or in a covered container at the bottom of the fridge, strictly below and away from cooked or ready-to-eat foods to prevent drip contamination.
- **Preparation:** Eviscerate (gut) and wash the fish if not already done, as bacteria in the gut cause rapid spoilage.
- **Time Frame:** Consume fresh fish within 24 hours of purchase for optimum quality and safety.

### (c) Selection and Control of Cooking Methods to Conserve Nutrients
- **Steaming and Poaching:** Highly recommended as they are gentle, moist-heat methods. Because fish is not submerged in boiling water during steaming, water-soluble B-group vitamins are conserved. Poaching liquid can be used to make sauces so any leached nutrients are still consumed.
- **Grilling (Broiling) and Baking:** These dry-heat methods do not require the addition of fat, preserving the naturally low-calorie, healthy profile of fish. High heat in grilling quickly seals the exterior, keeping juices and nutrients locked inside.
- **Avoiding Deep-Frying:** Deep-frying adds significant amounts of saturated fats and high temperatures can damage heat-sensitive vitamins (like Vitamin B1) and degrade the beneficial omega-3 fatty acids in oily fish.
- **Temperature and Time Control:** Fish protein (myosin) coagulates at a low temperature (around 60°C). Overcooking causes the protein fibers to shrink and squeeze out moisture, leading to a dry texture and loss of water-soluble vitamins in the lost juices. Cooking should be brief, stopping as soon as the flesh turns opaque and flakes easily.

Award up to 15 marks in total, structured across the three parts (maximum of 5 marks for each section):

### Part (a) Comparison of Nutritional Value (Max 5 marks):
- 1 mark: Identifying both provide HBV protein for growth/repair.
- 1 mark: Explaining fat distribution (oily fish has fat in flesh, white fish in the liver/lean flesh).
- 1 mark: Highlighting Omega-3 fatty acids in oily fish and their health benefits (e.g., heart health).
- 1 mark: Noting fat-soluble vitamins (A and D) present in oily fish vs B vitamins in white fish.
- 1 mark: Mentioning minerals (e.g., calcium in small edible bones, iodine in both).

### Part (b) Buying and Storing Fresh Fish (Max 5 marks):
- 1 mark per valid buying point with explanation (up to 3 marks):
- Bright, bulging eyes (not sunken).
- Red/pink gills (not grey/brown).
- Firm, elastic flesh (springs back).
- Fresh sea smell (no ammonia).
- 1 mark per valid storage point with explanation (up to 3 marks):
- Store in refrigerator (0-4°C) / on ice to slow bacterial growth.
- Keep covered on the bottom shelf to prevent cross-contamination.
- Use within 24 hours of purchase.

### Part (c) Selection and Control of Cooking Methods (Max 5 marks):
- 1 mark: Steaming/poaching prevents loss of water-soluble B vitamins.
- 1 mark: Baking/grilling avoids adding extra fat / maintains low-fat profile of white fish.
- 1 mark: Avoiding deep-frying as high heat damages omega-3 and adds unhealthy fat.
- 1 mark: Preventing overcooking which causes loss of moisture/nutrients and toughens protein.
- 1 mark: Use of poaching liquid in sauces to retain leached nutrients.