2024 Cambridge IGCSE Food and Nutrition (0648) 模擬試題連答案詳解

(a) Proteins are organic macromolecules consisting of amino acids. The four essential chemical elements present in all protein molecules are Carbon, Hydrogen, Oxygen, and Nitrogen. Some proteins also contain Sulfur or Phosphorus. (b) High Biological Value (HBV) proteins contain all of the essential (indispensable) amino acids required by the body in correct proportions. Animal sources like milk, cheese, meat, and eggs are typical examples. Low Biological Value (LBV) proteins are deficient in one or more essential amino acids. Plant sources like beans, lentils, peas, and cereals are typical examples.

Part (a): [4 marks total] - 1 mark for each correct element: Carbon, Hydrogen, Oxygen, Nitrogen (accept Sulfur or Phosphorus as alternatives). Part (b): [4 marks total] - 1 mark for correct definition of HBV protein (contains all essential/indispensable amino acids). - 1 mark for a correct HBV food source (e.g. egg, milk, fish, meat, soya). - 1 mark for correct definition of LBV protein (lacks one or more essential/indispensable amino acids). - 1 mark for a correct LBV food source (e.g. peas, beans, lentils, wheat, nuts).

(a) Bacteria require specific conditions to multiply rapidly: warmth (usually between 5 degrees C and 63 degrees C, known as the danger zone), moisture (water is essential for cellular functions), food/nutrients (particularly high-protein foods), and time (bacteria double every 10-20 minutes). Oxygen and neutral/slightly acidic pH are also conditions for most common pathogens. (b) Personal hygiene is critical: washing hands removes transient bacteria; tying hair prevents physical and bacterial contamination; wearing an apron protects food from dirt on clothing; removing jewellery prevents dirt/bacteria from harboring in crevices and falling into food.

Part (a): [4 marks total] - 1 mark for each correct condition listed: Warmth (or correct temperature), Moisture (or water), Food (or nutrients), Time (or neutral pH/oxygen). Max 4 marks. Part (b): [4 marks total] - 1 mark for each correct personal hygiene practice described: Handwashing with soap, tying/covering hair, wearing clean protective clothing/apron, removing jewelry/watches, covering cuts with waterproof blue dressing. Max 4 marks.

(a) Food preservation is done to prevent the action of enzymes and micro-organisms, thereby extending shelf-life. It also prevents food wastage by utilizing glut crops, allows food to be transported safely over long distances, makes out-of-season foods available year-round, and adds variety to the diet (e.g., turning milk into cheese or fruit into jam). (b) Freezing at -18 degrees C or below works by converting liquid water inside the food into ice crystals. Bacteria require liquid water to grow and reproduce; without it, their growth is halted. The extreme cold also inactivates or slows down enzyme activity and bacterial metabolism, rendering the bacteria dormant (though not killed).

Part (a): [4 marks total] - 1 mark for each valid reason explained: Extend shelf-life/prevent spoilage, prevent waste/utilize surplus, make out-of-season foods available, ease of storage/transport, provide variety/convenience. Max 4 marks. Part (b): [4 marks total] - 1 mark for stating that water is frozen into ice/is unavailable to microbes. - 1 mark for stating that bacteria become dormant/cannot reproduce. - 1 mark for stating that chemical/enzyme reactions are slowed down. - 1 mark for mentioning the preservation temperature (-18 degrees C or lower).

(a) Radiation is the transfer of heat energy by electromagnetic waves (rays) directly from the heat source to the food, without the need for a solid, liquid, or gas medium to conduct it. Examples include grilling, toasting, or microwave cooking. (b) Steaming cooks food in the steam rising from boiling water. Advantages: water-soluble vitamins (such as Vitamin C and B-group) and minerals are not dissolved (leached) into the cooking water; color, texture, shape, and flavor of the vegetables are better preserved; it is a gentle method that prevents food from breaking up; multiple tiers can save energy. (c) Water-soluble vitamins are easily destroyed by heat, oxygen, and leaching. To minimize loss: cook for the shortest time possible; use minimal cooking liquid; keep the lid on the pan to exclude oxygen and speed cooking; do not cut vegetables too small (less surface area exposed); do not add bicarbonate of soda (alkali destroys vitamin C); use the cooking liquid in sauces/gravies.

Part (a): [2 marks total] - 1 mark for defining radiation (heat transfer via electromagnetic waves/rays/no medium). - 1 mark for correct method (grilling, toasting, or microwaving). Part (b): [3 marks total] - 1 mark for each valid advantage: Retention of nutrients/vitamins, retention of color/flavor/texture, energy-efficient/multi-tier cooking, gentle method (no breaking of food). Max 3 marks. Part (c): [3 marks total] - 1 mark for each valid prevention method: Avoid over-chopping/peeling, cook for minimum time, use minimal water, keep lid on saucepan, avoid bicarbonate of soda, reuse cooking water. Max 3 marks.

(a) Meal planning for the elderly must address physiological changes: 1. Calcium and Vitamin D are needed to maintain bone density and prevent osteoporosis. 2. Dietary fibre (NSP) and fluids are essential to prevent constipation caused by a slower metabolic and digestive system. 3. Reduced energy intake (fewer carbohydrates/fats) is required because of a lower basal metabolic rate and reduced physical activity, preventing obesity. 4. Iron and Vitamin C are needed to prevent anaemia and maintain immune function. 5. High-quality protein is needed for cell repair. (b) Practical factors for family meal planning include: budget (cost of ingredients), time available for preparation and cooking, cooking facilities and equipment available, personal likes/dislikes or food allergies, cooking skills of the preparer, and seasonal availability of food.

Part (a): [4 marks total] - 1 mark for each correct nutrients/requirement and valid reason pair: e.g., Calcium/Vitamin D for bone strength; Fibre/NSP for digestive health/prevent constipation; Lower calories/energy because of lower metabolism/activity; Iron for preventing anaemia; Protein for tissue repair. Max 4 marks. Part (b): [4 marks total] - 1 mark for each correct non-nutritional factor: Budget/money, preparation/cooking time, kitchen equipment/facilities, individual preferences/dislikes, culinary skill of the cook, seasonal availability of ingredients. Max 4 marks.

The scientific principles of refrigeration: At 1 °C to 5 °C (the standard refrigerator temperature), the rate of bacterial multiplication and enzyme activity is significantly reduced. This extends the shelf-life of perishable foods by slowing down spoilage, though it does not destroy pathogens. Five essential hygiene practices: 1. Use a designated red chopping board and knife solely for raw poultry. 2. Wash hands thoroughly with warm water and soap before and immediately after handling raw meat. 3. Store raw chicken in a sealed container on the bottom shelf of the refrigerator to prevent any drip-contamination. 4. Do not wash the raw chicken prior to cooking, as water droplets can spray Campylobacter and other pathogens up to a meter away. 5. Promptly clean and sanitize all knives, boards, and surfaces after contact with the raw poultry.

Refrigeration Science: (2.5 marks total) - 1 mark for stating that cold temperatures slow down bacterial multiplication/growth. - 1 mark for stating that it deactivates or slows down enzyme activity. - 0.5 marks for stating that low temperature does not kill or destroy bacteria, but keeps them dormant. Hygiene Practices: (5 marks total) - 1 mark for each of five distinct, correct practices (e.g., separate red board, handwashing, storage on bottom shelf, avoiding washing poultry, sanitizing surfaces).

Heat transfer mechanisms: 1. Convection: Heat is transferred by the movement of heated air currents circulating inside the oven cavity. 2. Radiation: Infrared heat waves travel directly from the oven elements to the surface of the baking tin and cake. 3. Conduction: Heat is transferred directly through molecular contact from the hot metal baking tin to the cake batter. Changes to Starch: 1. Gelatinisation: Starch granules absorb moisture from liquid ingredients (eggs, milk). As temperature rises to around 60 °C, they swell, and at about 80 °C they burst and form a gel, which contributes to setting the cake structure. 2. Dextrinisation: Dry heat on the cake surface breaks down starch into simpler sugars called dextrins, giving the crust its golden-brown color. Changes to Proteins: 1. Denaturation: Heat disrupts the chemical bonds of egg and flour proteins, causing them to unfold. 2. Coagulation: Between 60 °C and 70 °C, these unfolded proteins join together to form a solid, rigid network that traps air/gas bubbles, defining the permanent crumb structure.

Heat Transfer: (3 marks total) - 1 mark for explaining Convection (heated air currents). - 1 mark for explaining Radiation (direct heat waves from elements). - 1 mark for explaining Conduction (heat transfer through direct contact with the tin). Changes to Starch: (2.5 marks total) - 1.5 marks for explaining Gelatinisation (moisture absorption, swelling at 60 °C, bursting/gelling at 80 °C). - 1 mark for explaining Dextrinisation (dry heat breaking down starch into dextrins on the crust). Changes to Proteins: (2.0 marks total) - 1 mark for describing denaturation (uncoiling under heat). - 1 mark for describing coagulation (setting between 60-70 °C to form a firm network).

Scientific roles of ingredients: 1. Pectin: A natural carbohydrate polymer found in fruit cell walls. When boiled, it dissolves and forms a microscopic three-dimensional mesh or network that traps liquid, causing the jam to set upon cooling. 2. Acid: Crucial for extracting pectin from fruit cells. It lowers the pH of the mixture, neutralizing negative charges on the pectin molecules so they can come together to form the gel network. It also helps invert sucrose into glucose and fructose, preventing sugar crystallization. 3. Sugar: Dehydrates pectin molecules by binding with water molecules, allowing the pectin chains to interact and form the gel network. It also acts as a preservative by lowering water activity through osmosis, inhibiting microbial growth. Reasons for failing to set: 1. Fruit naturally low in pectin (such as strawberries) was used without added commercial pectin or high-pectin fruit juice. 2. Insufficient boiling time, meaning the mixture did not reach the critical setting temperature of 104-105 °C to drive off enough moisture and concentrate pectin.

Roles of Ingredients: (5.5 marks total) - 2 marks for Pectin: explaining it is a gelling agent from cell walls that forms a 3D network to trap liquid. - 1.5 marks for Acid: explaining it extracts pectin, lowers pH to help molecules join together, and prevents sugar crystallization. - 2 marks for Sugar: explaining it dehydrates pectin to allow gelation and acts as a preservative by lowering water activity via osmosis. Jam Failures: (2 marks total) - 1 mark for explaining low natural pectin in strawberries (without supplementation). - 1 mark for explaining insufficient boiling time / not reaching setting temperature (104-105 °C).

Scientific process of enzymatic browning: When raw fruits or vegetables are peeled, sliced, or bruised, cell membranes rupture, releasing the enzyme polyphenol oxidase (phenolase) and its substrates, phenolic compounds. In the presence of atmospheric oxygen, polyphenol oxidase catalyses the oxidation of these phenols into quinones, which then rapidly polymerise to form insoluble brown pigments called melanins. Prevention methods: 1. Acidulation (e.g., adding lemon juice): The citric acid lowers the pH below 3.0, which is outside the enzyme's optimum working range, thereby denaturing the polyphenol oxidase enzyme and halting the reaction. 2. Oxygen Exclusion (e.g., submerging in cold water): Placing the cut food under water physically prevents atmospheric oxygen from contacting the surface, removing an essential reactant for the oxidation process. 3. Blanching (thermal treatment): Boiling the food briefly denatures the protein structure of polyphenol oxidase, permanently inactivating the enzyme so that browning cannot occur.

Browning Process: (2.5 marks total) - 1 mark for explaining that mechanical damage releases polyphenol oxidase and phenolic compounds. - 1.5 marks for explaining that atmospheric oxygen oxidizes phenols into quinones, polymerising into brown melanin pigments. Prevention Methods: (5 marks total) - Method 1 (Acidulation): 1.5 marks (0.5 for description, 1 mark for explaining low pH denatures the enzyme). - Method 2 (Oxygen Exclusion): 1.5 marks (0.5 for description, 1 mark for explaining that water blocks oxygen required for oxidation). - Method 3 (Blanching): 2 marks (0.5 for description, 1.5 marks for explaining that heat permanently denatures/inactivates the enzyme protein).

Safety rules to prevent deep-frying fires: 1. Never fill the deep fryer or saucepan more than one-third full of oil to prevent overflow when food is added. 2. Dry all food items thoroughly with paper towels before placing them into the hot oil; moisture causes the oil to spit and boil over violently. 3. Never leave the cooking pan unattended under any circumstances, and keep a pan lid or fire blanket close by. First-aid treatment for an oil scald: 1. Instantly place the burned forearm under cold, gently running water for a minimum of 10 to 20 minutes to cool the tissue and halt the burning process. 2. Carefully remove rings, watches, or restrictive clothing from the arm before swelling begins, but do not touch clothing that is adhered to the burn. 3. Avoid applying domestic remedies such as butter, oil, grease, or ice, as these can trap heat in the skin or cause severe bacterial infection. 4. Wrap the scalded area loosely with a sterile, non-fluffy dressing or clean plastic cling film to protect the damaged skin from airborne pathogens.

Safety Rules: (3 marks total) - 1 mark for each of three distinct, correct preventative safety rules (e.g., maximum one-third fill, drying foods, staying attended). First-Aid Treatment: (4.5 marks total) - 1.5 marks for applying cold running water for at least 10-20 minutes to cool tissue. - 1 mark for removing rings, watches, or non-stuck clothing before swelling starts. - 1 mark for explicitly warning against home remedies like butter/grease/ice to prevent heat trapping/infection. - 1 mark for covering loosely with a sterile, lint-free dressing or clean cling film.

Active Chemical Components: 1. Sodium bicarbonate (baking soda), which acts as the alkaline component. 2. Cream of tartar (potassium hydrogen tartrate) or another dry acid (such as sodium acid pyrophosphate), which acts as the acidic component. Chemical Reaction and Raising Mechanism: - When liquid (moisture) is added to the cake or scone batter, the dry acid and alkali dissolve and begin to react. - As the mixture is heated in the oven, the reaction rate accelerates rapidly, producing carbon dioxide gas (\(\text{CO}_2\)), water, and a neutral salt. The chemical reaction is: \(\text{NaHCO}_3 + \text{acid} \rightarrow \text{salt} + \text{H}_2\text{O} + \text{CO}_2\). - The carbon dioxide gas forms tiny bubbles that expand inside the flour mixture, pushing it upward to rise. The structure is then permanently locked in place as starch gelatinises and egg/gluten proteins coagulate. Function of Cornflour: Commercial baking powder contains cornflour (starch) as an inert filler. Its critical function is to absorb any ambient or atmospheric moisture inside the packaging, which prevents the acid and alkali from reacting prematurely during storage.

Active Components: (2 marks total) - 1 mark for identifying sodium bicarbonate (alkali). - 1 mark for identifying cream of tartar / dry acid (acid). Chemical Reaction and Raising Action: (4 marks total) - 1 mark for explaining that dry ingredients dissolve and react when liquid is introduced. - 1.5 marks for explaining that baking heat speeds up the reaction to release carbon dioxide gas (\(\text{CO}_2\)). - 0.5 marks for stating the chemical products (carbon dioxide, water, and neutral salt). - 1 mark for explaining that expanding carbon dioxide gas bubbles lift the gluten/starch matrix, raising the mixture. Cornflour Function: (1.5 marks total) - 0.5 marks for identifying it as a dry filler. - 1 mark for explaining that it absorbs ambient/atmospheric moisture to prevent a premature reaction during storage.

The essay should be structured into three key areas. 1. General scientific principles of preservation: Micro-organisms (bacteria, yeast, mould) require specific conditions to multiply: warmth, moisture, food, oxygen, and a suitable pH. Preservation methods work by removing or altering one or more of these essential conditions. Enzymes, which are biological catalysts responsible for food spoilage and ripening, must also be inactivated or destroyed. 2. Application to freezing: Freezing stores food at -18 degrees Celsius or below, which halts the reproduction of micro-organisms, though it does not kill them. Liquid water in the fruit is converted into ice, making it unavailable for microbial growth. Fast freezing is critical because it forms small ice crystals, minimizing damage to cell walls and preserving texture upon thawing. Packaging must be airtight to prevent dehydration and oxidation, known as freezer burn. 3. Application to jam-making: Fruit is boiled at high temperatures (typically above 100 degrees Celsius), which kills active micro-organisms and denatures enzymes. A high concentration of sugar (usually 60-65 percent) creates high osmotic pressure, drawing water out of any microbial cells through plasmolysis, making water unavailable for microbial survival. The natural acidity of the fruit lowers the pH, creating an inhospitable environment for pathogenic bacteria. Finally, sealing the hot jam in sterile jars with airtight lids excludes oxygen and prevents airborne micro-organisms from re-contaminating the food.

Award 1 mark for each valid, clearly explained point up to a maximum of 15 marks. Candidates should address all three areas of the prompt. General Principles (maximum of 5 marks): - Micro-organisms need moisture, warmth, oxygen, and correct pH to grow; altering these prevents spoilage (1 mark). - Enzymes cause spoilage/decay and must be denatured (1 mark). - High temperatures destroy bacteria and yeasts (1 mark). - Low temperatures slow down or stop bacterial growth (1 mark). - Removal of moisture (dehydration) stops microbial metabolism (1 mark). - Exclusion of oxygen prevents aerobic spoilage organisms (1 mark). Freezing (maximum of 5 marks): - Low temperature (-18 degrees Celsius) halts microbial growth and reproduction (1 mark). - Micro-organisms become dormant but are not killed (1 mark). - Water turns to ice, reducing water activity for microbes (1 mark). - Fast freezing forms small ice crystals, preserving the texture of fruit cells (1 mark). - Addition of acid (e.g., lemon juice) or quick blanching stops enzymatic browning (1 mark). - Airtight, moisture-proof packaging prevents freezer burn and oxidation (1 mark). Jam-making (maximum of 5 marks): - Boiling fruit destroys micro-organisms and denatures enzymes (1 mark). - High sugar concentration (at least 60 percent) reduces water activity (1 mark). - Water is drawn out of microbial cells by osmosis/plasmolysis, killing or inhibiting them (1 mark). - Acid in fruit (or added lemon juice) lowers pH, preventing growth of harmful bacteria (1 mark). - Sealing jars while hot creates a vacuum, excluding oxygen and preventing airborne re-contamination (1 mark).