Food Production

Yeast is a single-celled FUNGUS (Saccharomyces cerevisiae) used in two key food/drink industries.

Anaerobic respiration in yeast. When oxygen is absent, yeast respires sugar (glucose) by FERMENTATION:

glucose → ethanol + carbon dioxide + (small amount of) energy

This is the basis of both brewing and bread-making — the products (ethanol, CO2) are exploited differently.

Brewing (beer + wine) (spec 5.1-5.2):

1. Sugar source — for beer this is malted barley (sprouted grain), broken down by amylase into MALTOSE. For wine, the sugar comes from grape juice.
2. Yeast is added to the sugary 'wort'.
3. The vessel is sealed (or fitted with an airlock) so oxygen cannot enter → ANAEROBIC.
4. Temperature is held at the yeast's optimum (~25-30 °C; lager strains lower).
5. Yeast respires the sugar to ETHANOL + CO2. The CO2 bubbles off; the ethanol stays in solution → this is the alcoholic drink.
6. Fermentation stops either when the sugar runs out or when the alcohol concentration reaches ~12-15% (toxic to yeast).
7. Hops (for beer) provide flavour and act as a preservative. The brew is filtered, sometimes carbonated, and bottled.

Bread-making (spec 5.1-5.2):

1. Yeast is mixed into a dough of flour, water and salt (plus a little sugar).
2. The dough is kneaded — gluten in the flour forms a stretchy protein network.
3. The dough is left in a warm place to PROVE (~25-30 °C).
4. Yeast inside the dough respires sugars anaerobically → CO2 + ethanol.
5. CO2 bubbles are TRAPPED by the gluten network → dough RISES (becomes light and spongy).
6. Bake in a hot oven (~200 °C):
   • CO2 expands further, fixing the spongy texture.
   • The ETHANOL EVAPORATES (boiling point 78 °C → no alcohol in finished bread).
   • High temperature kills the yeast and sets the structure.

Key contrast: brewing wants the ETHANOL (CO2 escapes). Bread-making wants the CO2 (ethanol evaporates in the oven).

Edexcel requires you to know how to investigate the rate of anaerobic respiration in yeast.

Apparatus:

• Boiling tube containing yeast suspension + glucose solution.
• Layer of oil floated on top — prevents O2 dissolving in → ensures ANAEROBIC respiration.
• Delivery tube into a second tube containing LIMEWATER (calcium hydroxide solution).
• Water bath to control temperature.
• Stopwatch.

How to measure rate of respiration:

• COUNT the BUBBLES of CO2 per minute in the limewater.
• OR time how long the limewater takes to TURN CLOUDY (faster = more CO2 = more respiration).

Method (varying temperature, the most common version):

1. Set up apparatus with same volume + concentration of yeast and glucose each time.
2. Place boiling tube in a water bath at 20 °C; allow 5 min to equilibrate.
3. Time how long for the limewater to turn cloudy, OR count bubbles in 1 minute.
4. Repeat at 30, 40, 50 and 60 °C.
5. Plot graph of rate (bubbles per min) against temperature.

Variables:

• Independent (IV): temperature.
• Dependent (DV): rate of CO2 production.
• Controlled: mass/concentration of yeast, volume + concentration of glucose, time of measurement, pH.

Expected results:

• Rate INCREASES from 20 → 40 °C — yeast enzymes work faster as temperature rises (collisions, kinetic energy).
• Rate is MAXIMUM around 40 °C — the optimum temperature for yeast enzymes.
• Above 40-45 °C the rate DECREASES sharply — high temperature DENATURES the enzymes.
• At ~60 °C the yeast is dead → no CO2 produced.

Why limewater? CO2 + Ca(OH)2 → CaCO3 + H2O. CaCO3 is insoluble, giving the cloudy/milky appearance. This is a classic test for CO2.

Bacteria are used to produce many cultured dairy foods. The most-examined is yoghurt.

Producing yoghurt:

1. Pasteurise the milk. Heat to ~80-90 °C briefly to kill unwanted bacteria + denature spoilage enzymes. Cool to ~40 °C.

2. Add starter culture of Lactobacillus. (Often a mix with Streptococcus thermophilus.)

3. Incubate at ~40 °C — optimum for Lactobacillus enzymes.

4. Anaerobic respiration of lactose — the bacteria convert lactose (milk sugar) into LACTIC ACID:

   lactose → lactic acid + (small amount of) energy

5. Lactic acid lowers the pH. As acid accumulates, pH drops from ~6.5 toward ~4.

6. Milk proteins COAGULATE. At low pH, casein (the main milk protein) loses its surface charge and clumps together → mixture thickens.

7. Cool, flavour and package. Yoghurt is cooled to slow bacterial activity, sweetened/flavoured with fruit, and packaged. Lactic acid gives yoghurt its sour taste.

Cheese-making (briefly). Same starting steps as yoghurt — bacterial fermentation of lactose to lactic acid lowers pH. Then a CLOTTING ENZYME (rennet, containing chymosin) is added → coagulates the casein into curds. Curds are separated from whey, pressed and aged. Different bacteria + ageing conditions give different cheeses.

Why bacteria + careful temperature?

• Lactobacillus enzymes work best at ~40 °C — this is the optimum.
• Cooler → fermentation too slow; warmer → enzymes denature.
• The acidic conditions produced also preserve the yoghurt by inhibiting spoilage bacteria.

Key keyword: COAGULATE (not 'denature'). Casein clumps together because the acid neutralises its surface charge — it does NOT change shape in the way an enzyme does when denatured.

Industrial fermenters grow microorganisms at huge scale (thousands of litres) under tightly controlled conditions. Used for: mycoprotein (Fusarium → Quorn), penicillin (Penicillium), recombinant insulin (GM E. coli), single-cell protein, biofuels.

Key features and functions:

Feature	Function
Stainless-steel vessel	Strong, unreactive, easily steam-sterilised.
Sparger (bottom)	Bubbles sterile AIR up through the contents for AEROBIC respiration.
Paddle / stirrer (motor)	Mixes contents — distributes O2, nutrients and heat; prevents settling.
Water jacket (outside)	Cool water flows through to REMOVE the heat produced by respiration → keeps temperature at optimum.
pH probe + alkali addition	Monitors and corrects pH (which would otherwise drop as acidic waste products build up).
Temperature probe	Monitors temperature, controlling the water jacket flow.
Nutrient inlet	Continuous supply of glucose, nitrogen (ammonia) and mineral salts.
Filter-sterilised air inlet	Prevents airborne microbes contaminating the culture.
Outlet / harvest tap	Removes product for downstream processing.
Steam port	Steam-sterilises the vessel between batches.

Why each condition matters:

• Temperature — must be at the enzyme optimum. Respiration releases heat; without the water jacket the contents would overheat → denature the enzymes → microbes die.
• pH — enzymes have a narrow pH optimum; metabolic acids would lower the pH if not corrected.
• Oxygen — most industrial cultures (e.g. Fusarium) are AEROBIC → need plentiful sterile O2 from the sparger. Without O2 they respire anaerobically (low yield) or die.
• Nutrients — without fresh glucose + nitrogen the microbes cannot grow.
• Sterility — contaminating microbes could outcompete the wanted species or produce toxins. Steam-sterilising + filter-sterilising inputs prevents contamination.
• Mixing — distributes O2, nutrients and heat evenly; prevents settling and hotspots.

Mycoprotein (Quorn) production:

• Fungus: Fusarium venenatum.
• Carbon source: glucose syrup made from wheat starch.
• Nitrogen source: ammonia (ammonium salts).
• Aerobic respiration → fungus grows as long mycelial threads.
• Harvested, processed and textured into a high-protein, low-fat meat substitute.
• Advantages: vegetarian, low in saturated fat, sustainable land/water use.

Glasshouses and polytunnels are enclosed environments used to grow crops under controlled conditions.

How they increase yield (link each to photosynthesis):

1. Higher temperature. Solar radiation passes through the glass and warms the inside (greenhouse effect). Warmer T → photosynthesis enzymes work faster → more glucose → faster growth.
2. Enriched CO2. CO2 can be added by burning paraffin heaters or releasing it from cylinders. CO2 is a raw material for photosynthesis — extra CO2 removes it as a limiting factor → faster photosynthesis.
3. Artificial lighting. Lamps extend the day length, especially in winter. More light → more energy for photosynthesis → year-round production.
4. Pest, pathogen and weather protection. Aphids, caterpillars and wind/hail/frost are kept out. Less crop damage; less plant stress; less pesticide needed.
5. Controlled water + nutrients. Drip irrigation delivers exact amounts of water + dissolved fertiliser. Hydroponics is growing plants with their roots in a nutrient solution (no soil) — used in commercial glasshouses for tomatoes, lettuce, herbs.

Polytunnels are similar but cheaper — made of polythene over a metal frame rather than glass. Less precise temperature control but easier to install and move.

Hydroponics in more detail:

• Plant roots dipped in (or sprayed with) a nutrient solution containing dissolved nitrate, phosphate, potassium, calcium, magnesium and trace minerals.
• No soil → no soil-borne diseases, no soil compaction.
• Less water used (recirculated).
• Yield per area can be very high.
• Drawback: needs careful balance of nutrients; expensive setup; vulnerable to power cuts.

Why it matters. Glasshouse + hydroponic agriculture is a key strategy for feeding growing populations on limited land, especially in countries with short growing seasons or arid climates. The Netherlands is a famous example — high-yield glasshouse production using hydroponics.

Intensive farming uses high inputs (feed, fertiliser, pesticides, housing) to maximise yield per area or per animal.

Examples:

• Battery hens — kept in small cages indoors with controlled light, feed and water. Allows very high egg-per-area production.
• Feedlot cattle — kept in pens and fed concentrated grain rather than grazing. Faster weight gain than free-range.
• Glasshouse crops — controlled climate as above.
• Pig sheds + broiler chicken sheds — similar to battery hens.

Advantages:

• High yield per area. Many animals or plants per hectare → more food from same land.
• Cheaper food. Economies of scale + automation (feeding, watering, climate, egg-collection) → low unit cost.
• Reduced energy loss. Restricted movement + climate control mean animals waste less energy moving + maintaining body temperature → more energy goes into growth, eggs, milk.
• Easier disease/pest monitoring. Animals in controlled space can be observed and treated promptly.

Disadvantages:

1. Animal welfare. Battery hens may be unable to spread their wings; cattle in feedlots cannot graze normally. Behaviours like feather-pecking, lameness, foot disorders are common. Many countries have banned the worst practices (EU banned battery cages in 2012).

2. Antibiotic resistance. Crowded conditions allow disease to spread rapidly. Antibiotics are often given as growth promoters or to prevent disease (prophylactic use). This applies a SELECTION PRESSURE → antibiotic-resistant bacteria evolve → resistant strains can transfer to humans (e.g. MRSA, resistant Salmonella). A major public-health concern.

3. Environmental damage.

   • Eutrophication — concentrated animal waste runs off into rivers, causing algal blooms.
   • Methane — cattle release methane (a greenhouse gas) from rumen fermentation.
   • Deforestation — large amounts of land converted to grow cereals as livestock feed.
   • Energy + water use — intensive systems use lots of fossil-fuel inputs.

4. Reduced biodiversity. Monoculture crop farming + pesticide use kills many non-target species.

Compare with extensive (free-range) farming: lower yield per area but generally better welfare + lower environmental impact. The trade-off is at the centre of food-policy debates.

Modern challenges. Feeding 9 billion people by 2050 will require either: (a) more intensive farming, with all its problems, or (b) a shift to less-resource-intensive proteins (e.g. mycoprotein, plant-based meats, insect protein), or (c) reducing global meat consumption — most likely a mix of all three.