Animal Physiology and Dairy Testing

The Activity of the Liver Enzyme Catalase

Cells generate toxic hydrogen peroxide ($2\text{H}_2\text{O}_2$) during normal metabolic reactions. If left to build up, it oxidises cellular structures and damages tissues. Liver tissue acts as a primary detoxification organ, producing large quantities of the intracellular enzyme catalase to break down this substrate:

Experimental Method

1. Chop fresh liver into small pieces and grind with a pinch of clean sand and a little water in a pestle and mortar. The sand acts as an abrasive that shears open cell membranes, releasing active catalase into the suspension.
2. Add $10\text{ ml}$ of hydrogen peroxide solution to each of three large boiling tubes labelled Test, Control 1, and Control 2.
3. Add a few drops (about $1\text{ ml}$) of washing-up liquid to each tube. The detergent traps evolved oxygen gas, producing a measurable foam column.
4. Place the tubes in a thermostatically controlled water bath held at the optimum temperature (37 °C) and maintain the mixture at $\text{pH } 7$ using a buffer solution.
5. Add $1\text{ g}$ of ground liver suspension to the Test tube.
6. To quantify the rate of reaction, measure the height of foam in mm after 60 seconds using a ruler.

Controls

• Control 1 (no tissue): Add no liver to the tube containing hydrogen peroxide and detergent. This confirms that hydrogen peroxide does not break down spontaneously under experimental conditions ($37\text{ }^\circ\text{C}$, $\text{pH } 7$).
• Control 2 (boiled liver): Add $1\text{ g}$ of boiled liver (denatured). This produces no foam, proving that the reaction is driven by the biological catalase enzyme and not simply by the physical presence of animal tissue.

Environmental Factors on Catalase

• Temperature: Testing across temperatures ($0\text{ }^\circ\text{C}$, $20\text{ }^\circ\text{C}$, $37\text{ }^\circ\text{C}$, $60\text{ }^\circ\text{C}$, $80\text{ }^\circ\text{C}$) shows foam production rises to a peak at $37\text{ }^\circ\text{C}$. Above about $40\text{ }^\circ\text{C}$, heat disrupts the protein structure; the active site changes shape so the substrate no longer fits, leaving the enzyme denatured.
• pH: Repeating the procedure with buffers from $\text{pH } 3$ to $\text{pH } 11$ demonstrates an optimum at $\text{pH } 7$, with extreme acidic or alkaline conditions denaturing the enzyme.
• Surface area: Ground liver yields a taller foam column than cubed liver because rupturing the cells exposes a greater surface area of enzyme to substrate.
• Validity: Keep peroxide volume, liver mass, and incubation time constant. Repeat each test three times and calculate the mean.

Safety and Farm Relevance

Wear safety goggles and gloves because hydrogen peroxide is an irritant and corrosive. Handle fresh liver hygienically and dispose of it in a sealed biological waste bin. In agriculture, liver function is vital for animal performance; parasite burdens such as liver fluke (Fasciola hepatica) destroy functional liver tissue, drastically reducing metabolic detoxification and liveweight gain.

Dissect a Sheep Heart and Cardiovascular Circulation

A sheep heart provides a practical model of mammalian circulation because its structure matches the hearts of cattle and humans.

Dissection Method and Safety

Place the sheep heart on a dissecting board. Before picking up your tools, orient the heart on the dissecting board. You can spot the front (anterior) surface immediately by finding the coronary arteries, which show up as pale vessels running diagonally across the ventricles. Feel both sides of the heart; the left side feels noticeably firmer and thicker than the right. Wear disposable gloves and a lab coat, and always cut away from your body.

When you are ready to expose the interior chambers, insert your scalpel or scissors into the thick wall of the left ventricle near the top (the base) and cut straight down towards the pointed apex at the bottom. Repeat this downward cut on the right ventricle. Open out both muscular flaps to inspect the chambers, valves, and central dividing septum. Wash all dissection instruments with disinfectant and dispose of biological tissue in clinical waste bags.

If you want to trace coronary circulation, take a dropper of green food dye or water and insert it into the coronary opening right at the base of the aorta. As you gently squeeze the bulb, you will see the dye branch through the surface vessels that feed the heart muscle with oxygenated blood—and in live animals or humans, an obstruction in these arteries causes a myocardial infarction, or heart attack.

The Path of Blood Flow

Blood moves continuously through a double circulatory system, passing through the heart twice per complete body circuit:

Vena cava $\rightarrow$ right atrium $\rightarrow$ tricuspid valve $\rightarrow$ right ventricle $\rightarrow$ semi-lunar valve $\rightarrow$ pulmonary artery $\rightarrow$ lungs $\rightarrow$ pulmonary vein $\rightarrow$ left atrium $\rightarrow$ bicuspid (mitral) valve $\rightarrow$ left ventricle $\rightarrow$ semi-lunar valve $\rightarrow$ aorta $\rightarrow$ body tissues.

Watch out for two vascular exceptions that examiners love to test: the pulmonary artery is the only artery carrying deoxygenated blood, and the pulmonary vein is the only vein carrying oxygenated blood. You will also notice structural differences between the main vessels; the aorta has a thick, elastic wall built to withstand high systolic pressure, while the vena cava needs only a thin wall around its wide lumen because venous blood returns under low pressure. On a farm, efficient circulation sustains high milk production, and bulls showing cardiovascular defects are culled from breeding programmes.

Ruminant Digestion, the Calf's Stomach, and Milk Synthesis

Adult cattle rely on a complex four-chambered stomach to convert fibrous forage into the precursor metabolites needed for milk synthesis.

The Four Stomach Chambers

• Rumen: The largest chamber in an adult; functions as an anaerobic fermentation vat where symbiotic bacteria and protozoa ferment cellulose into volatile fatty acids (VFAs), primarily acetate, propionate, and butyrate. These VFAs provide the animal's primary energy supply.
• Reticulum: Features a distinct 'honeycomb' lining; traps dense foreign objects (wire, stones) and forms fibrous digesta into a bolus (cud) for regurgitation and rechewing.
• Omasum: Contains many muscular leaves ('many plies') that compress digesta to absorb water and grind fine food particles.
• Abomasum: The true stomach; secretes gastric juice containing hydrochloric acid, pepsin, and rennin to digest microbial and dietary protein enzymatically.

Monogastric livestock like pigs cannot digest grass because they possess a single simple stomach without rumen microbes or endogenous cellulase enzymes, requiring concentrated cereal-based feeds.

The Calf's Stomach and Rumen Development

At birth, the calf's stomach chambers are all present, but the abomasum is the largest (about 70% of total volume), while the rumen is small and non-functional. When the calf suckles, nervous stimulation triggers the oesophageal groove—a muscular fold of tissue—to close reflexively. This channels milk straight from the oesophagus into the abomasum, bypassing the rumen. If milk entered an undeveloped rumen, it would putrefy and trigger nutritional scour.

Developing the calf into a functioning ruminant follows a clear dietary sequence:

1. Colostrum fed within the first two hours to provide maternal antibodies.
2. Whole milk or milk replacer.
3. Dry calf concentrates introduced from week one. Microbial fermentation of concentrates yields VFAs (especially butyrate) that stimulate the growth of absorptive rumen papillae.
4. Clean straw or hay provides physical 'scratch factor', which stimulates the muscular expansion and motility of the rumen wall.
5. Weaning occurs once the calf reliably consumes $1\text{ kg}$ of concentrates daily (typically at 8–10 weeks).

Milk Secretion and Let-Down

Milk synthesis occurs within microscopic epithelial alveoli in the udder. Producing one litre of milk requires approximately 400 to 500 litres of blood to flow through the mammary tissue. Teat stimulation, suckling, or parlour noises stimulate the posterior pituitary gland to release the hormone oxytocin into the bloodstream. Oxytocin causes myoepithelial cells surrounding the alveoli to contract, forcing stored milk into the cisterns for collection. This let-down reflex lasts only 6 to 8 minutes, meaning milking clusters must be attached promptly. If cows encounter stress, loud noise, or rough handling, the adrenal glands release adrenaline, which constricts mammary blood vessels, blocks oxytocin action, and leaves residual milk in the udder.

Lactation Cycle, Energy Balance, and Mastitis Management

Managing lactation dynamics and herd health ensures milk output remains commercially viable.

The Lactation Curve

Following calving, daily milk yield increases rapidly to an initial peak at roughly 6 to 8 weeks before declining at a steady rate over a standard 305-day lactation. At the close of this cycle, the cow enters a deliberate dry period of about 60 days before her next calving date.

Milk Yield & Intake
       ^
       |        Peak Yield (6-8 wks)
 High  |             /\
       |            /  \           Lactation Curve (~305 days)
       |           /    \--------------------------------\
       |          /      \                                \
       |         /        \   Peak Feed Intake (10-12 wks) \    Drying Off
  Low  |        /          \- - - - - - - - - - - - - - - - \  (~60 days)
       +-------+------------+--------------------------------+------------->
       0       4            8               12              44       52 Weeks
              Calving      [NEGATIVE ENERGY BALANCE]

Negative Energy Balance

In early lactation, a cow enters negative energy balance because her energy output in milk peaks at 6 to 8 weeks, whereas her voluntary feed intake does not peak until 10 to 12 weeks. She physically cannot consume enough dry matter to meet her energy needs. To compensate, she mobilises body fat reserves, which leads to loss of body condition score, poor reproductive cycling, and an increased risk of ketosis. Dairy farmers manage this deficit by feeding high-energy concentrates alongside high-digestibility silage or spring grass.

Drying Off

Drying off refers to ceasing milking deliberately for roughly 60 days prior to the next calving. This break allows damaged secretory udder tissue to repair and regenerate, restores cow body condition, enables the application of dry cow antibiotic therapy, and supports rapid foetal growth during the final trimester.

Mastitis: Causes, Symptoms, and Control

The definition examiners look for is that mastitis is an inflammation of the mammary gland caused by bacterial pathogens (such as Staphylococcus aureus or Streptococcus uberis) entering via the teat canal.

• Symptoms: Hard, hot, swollen udder quarters; visible clots, flakes, or watery discolouration in the milk; pain and kicking during cluster attachment; and an elevated Somatic Cell Count (SCC), which reveals subclinical infection where visual signs are absent.
• Treatment: Completely strip out the infected quarter, disinfect the teat opening, infuse an intramammary antibiotic tube, mark the cow clearly with spray or leg bands, and withhold her milk from the bulk tank for the full statutory withdrawal period.
• Prevention: Dip or spray all teats with disinfectant immediately after every milking; keep cubicles clean, dry, and limed; service the milking machine regularly to maintain correct vacuum and pulsation; milk infected cows last; and cull chronically infected cows with repeat high cell counts.

Milk Composition, Breed Differences, and Dairy Quality Testing

Standard whole cow's milk contains roughly $87\%$ water and $13\%$ total solids, split between butterfat ($3.5\text{--}4.0\%$), protein ($3.3\text{--}3.5\%$, predominantly casein), lactose ($4.6\text{--}4.8\%$), and mineral ash ($~0.7\%$).

Breed Comparisons

Under Irish processing contracts, payments follow the A + B – C formula, where farmers are credited for kilograms of protein (A) and butterfat (B), but penalised with a processing deduction per litre of water transported (C). Higher solids make milk substantially more valuable for cheese and powder manufacture.

Milk Quality Testing

Every collection from a farm's bulk tank is tested. The co-operative uses these results to calculate bonuses or financial penalties.

Estimate the Percentage of Water and Solids in a Sample of Milk

This gravimetric practical determines the moisture and dry matter proportions of raw milk.

Method

1. Weigh a clean, dry evaporating basin on an electronic balance and record its mass as $m_1$.
2. Pipette roughly $25\text{ ml}$ of fresh whole milk into the basin and record the combined mass as $m_2$.
3. Calculate the starting milk mass: $\text{Mass of milk} = m_2 - m_1$.
4. Place the basin on a wire gauze on a tripod stand over a Bunsen burner, or place it in a drying oven set at $100\text{ }^\circ\text{C}$. Heat gently with a low flame to evaporate water slowly without causing charring or spitting of the residue.
5. Once steam ceases to rise and a dry crust forms, use metal tongs to transfer the hot basin into a desiccator containing silica gel. Allow it to cool to room temperature without absorbing ambient atmospheric moisture.
6. Reweigh the cooled basin on the balance and record the mass as $m_3$.
7. Return the basin to the heat source, cool in the desiccator, and reweigh. Repeat this heating, cooling, and weighing cycle until consecutive readings show a constant mass, proving that all moisture has been driven off.

Calculations

Experimental Precautions and Accuracy

Always use tongs to handle hot porcelain dishes. Avoid charring the sample during evaporation, as burning destroys organic compounds and gives a falsely low solids measurement. Do not weigh the basin while warm; rising convection currents destabilise balance pans, causing weighing errors. Verify experimental accuracy by checking that the calculated solid content falls within expected physiological parameters (roughly 12–13% solids and 87% water).

Test for the Presence of Lactose in Milk

Lactose is a disaccharide carbohydrate ($4.6\text{--}4.8\%$ of milk) that functions as a reducing sugar. It contains a free chemical group that readily donates electrons, allowing it to reduce blue copper(II) ions into an insoluble precipitate.

Experimental Procedure

1. Label three clean test tubes: Tube A (Milk), Tube B (Negative Control), and Tube C (Positive Control).
2. Add $2\text{ ml}$ of fresh milk to Tube A.
3. Add $2\text{ ml}$ of distilled water to Tube B.
4. Add $2\text{ ml}$ of glucose solution to Tube C.
5. Add $2\text{ ml}$ of bright blue Benedict's solution to every tube and swirl to mix.
6. Place all three tubes into a boiling water bath held at $80\text{--}100\text{ }^\circ\text{C}$ for 5 minutes.

Observations and Controls

• Tube A (Milk): The initial blue colour shifts through green, yellow, and orange, forming a distinct brick-red precipitate of copper(I) oxide ($\text{Cu}_2\text{O}$), confirming the presence of a reducing sugar.
• Tube B (Distilled water): The solution remains blue with no precipitate, confirming that heat alone does not reduce copper(II) ions.
• Tube C (Glucose positive control): Produces a brick-red precipitate, proving that the Benedict's reagent is active and functioning properly.
• Non-reducing disaccharides like sucrose (non-reducing) do not react and remain blue.

Semi-Quantitative Scale and Industry Relevance

Benedict's test provides semi-quantitative data: blue indicates no reducing sugar, green indicates trace quantities, yellow/orange indicates moderate levels, and brick-red confirms high concentrations. In dairy manufacturing, lactose serves as the primary energy substrate fermented by lactic acid bacteria during cheese and yogurt production. In dairy processing, factories also treat milk with lactase enzymes to break lactose down into glucose and galactose, creating lactose-free dairy ranges for lactose-intolerant consumers.

Key terms

Catalase

An intracellular enzyme concentrated in liver tissue that accelerates the breakdown of toxic hydrogen peroxide into water and oxygen gas.

Denatured

The structural state of an enzyme when excessive heat or extreme pH permanently alters its three-dimensional shape and active site, destroying catalytic function.

Optimum temperature (37 °C)

The specific temperature at which mammalian enzymes achieve their maximum rate of substrate conversion.

Active site

The region on the surface of an enzyme with a specific three-dimensional shape that binds the complementary substrate molecule.

Bicuspid (mitral) valve

The two-cusped atrioventricular valve located between the left atrium and left ventricle that prevents backflow during ventricular contraction.

Tricuspid valve

The three-cusped atrioventricular valve positioned between the right atrium and right ventricle that prevents blood backflowing into the atrium.

Semi-lunar valves

Crescent-shaped valves located at the base of the aorta and pulmonary artery that prevent backflow of blood into the ventricles during relaxation.

Septum

The central muscular dividing wall that separates the left and right sides of the heart to prevent mixing of oxygenated and deoxygenated blood.

Coronary arteries

Vascular vessels that branch across the outer surface of the heart to deliver oxygenated blood and nutrients directly to the cardiac muscle.

Double circulatory system

A circulatory layout where blood travels through the heart twice per complete body circuit, dividing into pulmonary and systemic pathways.

Oesophageal groove

A muscular fold in young calves that closes reflexively during suckling to route milk directly to the abomasum, bypassing the undeveloped rumen.

Abomasum

The true, fourth stomach chamber of a ruminant, which secretes acid and digestive enzymes to break down protein.

Negative energy balance

A physiological state in early lactation where energy demand for milk output exceeds dietary energy intake, forcing the cow to mobilise body fat.

Mastitis

An infectious inflammation of the udder caused by bacterial pathogens entering the teat canal, leading to altered milk and raised cell counts.

Somatic Cell Count (SCC)

A milk hygiene test measuring white blood cells per ml of milk, where counts above 200,000 cells/ml indicate subclinical mastitis.

Total Bacterial Count (TBC)

A milk hygiene quality test measuring the total number of living bacterial colony-forming units per ml of bulk milk.

Total solids

The dry residue remaining after completely evaporating all water from whole milk, consisting of butterfat, protein, lactose, and minerals.

Constant mass

A laboratory standard achieved when consecutive heating, cooling in a desiccator, and reweighing cycles show identical mass readings, confirming complete drying.

Desiccator

An airtight glass container holding a drying agent like silica gel, used to cool heated laboratory samples without moisture absorption.

Reducing sugar

A sugar containing a free functional group that donates electrons to reduce copper(II) ions in Benedict's solution to copper(I) oxide.

Copper(I) oxide

The insoluble brick-red precipitate (Cu₂O) formed when reducing sugars reduce copper(II) ions during Benedict's test.

Oxytocin

The pituitary hormone released during milking that causes myoepithelial cells around mammary alveoli to contract and trigger milk let-down.