Refrigeration and Freezing Quiz
What this quiz covers
Refrigeration is the cold chain that modern food safety stands on — and it is pure thermodynamics wearing work clothes. Every refrigerator is a heat engine running backwards: work goes in, heat gets pumped from the cold box to the warm room. The coefficient of performance tells you how good the bargain is, and the Carnot cycle sets a ceiling nobody can beat. Get the physics right and food stays safe; get it wrong and psychrotrophs — the cold-loving spoilage bacteria — do their quiet work at 4 °C.
These 20 questions split between the machine and the food. On the machine side: COP, the Carnot cycle as efficiency champion, why charging goes in through the suction line with the compressor running, and the reversed heat-engine description. On the food side: freezing-point depression and water’s 1.86 °C cryoscopic constant, milk adulteration detected by cryoscopy, latent heat (144 BTU per pound), cold shock, and why near-freezing milk still turns bitter.
Two halves, one logic
For the refrigeration questions, think energy: the evaporator absorbs heat, the compressor spends work, and COP is heat moved divided by work spent — above one in any working machine, highest for the ideal Carnot cycle. For the food questions, think colligative properties: dissolved particles — salt, sugar, milk solids — lower the freezing point in proportion to their number, which is exactly why added water raises milk’s freezing point toward 0 °C and betrays the adulteration.
How to use this quiz
Twenty seconds per question, one attempt. The formula questions (added-water percentage, freezing-point depression) go quickest if you write the expression down first. Note where you stumble — machine physics or food chemistry — and review that half before retaking.
Answer explanations
Quiz finished? Each answer below comes with the reason — the thermodynamics or food chemistry that decides it.
- Freezing, thawing, or churning breaks the milk-fat emulsion by disrupting which membrane components?
Correct answer: Both peripheral and integral proteins — The milk fat globule membrane’s protein architecture — peripheral and integral proteins alike — is what freezing, thawing, or churning tears apart, destabilizing the emulsion. - Which factor does NOT influence the freezing time of ice cream?
Correct answer: The number of operators — Refrigerant temperature, dasher speed, and flow velocity all change heat transfer; the headcount does not. - The death of some microorganisms caused by sudden, rapid chilling is known as what?
Correct answer: Cold shock — A rapid temperature drop kills a fraction of the microbial population outright — survivors may be injured but can recover. - Where should unopened canned food be stored?
Correct answer: In a cool, dry store — Shelf-stable cans need no refrigeration; a cool, dry store protects the can body and the seams. - What is a key difference between cryogenic freezing and immersion freezing?
Correct answer: In cryogenic freezing the refrigerant changes phase — Liquid nitrogen or CO₂ boils as it freezes the food — the phase change absorbs the heat — while immersion coolants like brine stay liquid. - When water is added to milk, the freezing point of the milk
Correct answer: Rises — Dilution lowers the solute concentration, so the freezing point moves up toward 0 °C — the basis of cryoscopic adulteration testing. - Which of these psychrotrophic bacteria can grow under refrigeration?
Correct answer: All three grow under refrigeration — Flavobacterium, Achromobacter, and Pseudomonas all include species that grow at refrigeration temperatures and spoil chilled foods. - What is the freezing point of an average ice-cream mix?
Correct answer: 27.5 °F — Dissolved sugars and milk solids depress the mix’s freezing point to about −2.5 °C. - Holding milk at near-freezing temperatures can bring about
Correct answer: Proteolysis by cold-tolerant bacteria — Psychrotrophic bacteria grow slowly in the cold and release heat-stable proteases that survive pasteurization and act later. - Milk kept at chilling temperatures may turn proteolytic or bitter because of species of
Correct answer: Flavobacterium, Bacillus, and Pseudomonas — All three genera include psychrotrophic species whose proteases and lipases cause bitterness and off-flavours in chilled milk. - How much latent heat of freezing does one pound of water release at sea level?
Correct answer: 144 BTU — Freezing one pound of water at 0 °C releases 144 BTU — and melting absorbs exactly the same. - Adding salt to water causes its freezing point to be
Correct answer: Lowered — Dissolved salt is a colligative effect: more particles, lower freezing point — the reason roads get salted and ice cream gets rock salt. - What is added to a car’s cooling system so the coolant will not freeze in winter?
Correct answer: Antifreeze — Ethylene or propylene glycol depresses the coolant’s freezing point while raising its boiling point. - If T₁ is the measured freezing point of a milk sample and T₂ the normal average freezing point, the percentage of added water W is found from
Correct answer: W = 100(T₂ − T₁)/T₂ — This is the standard cryoscopic formula, comparing the measured freezing-point depression against the normal one. - Dissolving one mole of a non-volatile solute in 1 kg of water depresses its freezing point by
Correct answer: 1.86 °C — Water’s cryoscopic constant: one mole of a non-volatile, non-dissociating solute per kilogram of water depresses the freezing point by 1.86 °C. - What determines how far the freezing point of a solution drops?
Correct answer: The number of dissolved solute particles — Colligative means particle count matters — not size, not mass. - Which refrigeration cycle gives the highest coefficient of performance?
Correct answer: Carnot cycle — The reversible Carnot cycle is the theoretical efficiency ceiling no real cycle can beat. - In a real working refrigerator, the COP is always:
Correct answer: More than one — A working refrigerator moves more heat than the work it consumes — real COPs typically run 2–4. - When a refrigeration system is being charged, the usual practice is to feed refrigerant into the suction line with:
Correct answer: The compressor running — Feeding vapour into the suction line with the compressor running draws refrigerant in safely; liquid into a stopped compressor risks slugging damage. - A refrigeration cycle is best described as:
Correct answer: A reversed heat-engine cycle — A heat engine turns heat into work; a refrigerator spends work to move heat — the same cycle run backwards.