Difference Between Freezing and Chilling

FOOD PROCESSING AND PRESERVATION: COLD PRESERVATION

Figure 1: Chilling and freezing both use cold, but freezing locks the water away as ice. The two boxes are alternatives, not steps in order.

Introduction

Chilling and freezing both preserve food with cold, but they work at different temperatures and buy very different amounts of time. Chilling holds food just above freezing, usually 0 °C to 5 °C, and keeps it close to fresh for days or weeks. Freezing turns most of the food’s water to ice, usually at minus 18 °C or colder for storage, and keeps it for months. Neither method kills most microbes: cold is a pause button, not a kill switch, and the microbes wake up and grow again when the food warms.

Quick comparison

Table 1: Freezing compared with chilling

Scientific explanation

Definition

Chilling is holding food just above freezing, usually 0 °C to 5 °C, to slow microbes, enzymes, and chemical change for days or weeks. Freezing is turning most of the water in a food to ice, usually at minus 18 °C or colder for storage, so microbes cannot grow while the food stays frozen.

Working principle

Lower temperature slows every chemical reaction and every microbial process. Enzymes work more slowly, microbes divide more slowly, and chemical spoilage crawls. Freezing works mainly by turning water into ice. Ice is water that microbes cannot use, so freezing drops the usable water, the water activity, far below the level any microbe needs, and it stops growth completely while the food stays frozen. Chilling only slows growth: a few microbes, called psychrotrophs, grow slowly even in a fridge, which is why chilled milk still sours after a week and chilled meat still goes slimy. Chilling also has a limit that surprises people: some foods are damaged by cold itself. Bananas, tomatoes, and some tropical fruits suffer chilling injury below about 10 °C, turning dark, soft, and flavourless, even though no microbe is involved.

Apparatus

Chilling uses fridges, chill rooms, and refrigerated trucks that hold a steady 0 °C to 5 °C. Freezing uses blast freezers, in which a strong blast of very cold air freezes food fast, plate freezers, in which food is pressed between cold plates, and cold stores held at minus 18 °C or colder. Factory freezers freeze fast, in a blast of cold air, on a cold plate, or in a bath of cold liquid, while a home freezer, which freezes slowly, gives a poorer result.

Measurement procedure

Cold storage is checked by measuring temperature, not by guessing. A temperature log that shows a steady 4 °C is worth more than a promise that the food was kept cold. The cold chain is the unbroken series of cold steps from harvest or production to the table, and it is checked at each link: the packing house, the truck, the warehouse, the shop, and the journey home. If any link warms up, the food’s clock starts again, and cooling it down afterwards does not undo the growth that happened during the warm gap. Freezing time is checked with Plank’s equation, given in the formulas section below.

Factors affecting readings

How fast the food freezes decides its quality. Fast freezing makes many small ice crystals, which sit between cells and do little damage. Slow freezing makes a few large crystals, which grow inside cells, break cell walls, and let juice leak out when the food thaws. That leaked juice, called drip loss, is the mushy texture of slowly frozen fruit and the dry bite of badly frozen meat. Thickness is the main lever: doubling the thickness quadruples the conduction term in Plank’s equation, so thick pieces take far longer than thin ones. Fat content matters too: fat in frozen food slowly turns rancid even at minus 18 °C, which is why fatty fish has a shorter frozen life than lean fish.

Formulas and calculations

Plank’s equation gives freezing time: t = (ρλ / (Tf – Ta)) x (P x a / h + R x a² / k), where t is the freezing time in seconds, ρ is the density of the food, λ is the latent heat of crystallisation, about 267 kJ per kg for a food that is about 80 percent water, Tf is its freezing point, Ta is the temperature of the freezing medium, a is the half-thickness, h is the surface heat transfer coefficient, k is the thermal conductivity of the frozen food, and P and R are shape factors: for a slab, P = 1/2 and R = 1/8; for a cylinder, P = 1/4 and R = 1/16; for a sphere, P = 1/6 and R = 1/24.

Worked example: a fish fillet. Take a slab fillet with half-thickness a = 0.01 m, density ρ = 1,050 kg/m³, latent heat λ = 267,000 J/kg, freezing point Tf = -1 °C, freezer air Ta = -35 °C, surface coefficient h = 30 W/m²K, and frozen conductivity k = 1.5 W/mK. First, ρλ = 1,050 x 267,000 = 2.80 x 10⁸ J/m³, and Tf – Ta = 34 K, so ρλ / (Tf – Ta) = 8.25 x 10⁶. Next, the surface term is P x a / h = 0.5 x 0.01 / 30 = 1.67 x 10⁻⁴, and the conduction term is R x a² / k = 0.125 x 0.0001 / 1.5 = 8.33 x 10⁻⁶. Adding them gives 1.75 x 10⁻⁴, and multiplying gives t = 8.25 x 10⁶ x 1.75 x 10⁻⁴ = 1,443 seconds, about 24 minutes. With a = 0.02 m, twice as thick, the time rises to about 50 minutes.

Industrial applications

• Freezing: frozen peas and vegetables on belts in blast freezers, fish fillets, meat, bread, ice cream, and ready meals kept for months in cold stores at minus 18 °C or colder.
• Chilling: fresh milk, meat, and salad in fridges and chill rooms; chilled ready meals; fruit held under controlled atmosphere, where lowered O₂ and raised CO₂ slow ripening beside the cold.

Advantages and limitations

Freezing, advantages: keeps food for months; stops microbial growth while frozen; keeps colour and flavour well when freezing is fast. Limitations: needs a costly cold chain at minus 18 °C; slow freezing damages texture; thawing is the risky step, because the outside warms into the danger zone while the inside is still frozen; thawed food is never refrozen without cooking.

Chilling, advantages: keeps food close to fresh in taste and texture; cheap and simple; no thawing needed. Limitations: shelf life is only days to weeks; psychrotrophs still grow in a fridge; some fruits suffer chilling injury below about 10 °C; one warm gap in the cold chain loses shelf life that later chilling cannot restore.

FAQ

Does freezing kill microbes? No. Most microbes survive chilling and freezing, and they wake up and grow again when the food warms. Freezing stops growth while the food stays frozen, it does not sterilize it.

Why do slowly frozen strawberries go mushy when they thaw? Because slow freezing makes large ice crystals inside the cells that break the cell walls, so juice leaks out as drip loss when the fruit thaws.

Why does chilled milk still go sour after a week? Because some microbes, called psychrotrophs, grow slowly even at fridge temperature, so chilling slows spoilage but does not stop it.

Why is thawing on a warm counter risky? Because the surface warms into the danger zone of about 5 °C to 60 °C and bacteria grow fast there while the centre is still frozen.

What is the cold chain? The unbroken series of cold steps from production to the table. One warm gap lets microbes grow, and later chilling cannot undo that growth.

References and further reading

FTN subjects: Principles of Food Preservation, Unit 3 (Preservation by Cold); Principles of Food Processing, Unit 6 (Refrigeration and Freezing). Books used: Fellows, P. J., Food Processing Technology: Principles and Practice; Rahman, M. S., Handbook of Food Preservation; Chakraverty, A., Mujumdar, A. S., Raghavan, G. S. V., and Ramaswamy, H. S., Handbook of Postharvest Technology: Cereals, Fruits, Vegetables, Tea and Spices; Frazier, W. C., and Westhoff, D. C., Food Microbiology.