🍲 Thermal Processing

This lesson explains core food science and nutrition concepts with practical relevance to food quality, safety, and human health.

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Thermal Processing

Principles

Thermal processing is the most widely used method of food preservation. It involves the application of heat to foods to destroy pathogenic and spoilage microorganisms, inactivate enzymes, and extend shelf life. The effectiveness of thermal processing depends on the temperature applied, duration of heating, and the heat resistance of the target microorganism.

The thermal death time (TDT) is the time required to destroy a specific number of microorganisms at a given temperature. The D-value (decimal reduction time) is the time at a given temperature to reduce the microbial population by 90% (one log cycle). The z-value is the temperature change required to alter the D-value by one log cycle.

Blanching

Blanching is a mild heat treatment applied to fruits and vegetables before freezing, drying, or canning. It is typically done using hot water (85–100 degree C for 1–10 minutes) or steam.

Purposes of blanching:

1. Inactivation of enzymes (peroxidase, polyphenol oxidase) that cause browning, off-flavors, and nutritional loss during storage
2. Removal of intercellular gases to improve vacuum in canned products
3. Softening of tissue to facilitate packing
4. Reduction of surface microbial load

Pasteurization

Pasteurization is a heat treatment designed to destroy all pathogenic microorganisms without significantly altering the food's characteristics.

• LTLT (Low Temperature Long Time) — 63 degree C for 30 minutes; batch process; used for small-scale dairy operations
• HTST (High Temperature Short Time) — 72 degree C for 15 seconds; continuous process; most common commercial method for milk
• Pasteurization of fruit juices is typically at 85–95 degree C for 15–30 seconds

Pasteurized products must be refrigerated as thermophilic and thermoduric organisms may survive.

Sterilization

Commercial sterilization involves heating food at temperatures above 100 degree C to destroy all viable microorganisms including spores of Clostridium botulinum (the most heat-resistant pathogenic spore-former). Canned foods are typically processed at 121 degree C (250 degree F) in retorts or autoclaves. The target is a 12D process (bot cook) ensuring a 12 log reduction of C. botulinum spores.

UHT (Ultra-High Temperature) Processing

UHT involves heating food to 135–150 degree C for 2–5 seconds followed by rapid cooling and aseptic packaging. This produces commercially sterile products with minimal nutritional and sensory changes compared to conventional sterilization. UHT milk has a shelf life of 6–9 months at room temperature without refrigeration, making it particularly suitable for regions with limited cold chain infrastructure.

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Thermal processing of foods — blanching, pasteurization, sterilization, and UHT.
Section context	Revise this lesson with the rest of Principles of Food Science and Nutrition for stronger conceptual continuity.