Thermal Preservation of Food

Principle of Thermal Preservation

Regulated processes using heat that are performed commercially to control chemical, enzymatic and microbial reactions

3 Types of Thermal Preservation

1. Blanching

2. Pasteurization

3. Commerical Sterilization

Blanching

• Exposing produce to boiling water/steam for a short period of time

• To inactivate endogenous enzymes and drive off gases

Pasteurization

• Boiling below boiling point of water (60-80°C)

• low acid and acid foods

Pasteurizing Low Acid foods

• Milk & Eggs

• to destroy pathogenic (disease-causing) bacteria & viruses

• to inactivate milk and enzymes

Pasteurizing Acid foods

• Beer, wine, juice

• To extend product-shelf life

• Destroys spoilage-causing MOs & enzymes

Commercial Sterilization (CS)

• Canning

• Requires minimum of 131°C heat for 15 mins

• destroys spoilage causing & pathogenic MOs

• ensures Clostridium Botulinum are destroyed

 Clostridium botulinum (C. botulinum)

• Bacteria present in soil, water, air

• Anaerobic (NO oxygen)

• Thrives in low-acid foods

What can C. botulinum do?

• produces a neurotoxin that affects the central nervous system

• results in vertigo, blurred vision, difficulty breathing & swallowing, slurred speech

• worst cases: death from respiratory collapse or cardiac arrest

Characteristics of CS containers

• CS conducted after packaging

• must withstand high temp and pressure

• hermetically sealed / airtight

• impermeable by gases, liquids, and microorganisms

• shelf life of 2+ years

UHT processing and Aseptic Packaging = Commercial Sterilization (what are they)

1. UHT: injection of hot steam under pressure (140 - 150°C) for a short period of time (4-6 secs); followed by immediate cooling

2. Aseptic packaging: commercially sterilized product is placed into pre-sterilized container and sealed in an aseptic environment

   • ex. shelf-stable milk, juice

Why select CS?

longer storage time at room temp

Goal of Selecting Heat Treatments (HT)

• To minimize nutrient loss and ensure appropriate storage

  • Sufficient & Mild Treatments

Why select Sufficient Heat for treatment?

To destroy microorganisms & food enzymes

Mildest HT

Guarantees freedom from pathogens and toxins while producing desired storage life

Milk Processing Process (3)

1. Clarification

2. Homogenization

3. Pasteurization

Clarification

Raw milk is passed through an industrial centrifugal clarifier

Purpose:

• To remove impurities that may have accidentally entered the milk at farm level, ensuring it is physically clean before chemical processing

Homogenization

The milk is pumped under extreme physical pressure through a microscopic valve

Purpose:

• To break down the large fat (cream) into smaller droplets

• Shrinking the size of the droplets prevents the cream from spreading and rising to the top of the container during storage, providing a smooth, consistent mouthfeel

Pasteurization

A moderate-intensity thermal process that heats fluid milk to a specific temperature below boiling point

Purpose:

• The primary safety hurdle to completely destroy pathogenic bacteria and viruses

• Inactivates naturally occurring enzymes to prevent spoilage

Pasteurization of dairy products other than milk- the effect of other ingredients (e.g. in egg nog)

• Other ingredients in pasteurization of dairy products other than milk affect require a higher temperature and holding time

• Sugars, starches and fats protect bacteria from heat

Tetra Pak

Container with a Hermetic seal and a 7 layers

Thermal Death Curves (TDC)

• graphs used by the food industry to determine how much treatment a food needs to be safe and high quality

• Microorganisms are not killed instantly when exposed to heat, but instead die in a predictable logarithmic order of death

• Under constant thermal conditions, the same % of microbial population will be destroyed in a given time interval, regardless of size

2 Types of Thermal Death Curves

1. Thermal Death Rate Curves (TDRC)

2. Thermal Death Time Curves (TDTC)

Thermal Death Rate Curves (TDRC)

• graph that looks at a microbial population heated at one specific, constant temperature (ex. 121°C)

• one specific temperature that allows you to calculate the D-value by measuring how much time it takes for the population to drop

D-Value (decimal reduction time)

• time (mins) at a particular temp required to kill 90% of a microbial population

• If u increase the temp, the time required to kill the pathogen is decreased

Thermal Death Time Curves (TDTC)

• time required for destruction of a microbe under specific conditions at different temperatures 

• z-value (°C) and F-value (min)

Z-Value

• Allows a comparison of heating processes at different temperatures because in the real world, we don’t have instant heat at 121ºC

• Resistance of MO to temperature Variation

F-Value

Time (min) required ot kill MOs at 121ºC; lethality of heat treatment

Margin of Safety (MS)

• guarantees that even if a food is heavily contaminated with highly dangerous microbial spores, the heat treatment will thoroughly destroy them

• probability of surviving C. botulinum spores

• intention is to have a wide margin of safety to minimize the probability of survivors

MS in Low Acid foods

12D thermal process

MS in Acid Foods

5D thermal process

How can we determine if C. botulinum spores have been destroyed?

1. Inoculated pack studies

2. Clostridium sporogenes PA3679

Inoculated pack studies

• Experiment to check if a canning process (time & temp) works

• scientists contaminate (inoculate) a test batch with harmless bacterial spores

• run these test cans through industrial cooker, incubate, and see if any spores survive

• if spores are 100% dead, heat treatment is setup and safe

Clostridium sporogenes PA3679

• Harmless strain of bacterium that is a putrefactive anaerobe

• more heat resistant than C. botulinum, so they require a higher temp an time

The “Cold Point”

• When a sealed can is cooked, the heat travels from the outside walls inward

• The cold point is the absolute last part of the container to reach the required sterilization temperature

• Temperature sensors must be placed directly at the cold point because if this spot reaches the safety target, everything else in the can is fine

  • location determined by the mechanism of heat transfer

Mechanism of heat transfer is determined by (2)

1. Consistency of Food

2. Chemical Composition

Consistency of Food