Food Studies (FDST 3000), Dr. Kucha, UGA, Exam 3 Topic: Thermal Food Processing Technology and Novel Techniques

What is thermal processing?

The controlled use of heat to increase (or reduce) the rates of reactions in foods; in particular, it is the use of heat to detroy pathogenic microorganisms in foods

What are the main objectives of thermal processing?

1) Make the product safe and shelf-stable

2) Reduce the number of microorganisms of public health concern to a statistically small level

3) Package the food in an environment to prevent recontamination and suppress the growth & activity of spoilage microorganisms

What are the modes of heat transfer?

1) Conduction

2) Convection

3) Radiation

What is conduction heat transfer?

Heat transfer through a barrier that blocks the inside from the outside (heat flows from outside to inside)

What is thermal conductivity (k)?

The quantity of heat a given material (with a given unit area of cross-section) can transfer a unit temperature gradient in the direction of the heat flow

What is the formula for conductive heat transfer?

q = - kA x (change in T/change in x)

q = heat in Watts

k = thermal conductivity

A = cross-section area of the wall (found by length times width)

change in T = temp difference between the inside & outside (C)

change in x = wall thickness (m)

What is convective heat transfer?

Heat transfer from a solid to a liquid

What is the heat transfer coefficient?

Used for convective heat transfer; it is the quantity of heat transferred across a unit surface area to a fluid when a unit temp gradient exists between them

What is the formula for convection heat transfer?

q = hA(Ts - T∞)

q = heat transferred in Watts

A = area

h = heat transfer coefficient

Ts = surface temperature

T∞ = temp at a point away from surface

What should be considered before deciding to thermal process?

1) Physico-chemical properties of food

- acidity (pH)

- state of food (liquid/solid)

2) Target microorganism

- most heat-resistant spoilage microorganisms

- vegetative bacteria vs. endospores

3) Location of cold spot (the slowest heating point should receive enough heat)

4) Shelf-life requirement

- Longer shelf-life requires more severe treatment

- Storage condition (refrigerated or ambient temperature)

What are the types of thermal processing?

For Product Quality

- Blanching

For Product Safety

- Sterilization (at temps > 100 C)

- Pasteurization (at temps < 100 C)

What is blanching?

- Involves rapidly heating vegetables and fruits to predetermined temps and maintaining them for a specified time (usually 1-10 min)

- Done usually prior to food processes such as drying, freezing, frying, and canning

- It inactivates enzymes and destroys microorganisms that might contaminate raw fruits and vegetables

- The blanching medium may include water, steam, hot gas, and microwave

How to know when to use pasteurization or sterilization?

1) Determine pH

- if a high acid food (pH < 4.5), use pasteurization

- if a low acid food (pH > 4.5), go to step 2

2) Are microorganisms pathogens or non-pathogens

- if pathogens, use pasteurization

- if non-pathogens, use sterilization

What does pasteurization do?

Since it is a mild heat treatment (55 C - 100 C), it causes the destruction of pathogenic microorganisms, but it isn't high enough to destroy all spoilage bacterial spored (product is refrigerated to control spoilage and is only a temporary shelf-life extension)

How to perform pasteurization

Batch (Holding) process

- every particle must be heated at least 63 C and held for at least 30 minutes

- low temp for a long time (LTLT/Vat treatment)

- traditional (not used commercially nowadays)

Continuous process

- every particle must be heated to at least 72 C and held for at least 15 seconds

- high temp for a short time (HTST treatment or flash pasteurization)

- less nutritional and sensory damage

What does sterilization do?

- Goal is to extend the shelf-life of a food while promoting its safety

- Accomplishes this by a more extreme heat treatment (100 C - 150 C)

- "Commercially Sterile" (name is a "Misnomer" cuz product is not truly sterile)

- Product will be stored in air-tight (hermetically sealed) packages usually at room temp

- The container environment will prevent growth of microorganisms and thus spoilage

What are the different types of sterilization?

Canning (Retorting)

- product is packaged before heating

- solid product (meat, seafood, vegetables)

Aseptic Processing (Ultra High Temperature (UHT) Processing)

- Product and package are sterilized separately

- Liquid product (milk, juice, soup)

How is the amount of heat required to destroy microorganisms determined?

Determines through thermal death time (TDT) tests conducted by thermo-bacteriologists in a laboratory

How to Sterilize: Canning

- Heat at high temps (116 C - 121 C)

- Heat for a long time (50-90 minutes)

- Use pressure vessels (retorts)

- Substantial changes in nutritive and sensory characteristics

What is a retort?

- A closed, pressurized vessel used for heating foods sealed in containers

- It is a pressurized system, so the temperature is much higher than boiling water

- Needs heating medium (hot water, steam, steam-air mixture)

Why is steam used in a retort?

The steam can carry more energy (due to having stored energy from when it converted from water to vapor) than hot air

What is a Still Retort?

- The containers don't move during thermal process

- They may be vertical (crates stacked) or horizontal (crates side by side)

What do Agitating Retorts do?

Improves the heat transfer by enhanced convection, so heating time is reduced and the quality is improved

What is Aseptic Processing?

Aseptic (meaning free of microorganisms) Processing is the application of the thermal process in a continuous system using rapid heating to ultra-high temps (UHT, 140 C - 150 C) for a short time (4-5 seconds), followed by immediate cooling

What is Aseptic Packaging?

UHT food is aseptically placed into pre-sterilized containers and hermetically sealed

What is Aseptic Processing and Packaging practical for?

Only practical for liquid or fluid products (shelf-life stable milk and juices)

What are the requirements of Aseptic Processing?

- A pump-able product

- Equipment that can be sterilized

- Sterile product

- Sterile packages

- Sterile environment

- Monitoring, recording, and controlling critical factors

- Proper handling of finished packages to maintain container integrity

What does a heating profile depend on?

1) Heating process

- Still vs. Agitated retort

- Aseptic vs. In-package

2) Heating medium

- Steam

- water (immersion or spray) with or without overpressure

- steam-air

3) Heating conditions

- retort temp

- initial temp

- loading pattern

4) Product type

- solid

- semisolid

- liquid

- particulate liquid

- thermo-physical properties of the product

5) Container

- material/type (metal, glass, platic)

- shape

- size

What does the success of the thermal process depend on?

1) Removing the oxygen from the package

2) Controlling the pH

3) Giving adequate heating treatment

4) Controlling the storage temperature

(foods must also be stored in hermetically sealed containers to prevent recontamination)