Food Preservation Techniques and Advanced Thermal Processing

Flashcards covering thermal and non-thermal food preservation methods, including dehydration, concentration, chemical additives, microwave heating, irradiation, ohmic heating, induction cooking, and high-pressure processing.

What is the primary intent of food dehydration as a preservation method?

The primary intent is to reduce moisture content and prevent or preclude the possibility of microbial growth, such as bacteria, mold, and yeast, as a decrease in relative humidity (RH) leads to a decrease in microorganism growth.

How does freeze drying remove moisture from food products?

Freeze drying freezes the food and subsequently applies a vacuum to evaporate moisture through the process of sublimation, in which ice is converted directly into a vapor without passing through the liquid phase.

What processing mechanism is used in drum drying, and what are typical food examples?

The food product is dried on two heated stainless steel drums before being scraped off; examples include milk, juices, and purees.

How does puff drying create a puffed structure in foods such as ready-to-eat cereals?

It occurs either through heat and subsequent vacuum (increasing the internal and external pressure difference), combining vacuuming and steaming, or by raising the food's water temperature above $212\,^{\circ}\text{F}\,(100\,^{\circ}\text{C})$ and quickly releasing the external pressure.

How does spray drying operate, and which foods commonly undergo this process?

The product is dried as it is sprayed into a chamber concurrently with hot air; common examples include eggs, instant coffee, and milk.

How can detrimental enzymatic color, flavor, or textural changes be controlled prior to dehydrating foods?

They can be controlled by deactivating enzymes, by blanching, or by adding sulfur compounds prior to dehydration.

What is the primary cause of overall deterioration in dried foods?

Oxidative spoilage, which involves chemical changes caused by the oxidation of fats.

Why is concentration generally not considered an effective method of food preservation on its own?

Because the water activity is not reduced sufficiently to prevent bacterial growth, requiring additional preservation methods such as pasteurization, refrigeration, or canning (with the exception of high-sugar jams and jellies).

How do flash evaporators concentrate food, and what is the primary drawback of this method?

Heated steam ($150\,^{\circ}\text{C}$) is injected into the food and later removed along with water vapor; while heating time is reduced, the high temperatures can cause the loss of volatile flavor constituents.

What is the operational difference between ultrafiltration and reverse osmosis in liquid concentration?

Ultrafiltration pumps a dispersion under pressure against a membrane that retains proteins but allows smaller molecules like salts and sugars to pass; reverse osmosis uses higher pressures and smaller membrane pores to hold back salts, sugars, and larger protein molecules.

How do heavy sugar syrups and brines preserve food against bacterial cells?

They compete with bacteria for water; through osmosis, water moves out of the bacterial cells into the surrounding lower-water medium to equalize concentrations.

What nutrient does microwave heating report to inactivate according to Watanabe et al. (1988)?

Microwave heating inactivates vitamin $B_{12}$, a nutrient that plays an important role in maintaining nerve tissue.

What is the frequency of microwave electromagnetic waves, and how do they generate heat in food?

Microwaves operate at a frequency of $2.5\,\text{GHz}$, generating heat through molecular friction as polar molecules such as water try to align with the constantly changing electromagnetic field.

How far do microwaves typically penetrate into solid foods before heat transfer shifts to conduction?

Microwaves penetrate $1\text{--}2\,\text{in.}$ into the food; beyond that depth, heat is transferred by conduction.

In microwave heating terminology, what do 'shielding' and 'thermal runaway' refer to?

'Shielding' is the protection of portions of food (such as cylindrical ends) that readily overcook, while 'thermal runaway' refers to the differential heating of food without reaching thermal equilibrium.

Does food subjected to irradiation become radioactive or retain radiation residues?

No; scientific evidence demonstrates that foods do not become radioactive and that no radiation residue remains in the food.

What are the two distinct forms of irradiation sources used for food preservation?

An isotope-sourced form using gamma rays, and a machine-sourced form that is electronically generated, known as e-beam (Higgins 2000).

What dosage difference applies when using irradiation against Trichinella spiralis in pork?

A low dose is required to stop the reproduction of Trichinella spiralis, whereas a much higher dose is needed to completely eliminate the parasite from the pork.

What symbol and phrasing are required for whole irradiated foods in the United States, and what items are exempt?

Whole irradiated food items must display the radura symbol along with the words 'Treated with Radiation' or 'Treated by Irradiation'; spices, restaurant foods, and processed foods containing irradiated ingredients are exempt from this labeling.

What is ohmic heating, and by what other names is it known?

Ohmic heating is an advanced thermal processing method in which the food material acts as an electrical resistor and is heated by passing an electrical current through it; it is also called electrical resistance heating, Joule heating, or electro-heating.

What is a major processing advantage of ohmic heating over conventional thermal heat exchangers regarding particulates and cleaning?

Ohmic heating can volumetrically heat large particulate foods up to $1\,\text{in.}$ and requires comparatively less cleaning due to reduced product fouling on food contact surfaces.

How does induction cooktop heating generate heat in cookware?

An alternating current in a coil beneath the cooktop creates a high-frequency electromagnetic field that generates eddy currents in a ferromagnetic (iron or stainless steel) pan; because the metal has high electrical resistance, the eddy currents are converted directly into heat.

What is High-Pressure Processing (HPP)?

High-pressure processing (HPP) is a nonthermal food preservation method that uses physical pressure rather than heat or chemicals to preserve food.