14-Physical Preservation ew Flashcards
Modern food preservation designed to
extend shelf life but also enusre its safety by inactivating pathogens or in some cases preventing growth
inactivation (bacteria)
defined by the destruction of the organisms as judged by its inability to recover on microbiological media
most common modern food preservation method?
physical
Name (6) Physical Methods of Food Preservation
- high temp treatment (cooking)
- low temp treatment (freezing)
- decreasing water availability
- high pressure processing
- ionization radiation
- pulsed e field
what are the factors that affect heat transfer in foods?
- product type
- container materials
- container shape
- container size
- agitation
- temp of heating medium
T or F: liquid absorbs heat faster than solid
T
why is container size important in heating?
because critical temp (middle of container)
What is the relationship between D, Z, and F value? (LONG ONE)
D- decimal reduction value- time needed to reduce population by 1 log ex. 1010 to 109
Z- temp increase needed to decrease D value by 10
F- time, in minutes at a specified temperature required to achieve targeted reduction in a homogenous population having a specific z value
D- decimal reduction value- time needed to reduce population by 1 log ex. 1010 to 109
Z- temp increase needed to decrease D value by 10
F- time, in minutes at a specified temperature required to achieve targeted reduction in a homogenous population having a specific z value
what is most important in preservation by high temp?
temp and time
how to determine kinetics of microbial inactivation at a given temperature
observing decline in population survivors during heating time
what are survivor plots?
depict the logarithmic nature of population inactivation over time
what is the decimal reduction time?
defined as the time it takes for a 10 fold reduction in the number of survivors at a given temperature
the greater the D value the..
more resistant that organism is to heat
D value ___ as processing temperature increases
decreases
what is a thermal resistance plot
When D value is plotted against temperature, it is known as a thermal resistance plot, it is generally linear, and is also useful for comparing the difference resistances of a microorganism at different temperatures
The thermal resistance plot can be used to calculate the z value known as the thermal resistance constant, which is mathematically equal to the negative reciprocal of the slope
what is the z value
The z value represents the change in temperature required to change the D value of a microorganism by an order of magnitude (10-fold)
a large z value indicates
a more heat resistant organism
thermal lethality measurements
are taken to determine the amount of time required to commercially sterilize food at a given temp
what is the F value
the time in minutes at a specified temp, required to achieve a targeted reduction in homogeneous population having a specific z value
what is the definition of F and D value?
The difference between D value, which defines the time it takes to reduce the population by 1 log, F value represents the time it takes to get the population to a specific level (i.e. 101 or sterile)
Explain how the F value is important in the commercial canning industry
It is common for the canning industry to calculate F value with the time at a specific temperature to achieve commercial sterility
It is assumed that this process can reduce a C. botulinum population by 12 log
C. botulinum is the key pathogen of concern for canned low acid foods
C. botulinum has a D121C value of 0.21 minutes, and a Z value of 10C. Therefore, to achieve commercial sterility at F121C you multiply 0.21 minutes by 12 which equals 2.52 minutes
What is the difference between sterilization and commercial sterilization?
Sterilization: process of rendering a product free of any living organisms. Proper autoclaving, for ex. renders microbiological media sterile. application of sterilization to food require exposing products to excessive heating. Commercial sterilization is the application of heat renders the food free of: 1. microorganisms capable of reproducing in the food under normal non refrigerated storage and distribution temp. viable microbial cells or spores of public health significance. commercially sterile foods are heat treated just enough to produce a safe and shelf stable product.
what are the mechanisms used by bacteria to render them resistant to heat?
- protein cross linking (disulfide bonding), DNA structural changes (higher GC)
- state of dormancy; stationary cells are more resistant to heat than exponential cells . spore formation = heat resistance
- aw and ph, if the aw or pH is already stressing the bacteria out, then u will need less heat
explain the process of asepting processing
is used industrially, where foods and cans are sterilized separately and then foods are packaged under aseptic conditions
This avoids having to heat the food to excess to insure that the center of the can reaches temperature, and improves food quality and nutrition
The product is heated by passing through a set of heat exchangers until the holding time and temperature have been reached, then the food is immediately passed through a set of cooling heat exchangers
The cooled product is placed in pre-sterilized packages and hermetically sealed
Aseptic processing is generally used for fruit juices, dairy products, and sauces
It is not used for low-acid, or particulate-containing foods.
Why aren’t microwaves used in processing, and why aren’t they a good idea for foods that require cooking for safety?
Microwaves cause rotation of water molecules in foods, and these rotations generate friction and therefore heat. Unlike convectional heat, which penetrates from the outside inward, microwaves generate heat throught out the material leading to faster heating and shorter processing times
Distribution of the heat is very uneven, leading to uneven lethality of microorganisms (don’t microwave your chicken fingers)
This has hindered the use of the microwave in commercial food processing.
summarize preservation by low temperature
freezing stops metabolic activity of most food borne microorganisms, although cryophillic, psychrophillic, and psychotropic food microbiota will still alter foods at sub zero temps.
toxins are not affected by freezing
Freezing also retards chemical and biochemical deteriorations like lipid oxidation, enzymatic browning, proteolysis and lipolysis.
Freezing also decreases aw
explain the effects of freezing on bacteria-microbes
Freezing also decreases the aw therefore, only cold-tolerant and xerotolerant microorganisms can grow, but some yeast and fungi have these characteristics
Gram-positive bacteria survive freezing better than Gram-negative bacteria
Parasites are readily destroyed via freezing
Trichinella spiralis (pork worm) is eliminated if the raw product is held briefly at -23C
Rapid freezing produces smaller ice crystals (cells don’t get punctured), and exposes cells to osmotic stress for shorter time periods, therefore, microbial cells are more susceptible to injury and death during slow freezing (like in your home freezer)
what are the forms of water in food? what is the form of water essential for microbial growth
unbound and bound, water can be bound by hydroxyl, amino and salt groups. Unbound water is essential for microbial growth and survival. Removing or restricting unbound water can preserve the food by supressing microbial activity and extending product shelf life.
what is a food vapour pressure?
when a moist food is contained within a package with headspace the water molecules will eventually leave the food and enter the surrounding atmosphere until a state of equilibrium is obtained with the air and surrounding food.
what are the 3 categories of foods with respect to aw
- high moisture; aw> 0.9
- intermediate 0.65-0.9
- dry foods; less than 0.65 - dont support growth of food born microorganisms and have long shelf life
spoilage bacteria tend to need a very __ aw for growth
high
spore forming bacteria L monocytogenes and V parahawmolyticus can grow in the range of _____
0.91 and 0.95
yeasts have an aw requirement ____ than those of bacteria
lower
molds are ___ and grow at ___ moisture content than other types of spoilage microorganisms
xerotolerant and grow at lower moisture contents
T or F: mycotoxin production is restricted under xerophillic conditions
True
why do living cells require water availability?
for nutrient acquisition, waste removal, and as a medium for metabolic reactions to take place in
T or F: growth of microorganisms can also be restricted based on the humectant used.
True; glycerol can permeate the membrane and have diff inhibitory effect
T or F: lowering the aw kills the microbes and prevents their growth.
F; inhibits microbial growth but may not kill the microorganisms in the food. These microbes may remain dormant until the food is rehydrated and then resume their metabolic activity and multiply (i.e. Salmonella in chocolate bars)
Define drying
: is the process of mobilizing water present in a food matrix to its surface and then removing it from the surface by evaporation. It often involves simultaneous heat and mass transfer. Most drying operations involve converting liquid water in the material to vapor and then removing it by passing hot air over the product (i.e. beef jerky).
Define freeze drying
During freeze-drying the product is frozen first and the moisture is removed via sublimation. Freeze-drying has a minimal impact on the structure and flavor of the food compared with other drying techniques, but has a relatively high cost, and is therefore used as a value added technique (i.e. camp dinners, instant coffee, infant formulas).
Define osmotic drying
is a procedure that involves removing water from fresh food using a hypertonic solutions (sucrose, salts, glycerol). It relies on the ability of the food cells to selectively permit the passage of water out of the food without allowing surrounding solutes to enter these cells. As a result food is dehydrated. It cannot remove all the moisture from the food, so it is mainly used as a pretreatment to other methods of dehydration. Primarily used for dehydrating fruits and vegetables (apples, blueberries, pineapples, mangoes etc.).
define HPP
involves treating food with pressures in the range of 100 to 600 MPa, which will inactivate most bacteria, yeasts, molds, and viruses (spores survive)
Food materials containing air pockets may be deformed due to the differences in compressibility between the air and food material
Appropriate packaging must also be chosen
define pulse electric
processing is designed to treat pumpable food and involves the application of a short burst of high voltage to a product between two electrodes
The treatment time is between microseconds and milliseconds
This treatment will eliminate vegetative bacterial cells
The success of the treatment relies on food composition, pH, and electrical conductivity
Air bubbles are also poor conductors and can create process non-uniformity (i.e. pumpable food)
what is ionizing radiation
electromagnetic radiation wiht shorter wavelengths have lethal effects on microorganisms. The cells DNA is the primary target so the smaller the genome, the greater resistance to ionizing radiations.
DNA damage due to ionizing radiation is caused by ___
by the reaction of reactive oxygen species (ROS) that are generated during radiation
types of ionizing radiation
gamma and x rays
define x rays
- produced when electron beam strikes metal
- irradiate food like fruit and veg
- high power requirement
define gamma radiation +advantages
is emitted by an excited nucleus of a radioisotope, which permits the nucleus to go to its lowest energy or ground state
The radiation sources used for food are cobalt-60, and cesium-137
The source is stored under water, but during treatment it is contained in a concrete and steal structure to protect workers
The advantage is radiation has a high penetration power, and the energy is constant over the duration of the treatment, it is a continuous source that doesn’t need to be turned off
The process usually consists of exposing the prepackaged food material to a radiation dose for a certain time
explain the 3 different dosage levels of gamma radiaition in food preservations
- Redappertization. Involves a high dose (10 to 75 kGy) to produce commercial sterility
- Radicidation. Involves medium dose (1 to 10 kGy) to produce an equivalent effect to thermal pasteurization – effective against vegetative bacteria but spores are not inactivated
- Radurization. Involves low dose irradiation (0.05 to 1 kGy) to control the presence of insects in grains or stop the sprouting of potatoes, or delay fruit ripening
what does PEF processing depend on
food composition, pH and electrical conductivity. If there are air bubbles than poor conduction and create process of non uniformity.
