FOOD PRESERVATION
Although primarily used to delay food spoilage, many preservatives are also used to inhibit specific pathogens.
While it may be tempting to use chemotherapeutic compounds (e.g. antibiotics) as food preservatives, it is generally accepted that:
1. the costs of this practice outweigh the benefits
2. it invariably leads to microorganisms which are resistant to the agent
Instead, we use several means to preserve food ranging from modern chemical methods and radiation to more traditional techniques such as temperature control, fermentation or water removal.
I. CHEMICAL FOOD PRESERVATIVES
This group includes chemicals which are not used therapeutically in people or animals and which are generally recognized as safe (GRAS) by FDA.
A. Proton ionosphores
a. Organic acids that inhibit membrane transport functions.
b. In their undissociated state, these acids are soluble in the cell membrane and so will move into the cell by simple diffusion. Once inside the cell, they dissociate. As more and more protons are released, membrane potential is disrupted and the proton-motive force is lost.
c. As a result, cells are unable to utilize any of their PMF-dependent transport systems (e.g. uptake of amino acids, CHOs, and other substrates they require for growth). Although cells may not die, growth is arrested.
1. Benzoic acid. Na-benzoate was the first chemical food preservative approved in the U.S. by FDA:
structure – aromatic ring with COOH
Like all proton ionophores, antimicrobial activity is related to pH because only the undissociated molecule is inhibitory.
at pK; [A-]/[HA] = 1
pH = pK + log [A-]/[HA] (remember; log 1 = 0)
The pK for benzoate is 4.20 so at pH 6 only 1.5% is undissociated.
Consequently benzoate is only effective in high acid foods (apple cider, soda, catsup, salad dressings, margarine), where its action is primarily aimed at fungi (molds are inhibited more than yeasts).
Maximum permissible amount is 0.1%. In fruit juices, even this level may impart a peppery or burning taste.
1a. Parabens.
a. Esters of hydroxyparabenzoic acid.
b. Far less sensitive to pH than most other ionophores.
(pKs around 8.7; so effective at pHs as high as 8.0)
str: HO-aromatic ring-COO-R, R=CH3, (CH2)6CH3 [heptly], or (CH2)2CH3 [proply]- propylparaben is more effective than methyl, and heptyl may be most effective of the 3.
Propyl and methyl have the same max level as benzoate (0.1%), heptyl is allowed at 12 ppm in beer and 20 ppm in noncarbonated and fruit-based beverages….also, more effective against molds than yeasts.
2. Propionate. CH3CH2COOH;
a. Ca2+, Na+ salts permitted in breads, cakes, certain cheeses and other foods, mostly as a mold inhibitor.
b. Also most effective in acid foods, pK = 4.87.
c. Probably no effective at pHs around 7.0 or above. 0.32% max allowable level.
3. Sorbic acid. CH3CH=CHCH=CHCOOH;
a. Ca2+, Na+ or K+ salts used as a food preservative at levels not to exceed 0.2%.
b. pK is 4.80, also more effective in acid foods, ineffective at pH 6.5, but more inhibitory than benzoate when pH is between 4.0-6.0, same as benzoate but better than propionate at pH<3.0.
c. Can use higher levels in cakes than propionates without leaving off-flavor.
d. Although primarily effective against yeasts and molds.
e. Sorbic acid is also effective against a wide range of bacteria including Staph, salmonellae, coliforms, psychrotophic spoilage bacteria and Vibrio parahaemolyticus.
f. Lactic acid bacteria are resistant to sorbate, esp. at pH>4.5, so it is used to control fungi in lactic fermentations.
g. Widely used in cheese, bakery products, fruit juices, beverages, salad dressings, jellies.
4. Sulfur dioxide and sulfites.
a. SO2 sulfite (=SO3), bisulfate (-HSO3) and metabisulfite (=S2O5) appear to have a similar but poorly defined mechanism of inhibition.
b. 200-300 ppm is max allowable level in foods such as molasses, dried fruits, wine, and lemon juice.
c. These compound are more effective at acid pH.
100-200 ppm SO2 is bacteriostatic to Acetobacter spp. Lactic acid bacteria (higher levels are bactericidal to these and a few other species of bacteria.
0.2-20 ppm inhibits yeasts like Saccharomyces and Candida (some yeasts actually produce SO2 at levels between 500-1000 ppm). Molds are even more sensitive.
Problems:
Salad Bars
1. Sulfites also reduce S-S bonds and so can destroy activity in enzymes with these linkages.
2. Although it is still used to prevent browning in dried foods (where its presence is revealed on the product label), use in salad has been prohibited because 1-2% of severely asthmatic people are hypersensitive to sulfites (the food idiosyncrasy induces asthma attacks) and were inadvertently poisoned at salad bars.
3. Low levels of sulfites usually do not trigger an attack but sensitive asthmatics must also beware of wines because of some imported varieties contain high levels.
Bisulfite can be used to destroy aflatoxins AFB1 and AFB2. 1% bisulfate reduced 250 ppb of AFB1 28.2% in 72 h, and addition of 0.2% H2O2 increased aflatoxin degradation to 65.5%.
Meats
Sulfites were used as meat preservatives as early as 1813 but because they can remove decaying odor and impart a bright red color, that use has been banned.
Vitamins
Sulfurous acid and sulfites also destroy thiamine so they are not permitted in foods which are recommended sources of vitamin B1.
5. Nitrites and Nitrates. (NaNO2 and NaNO3)
Used in meat to cure to:
a. stabilize red meat color
b. inhibit spoilage and pathogenic organisms
c. contribute to flavor
Mode of Action
a. Nitrate is reduced by bacteria in the meat to nitrite, which is further reduced to nitric oxide (NO), a compound that reacts with heme or iron-sulfur groups.
b. In meat, NO reacts with the heme in myoglobin to form nitrosohemochrome which gives meat a red color.
c. In bacteria, NO reacts with and destroys and iron-sulfur enzyme, ferredoxin, that is involved in ATP synthesis. Without ferredoxin, the cells cannot synthesize energy and die.
Effect on Clostridia
1. Clostridia contain ferredoxin and one important function of nitrite is to inhibit C. botulinum in meat products.
2. Endospores will remain viable, however, and can germinate when transferred to nitrite-free foods.
3. Other bacteria which contain iron-sulfur enzymes are also inhibited. Enterobacteria, including coliforms and Salmonella, and lactic acid bacteria are not inhibited by nitrite because they do not contain ferredoxin.
From a public health standpoint, the antibotulinal effect is far more important than flavor and color, and it also requires more nitrite to attain.
a. 100 ppm or less will give meat the desired color and flavor
b. at least 120 ppm is required in the presence of salt, lactic acid bacteria, or lower pH values
The effectiveness of the preservative increases as the pH becomes more acid, but the reasons are probably not as direct as with the organic acids we just discussed.
Nitrite use in meats can lead to the formation of carcinogenic nitrosamines:
2o amine (R2NH2) + nitrite + acid (or heat_ = nitrosamine (R2N-NO) + water
-can also form from tertiary amines and quarternary ammonium compounds
-Several species of bacteria, including some lactic acid bacteria, can nitrosylate amines with nitrite near pH 7.0.
-Because of the increased potential for nitrosamine formation at high temps, only 120 ppm of nitrite can be added to cured bacon. Residual nitrite levels of 200 ppm are permitted in other processed meats like sausages, hams, and comminuted meat products. No nitrite is allowed in meat-based baby foods.
6. NaCl and sugar.
Both exert a drying effect on cells by creating a hypertonic medium that draws water out of cells.
(plasmolysis-causes growth inhibition or death)
Organisms that can grow in high salt conc. are halophiles, those that can withstand high salt but cannot grow are halodurics.
Similarly, the terms osmophile and osmoduric apply to comparable organisms that can survive in high sugar conc.
With respect to other microorganisms, it generally takes about six times as much sugar as salt to affect the same degree of inhibition.
Application:
1. salt – meats
2. sugar –fruit preserves, candies, cakes condensed milk
7. Ethylene and propylene oxides.
a. Antifungal gases used as fumigants to sterilize packaging containers for aseptically processed foods.
b. Also used on dried fruits and spices.
c. These unstable ring structures are alkylating agents that inactivate proteins with susceptible groups (-COOH, -NH2, -SH, and –OH).
8. Nisin.
a. Incorrectly called an antibiotic, actually a bacteriocin.
b. Used in processed dairy foods and low acid canned foods.
(some true antibiotics are used in other countries)
c. Useful properties:
a. nontoxic to humans
b. “natural”
c. heat stable (at low pH)
d. good storage stability
e. doesn’t create off odors or flavors
f. no cross resistance to other clinical antibiotics, no clinical applications
Use in low acid canned foods (e.g. green beans) allows a dramatic reduction in heat processing time and temp-gives increased quality to the food.
Use in U.S. is limited to pasteurized process cheese spreads to prevent botulism.
9. Indirect antimicrobials - Added compounds whose primary purpose in foods is not antimicrobial but which do contribute to antimicrobial activity.
-Antioxidants (e.g. butylated hydroxyanisole [BHA] and B-hydroxytoluene [BHT].
-Flavoring agents (e.g. diacetyl, wood smoke)
-Natural spices (several have antimicrobial oils)
II. THERMAL DESTRUCTION OF MICROORGANISMS
Two main types of heat treatment: Pasteurization or Sterilization.
A. Pasteurization
1. Treatment designed to destroy the most heat-resistant nonsporeforming pathogens
2. Kills yeasts, molds, gram-, and most gram+ bacteria.
3. Thermophilic and thermoduric organisms may survive
a. Thermoduric-commonly streptococci and lactobacilli.
b. Thermophilic-clostridia and bacilli are the most important.
in milk; 145oF (63oC)/30 min for LTLT or 161oF (72oC)/15s for HTST
in eggs; use 60oC for about 4 min.
B. Sterilization
1. Destruction of all viable organisms as measured by an appropriate enumeration method.
“commercially sterile” which means that no viable MO can be detected or else the number of survivors is so low that they are of no consequence under the intrinisic and extrinsic conditions of the product.
UHT milk; 140-150oC for a few sec.
Some foods, especially liquids like milk, are not sterilized in their packages and must be packaged aseptically after sterilization.
Heat resistance of MO is usually related to their optimal growth temp:
a. Psychrophiles are the most sensitive, thermophiles the most resistant.
b. Sporeforming bacteria are more resistant than nonsporeformers.
c. G+ in general are more resistant than G- bacteria.
d. Yeasts and molds are fairly heat sensitive, their spores are only slightly more heat-resistant than the vegetable cells.
Heat resistance of spores
a. The heat resistance of bacterial endospores is primary concern in the thermal preservation of foods.
b. The greatest single determinant of heat resistance in spores is the degree of protoplast dehydration.
c. Spore protoplasts are rich in Ca2+ and dipicolinic acid which together form a gel that enables spores to remain viable with very little cytoplasmic water.
d. The drier the spore, the more heat resistant it is.
Other factors affecting heat resistance in MO:
-Water; heat resistance increases with decreasing moisture; proteins denature more rapidly in water than in air.
-Fat; in the presence of fats, heat resistance increases, presumably because fat reduces cell moisture.
-Salts; some increase heat resistance while others can decrease it. The difference has been attributed to the ability of some salt to increase (e.g. Mg2+) or decrease (e.g. Na+) water activity.
-CHO; presence of sugars increases heat resistance, again as a consequence of lower water activity.
-pH; MO are most resistant at their optimal growth pH (about 7.0). As pH is raised or lowered, heat resistance decreases. This is why high acid foods require less heat processing than low acid foods.
-Proteins; proteins have a protective effect on MO. As a result, high protein foods need a higher heat treatment than low protein foods to obtain similar results.
-Number of MO; The larger the number, the higher the degree of heat resistance. May be linked to the protective effect of protein or to natural variation in a population with respect to heat resistance.
-Age of MO; most heat resistant in the stationary phase of growth, least in logarithmic. Old spores are also more resistant than young one.
-Growth temperature; heat resistance increases with incubation temp; may be due to changes in cell membrane composition. Also, the spores from a microbe grown at high temp are more heat resistant than those from the same microbe grown at a lower temp. Effects are probably a combination of genetic selection for more resistant MO and stress adaptation.
-Inhibitory compounds; heat resistance decreases in the presence of heat-resistant microbial inhibitors (e.g. nisin).
-Time/Temp relationship; many people mistakenly think that the longer the heating time, the greater the killing effect. It is more correct to say that higher temperatures kill more efficiently, thus, as temperature increases, less time is needed to cause the desired kill level.
Terminology in thermal processing:
Thermal Death Time (TDT) – time needed to kill a given number of MO at a specified temperature. Determined by placing a known number of cells or spores in sealed containers. The containers are put in an oil bath and heated for a test period then cooled quickly. The number of survivors from each test period is then determined by plate counts. Death is defined by the inability of MO to form visible colonies.
D value (the decimal reduction time) – time in min. needed at a specified temperature to kill 90% of the MO and thus drop counts by one log number.
When D is measured at 250oF, it is designated as Dr. Thus, D reflects the resistance of an organism to a specific temperature and can be used to compare the relative heat resistance among different organisms or spores; e.g. Spores of B. stearothermophilus have a Dr = 4-5.0 min, while those of C. botulinum types A&B have Dr values of 0.1-0.2.
D for the same MO can vary depending on the type of food in question (e.g. lower in more acid foods, higher in food with high protein content, etc.).
z Value – the degree F required for the thermal destruction curve to drop on log cycle. z gives information on the relative resistance of an organism to different destructive temperatures and allow us to determine equivalent thermal processes at different temperatures.
For example, if 3 min at 150oF gives you an adequate heat process and you find that z = 10oF, then 30 min at 140oF or 0.3 min at 160oF would provide an equivalent heat process.
F value – better way to express TDT. In simplest terms, F is the time in minutes that are required to kill all spores or cells at 250oF. F is calculated as follows:
Fo=Dr(log a-log b)
where a=initial load and b=final cell numbers
12-D Concept – Used in low acid canned foods (pH>4.6), where C. botulinum is a serious concern. Thermal processing requirement designed to reduce the probability of survival of the most heat resistant C. botulinum spores to 10-12. So, for the 12-D concept, Fo=Dr(lo9g a-log b); Dr=0.21 and log a – log b=12 so Fo=2.52 min.
III. RADIATION TREATMENT
In the U.S., irradiation is defined as an additive instead of a process, but it has been approved for use in spices, vegetable seasonings, papayas, strawberries, pork (to control Trichinella), poultry, and ground beef.
More applications are likely, but widespread acceptance of irradiated food will require consumer education to allay fears that this treatment produces radioactive products in food.
In some foods, radiation treatments can catalyze oxidative changes (e.g. free radical production) that produce:
a. product discoloration
b. tissue softening (in fruits)
c. rancidity (in high fat products)
Radiation at low temperature in the absence of oxygen can minimize these effects (except softening).
Terminology for radiation treatment of foods:
1. Radappertization. Sterilization treatment. Uses 30-40 kGray (Gray is a unit of absorbed dose measurement = 11 Joule/kg. Rad is another unit of absorbed dose: 62.4 MeV/g; 1 Gray = 100 rads).
2. Radicidation. Pasteurization treatment designed specifically to destroy all non-sporeforming pathogens, except. Levels used are 2.5-10 kGy.
3. Raduriztion. Thermization-like treatment aimed at reducing numbers of viable spoilage MO. Dose levels are 0.75-2.5 kGy.
Types of radiation used to preserve foods:
1. UV light. Powerful bactericidal (and virucidal) agent at wavelengths between 2000-2900 angstroms (2600 Å most effective).
UV is non-ionizing and is absorbed by proteins and nucleic acids, produces lethal mutations.
Poor penetrating capability so used only on surfaces – e.g. packaging materials.
2. Beta rays (electron rays). Also have poor penetration but better than UV.
Because electrons have charge, they act as direct ionizing particles.
As they pass through matter, electrons have enough kinetic energy to cause ionization of other molecules through impulses imparted to orbital electrons of atoms in the medium.
Some of the products from these collisions are radioactive so there is some concern over the use of high energy beta sterilization in foods.
3. Microwave. Uses a rapidly alternating electromagnetic field, polar molecules try to align and friction is created as they switch back and forth.
Basically a heat process but, due to uneven heat distribution, microwave radiation is less effective than conventional heat treatment.
4. Gamma and X rays. Basically the same thing just originate differently. Very effective sterilants.
These are uncharged particles that penetrate almost anything, forming directly ionizing particles (electrons) as they collide with nuclei.
Resistance to radiation
1. generally G+ > G- > yeasts > molds
2. Sporeformers are generally more resistant than non-sporeformers, so concerns exist over use of radiation to control botulism.
At present, the most widely used direct applications of radiation in foods are sprout inhibition of seeds and insect deinfestation, but recent outbreaks of E. coli 0157:H7 and Salmonella have generated renewed interest in the use of radiation for raw meats.
IV. LOW TEMPERATURE PRESERVATION
Foods that are going to be preserved by freezing undergo several processing steps prior to actual freezing.
1. Sorting, washing, blanching and packaging
Blanching is done by either immersion in hot water or by steam injection and accomplished the following:
a. inactivates enzymes that can cause undesirable changes during frozen storage
b. the amount of heat used to inactivate enzymes also reduces the total microbial load in the food.
c. it enhances or fixes the green color of certain veggies
d. wilts leafy veggies and displaces air that might be trapped in plant tissue which makes these products easier to package
To ensure a high quality frozen product, processor should only use fresh food that has no signs of spoilage or damage.
Once foods are prepared, one of two types of freezing processes can be used:
Quick or fast freezing; drop food to -20oC within 30 min.
Best for overall product quality, gives smaller ice crystals in the food which do not disrupt the cells as much as slow freezing.
Slow freezing; desired temp reached in 3-72 h.
a. This is the type of freezing done in the home.
b. Large ice crystals are formed that disrupt cells and can affect product te4xture and flavor. (Which do you think is more inhibitory to MO?).
V. PRESERVATION BY DRYING
1. Foods to be dried are prepared in ways similar to those used for frozen foods.
2. Vegetable to be dried are blanched to inactivate enzymes that can catalyze undesirable changes in the dried food.
3. Some light colored fruits may be treated with SO2 to maintain color (again by inactivating enzymes), conserve certain vitamins and reduce the microbial load.
4. Fruits are also pasteurized after drying (150-185oF for 30-70 min).
5. Meats are usually cooked before they are dried.
Types of dried foods:
Low moisture (LM); no more than 25% moisture, Aw between 0.00-0.60. Includes traditional dried foods like beef jerky and freeze-dried foods.
Intermediate-moisture (IM) foods; 15-50% moisture, Aw between 0.60-0.85. Examples are dried fruits, cakes, sugars, etc.
The “alarm water” content is a value for moisture content (%water) that should not be exceeded in order to prevent mold growth on dried foods.
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