Chemical substances, known as artificial preservatives, prevent or delay bacterial growth, spoilage, and discoloration in food. These preservatives can be added directly to food or sprayed onto its surface.
Common chemical preservatives include propionic acid, sorbic acid, benzoic acid, and sulfur dioxide. In contrast, biological preservation technologies, classified as “biopreservation,” utilize lactic acid bacteria (LAB), their metabolic products, or both to enhance the safety and quality of foods, particularly fermented ones.
This article explores the principles of chemical and biological food preservation, their types, applications, and associated benefits and risks.
Definition and Types of Chemical Preservatives
Overview of Chemical Preservatives
The quality of food products declines over time, from harvest or slaughter to consumption, due to microbiological, enzymatic, chemical, or physical changes. Food antimicrobial agents are chemical compounds added to or naturally present in foods to retard microbial growth or eliminate microorganisms.
These preservatives include antimicrobial agents (e.g., benzoic acid, nitrate, citric acid, and sulfur dioxide) and antioxidants (e.g., butylated hydroxyanisole and citric acid).
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Common Chemical Preservatives: Nitrates, Nitrites, and Sulfites
1. Nitrates and Nitrites: Nitrates convert to nitric acid, forming a stable red color in meat. Nitrites inhibit Clostridium botulinum in meat products but can form carcinogenic nitrosamines.
They are used in meats like sausage, ham, bacon, and beef. Side effects include allergies, asthma, nausea, vomiting, headaches, and potential cancer risk from sodium nitrite converting to nitrous acid in the body.
2. Sulfites (Sulfur Dioxide and Metabisulfite): Sulfites form sulfurous acid, an active antimicrobial compound that disrupts microbial cells by reducing sulfide linkages, forming carbonyl compounds, and inhibiting respiratory mechanisms.
Used at levels of 0.005–0.2%, sulfites prevent fungal spoilage and browning in peeled fruits and vegetables. Side effects include allergies, asthma, nausea, vomiting, joint pain, palpitations, and headaches.
Common Chemical Preservatives: Benzoates, Propionates, and Sorbates
1. Sodium Benzoate or Benzoic Acid: Effective against yeast and molds at concentrations up to 0.2%, benzoates inhibit enzymes necessary for oxidative phosphorylation, disrupt membrane protein function, and destroy membrane potential. They are added to carbonated drinks, margarine, flour, pickles, fruit purees, and juices. Side effects include severe allergic reactions and potential cancer risk.
2. Propionates: Calcium and sodium propionate, effective at 0.1–0.2%, combat mold and bacteria but not yeast at these levels. Their inhibitory action results from cytoplasm acidification and destabilization of the membrane proton gradient.
3. Sorbates: Sodium, calcium, or potassium sorbate, used at 0.05–0.2%, is more effective against yeast and mold than bacteria. Sorbic acid’s activity increases as pH decreases, and it is tasteless and odorless below 0.3%. It is used in non-alcoholic and alcoholic drinks, processed vegetables, fruits, and dairy desserts.
Common Chemical Preservatives: BHT, BHA, and Glycerides
1. Butylated Hydroxytoluene (BHT) and Butylated Hydroxyanisole (BHA): These antioxidants prevent fat oxidation (rancidity) in foods like fresh meat, pork, sausages, potato chips, crackers, beer, baked goods, drink powders, dry cereals, and frozen pizza. Side effects include potential cancer risk and liver disease.
2. Mono-glycerides and Diglycerides: Used in cookies, cakes, pies, bread, peanut butter, roasted nuts, shortening, and margarine, these preservatives may cause cancer and birth defects.
Common Chemical Preservatives: Salt and Sugar
1. Salt and Sugar: Long used to extend shelf life, salt and sugar bind water, reducing water activity (aw) and limiting microbial growth. Since most microorganisms require high water activity, they cannot survive in such environments.
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Biological Methods of Food Preservation
Overview of Biological Preservation
Biopreservation, or biological preservation, involves using “desired” microorganisms (cultures) and/or their metabolites to naturally enhance food safety and stability without altering sensory quality. Protective or antagonistic microorganisms are added to inhibit pathogens and extend shelf life while minimally impacting sensory properties.
Biopreservation can be applied through two methods:
- Adding crude, semi-purified, or purified microbial metabolites.
- Adding pure, viable microorganisms.
Biopreservation serves as a complementary approach in hurdle technology, which combines multiple preservation methods to avoid overly severe single-method impacts on nutrients.
Antimicrobial Metabolites of Lactic Acid Bacteria
1. Organic Acids: Lactic and acetic acids, the most significant antimicrobials produced by LAB, influence microbial activity in fermented materials. Acetic acid is more effective against yeasts than lactic acid. Some oxidative yeasts utilize organic acids, causing spoilage through deacidification in fermented plant material.
The inhibitory effect stems from the undissociated form of the acid, which diffuses across cell membranes, reducing intracellular pH and dissipating membrane potential.
2. Hydrogen Peroxide: Its antimicrobial activity results from a strong oxidizing effect on bacterial cells, destroying basic molecular structures of cell proteins. In raw milk, hydrogen peroxide, catalyzed by lactoperoxidase, oxidizes endogenous thiocyanate, producing toxic intermediary products that affect various bacteria. It is particularly effective against urogenital infections, such as bacterial vaginosis.
3. Carbon Dioxide: Carbon dioxide contributes to preservation through its antimicrobial activity and by creating an anaerobic environment, replacing molecular oxygen. Its antifungal activity results from inhibiting enzymatic decarboxylations and accumulating in membrane lipid bilayers, disrupting permeability.
4. Bacteriocins: Bacteriocins like nisin, pediocins, and sakacins are studied for their biopreservative effects, particularly in meat products. Nisin, produced by Lactococcus lactis subsp. lactis, is the only bacteriocin widely applied in food, preventing late-blowing of cheese by inhibiting Clostridium spores and controlling Clostridium and Listeria in pasteurized cheese spreads.
Applications of Protective Cultures in Food Preservation
Protective cultures, particularly bacteriocinogenic strains, are effective in controlling Listeria monocytogenes in various food products.
- 1. Milk and Dairy Products: Cheese is prone to spoilage from Clostridium spp. (late blowing) and contamination by L. monocytogenes, especially in cheeses with increasing pH during ripening (e.g., Taleggio, Gorgonzola, Mozzarella).
A nisin-producing starter system in cheddar cheese extended the shelf life of pasteurized processed cheese from 14 to 87 days at 22°C and controlled L. monocytogenes, Clostridium sporogenes, and Staphylococcus aureus.
Antilisterial effects were also observed with a bacteriocinogenic Enterococcus faecium strain during Taleggio production.
2. Vegetable Products: Bacteriocinogenic LAB are effective in biopreserving minimally processed vegetables (e.g., pre-packaged mixed salads, sprouts) and fermented vegetables (e.g., sauerkraut, olives). They act against coliforms, enterococci, and L. monocytogenes. Biocompetitive microorganisms, including non-aflatoxinogenic molds or antagonistic yeasts and bacteria, inhibit mycotoxin-forming molds.
Fermentation as a Preservation Technique
Fermentation involves the bioconversion of organic substances by microorganisms and/or enzymes (complex proteins) of microbial, plant, or animal origin.
One of the oldest food preservation methods, fermentation is applied globally and is strongly linked to cultural and traditional practices, particularly in rural households and village communities.
During fermentation, microbial growth and metabolism break down complex substances into simpler ones, producing diverse metabolites. For example, under limited oxygen or anaerobic conditions, yeasts convert sugars to alcohol and carbon dioxide, while Acetobacter converts ethyl alcohol to acetic acid in the presence of oxygen, a process also referred to as fermentation.
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