Chemical & Biological Preservation Methods

Chemical and Biological Preservation

• Traditional antimicrobials are small-molecular agents that target bacterial components to inhibit their growth or kill them.
• Antimicrobials can be synthetic or inorganic.
• Antimicrobials can be natural extracts or organic compounds.
• Antimicrobials generally recognized as safe include NaCl, acetic acid (vinegar), and lactic acid.
• Not all acids are antimicrobials.
• Organic acids/esters with high activity include acetic, lactic, propionic, benzoic acids, and parabens.
• Organic acids/esters with limited activity include citric, malic, and fumaric acids.
• Acetic acids turns into acetates.
• Benzoic acid turns into benzoates.
• Lactic acid turns into lactates.
• Propionic acid turns into propionates.
• Sorbic acid turns into sorbates.
• Parabens include methylparabens, propylparabens, and heptylparabens.
• pKa values indicate the strength of an acid.
• Weak acids have large pKa's due to little dissociation.
• The pKa range for weak organic acids utilized as antimicrobials is 3-5.
• In acid resistance, acid in the un-protonated form goes into the cell membrane and then becomes acidic.
• The faster bacteria can pump them out before it becomes acids, the more resistant they are.
• Salts act as food preservatives through plasmolysis, causing water loss.
• This inhibits the growth of harmful microorganisms that utilize water for nutrients.
• Halophiles/halotolerant bacteria may continue to grow (Staph, Listeria, Vibrio, Fungi)
• Nitrate (NO3) and Nitrite (NO2) act as antioxidants and antibacterials, for example, against C. botulinum.
• Nitrate and Nitrite gives a distinct pink color and flavor in cured meat such as ham, bacon, and sausages.
• Nitrate is naturally found in leafy greens and converts to nitrite in the mouth.
• Salt preservation reduces water activity (the amount of free water available) in food.
• Water moves from areas of high concentration to low concentration to dilute salt.
• Nitrosamines are nitrates + amines under acidic conditions.
• Nitrosamines form biosynthetically and by microbial activity.
• Nitrosamines are unintentional by-products found in processed foods during food preparation, for example, in cured meat and beer production.
• Frying may increase nitrosamine formation.
• Nitrosamines are linked to liver tumors in rats.
• Sodium erythorbate does not increase nitrosamines; instead, it is added to inhibit their formation.
• Sulfites (SO2) or bisulfites (NaHSO3) are typically used in wine as preservatives.
• Nitrates and phosphates might be found as preservatives in baked goods.
• Sulfites and organic acids are types of preservatives.
• Biopreservation is a method of food preservation using natural or controlled microbiota or antimicrobials to extend shelf life.
• It reduces/eliminates microbial spoilage.
• Examples of biopreservatives include lactic acid bacteria (LAB), probiotic bacteria, and bacteriophages (bacterial viruses).
• Bacteriophages kill bacteria, are diverse, and are species-specific.
• Lactic acid bacteria (LAB) promote food preservation by causing acidification of food due to fermentation.
• This lowers the pH, inhibiting the growth of specific spoilage organisms (SSOs) and pathogens.
• LAB also produce antimicrobial substances that inhibit the growth of pathogens and specific spoilage organisms (SSOs).
• Lactic Acid Bacteria (LAB) can inhibit the growth of C. botulinum by lowering pH levels during acidification.
• C. botulinum does not grow at a pH < 4.8.
• Bacteriocins act on vegetative cells by disrupting membranes, increasing permeability, forming pores, and/or exhibiting detergent activity.
• Bacteriocins act on spores by allowing them to germinate but preventing growth, requiring a higher concentration.
• Bacteriophages (phages) are bacterial viruses that are diverse, ubiquitous, abundant, and specific to hosts.
• Lytic infection (cell lysis) involves phages rapidly replicating and lysing (disintegrating) the host cell and is favored in biopreservation.
• Lysogenic infection (dormant) involves phage DNA being incorporated into the host genome and remaining there, replicating with the host cell itself, and passed on to multiple generations.
• Bacteriophages do not increase the growth of bacteria; instead, they are viruses that infect bacteria and decrease bacterial growth by killing or inhibiting them.
• Phages have been used in the fermentation industry for spoilage, specifically predation of starter-cultures, causing delays in the fermentation process.
• Phages have also been used in the fermentation industry of biopreservation, against Lactobacillus brevis, a fermenting bacteria that causes beer spoilage.
• Phages can be found on food and food processing facility contact surfaces.
• Limitations and challenges of using bacteriophages include large-scale production and purification, bacterial endotoxins (e.g. lipopolysaccharide or LPS), formulation and delivery, and environmental stability.
• Other limiting factors of using bacteriophages include specificity to host range, anti-phage defense mechanisms (mutation, CRISPR), and market and regulatory aspects.

Physical Preservation

• D-value stands for Decimal reduction time/ Thermal Death time.
• It is the time (usually in minutes) in a given temperature required for a 90% reduction of living cells (1 log).
• Z-value is the temperature increment needed for shortening D by a factor of 10.
• Strategies to control spoilage in foods include preventing initial contamination, preventing or slowing down microbial growth, and killing specific spoilage organisms.
• The most important extrinsic parameter is temperature (-2°C to 16°C).
• Under refrigeration temperatures, psychrotrophic bacteria may still grow such as Pseudomonads, Yersinia enterocolitica, vibrio parahaemolyticus, listeria monocytogenes, aeromonas hydrophilia, and salmonella enterica.
• Freezing does not kill pathogenic microorganisms; it merely slows their growth and activity.
• Refreezing thawed food products should be avoided to prevent bacterial growth and ensure food safety.
• Thawed food still contains bacteria that can multiply at room temperature.
• Refreezing thawed food products will increase its microbial count and only make it less safe to eat the next time it is defrosted.
• Lower temperatures induce an increase in unsaturated fatty acids in the bacterial membrane, preventing membrane crystals and maintaining fluidity in the cell.
• Freezing prevents the growth of microorganisms but does not kill them.
• How freezing works depends on rate, medium, and temperature.
• Cryoprotectives for cultures such as glycerin, saccharose, gelatin, and proteins.
• Freeze-drying involves freezing followed by vacuum sublimation of ice, gently removes water, achieves a very low Aw (which depletes microorganisms of nutrients), can be coupled with infrared drying, and can be used for microbial preservation in cultures as well.
• MAP packaging is obtained by replacing O2 with CO2 and N2.
• It is antimicrobial against aerobes and favors anaerobes (clostridium, LABs, yeast).
• High Hydrostatic Pressure (HHP) is also known as cold pasteurization.
• It involves uniform high hydrostatic pressure in a sealed vessel, which is influenced by salt, pH, and temperature.
• The general process of HHP involves unprocessed batch goes into a vessel, product loading, vessel pre-filling, pressurizing (high pressure), product unloading, and processed batch.
• HHP limits microbial growth and autolysis by inactivating enzymes.
• It may have physical impacts on certain foods like color, opaqueness, and shell opening in oysters.
• HHP has no negative impact on flavor and no loss of nutrients.
• Hot air drying is preservation under low humidity, low heat, and air flow.
• Water activity of dried fruits and powdered products = 0.2-0.6.
• Dried products are stable if relative humidity is < 70%.
• Bacteria can persist after food products undergo hot air drying.
• Infrared radiation drying involves energy converted to heat through increased vibrational energy in food molecules.
• It provides uniform heating of materials, ease of temperature control, maintains product quality, and has low energy cost.
• It is mostly used on high moisture foods.
• Ultraviolet (UV) radiation is a non-thermal disinfectant.
• It damages microbial DNA by creating thymine dimers.
• Gram-negative microbes are the most easily killed by UV radiation, while spores and fungi are more resistant.
• Viruses are more resistant than bacteria to UV radiation.
• UV radiation is mainly used for air/liquid disinfection.
• A limitation of UV radiation is its low and uneven exposure.
• Ionizing radiation includes gamma rays and X-rays.
• It inhibits DNA synthesis by DNA break.
• 0.5kGy inactivates vegetative cells
• 3.5kGy inactivates endospores
• Food irradiation is not commonly used because it poses a greater health risk to humans, this is false.
• Microbial adaptations to treatments include heat/cold/osmotic shock proteins.
• Spores and viruses are usually resistant.
• the Stationary phase is more resistant.
• Salt/pH/nutrients: food medium is important.
• VBNC is induced in Salmonella by high salt.

Yeasts and Fermentation

• Fermentation is the anaerobic breakdown of sugars by microorganisms (bacteria and yeast) to produce energy, and results in CO2 and either alcohol and/or acid.
• Glycolysis produces pyruvate.
• NAD (nicotinamide adenine dinucleotide) is an important coenzyme in many biological oxidation reactions.
• Louis Pasteur demonstrated the role of yeasts in fermentation.
• Benefits of fermentation include reducing pathogens and spoilage bacteria, destroying toxins, increasing shelf life, developing flavors and textures, improving digestibility (milk, legumes), creating new products from microbial metabolites (lactic acid, alcohol, acetic acid, etc), adding dietary value (vitamins and nutrients), and creating probiotic microflora.
• Intrinsic and extrinsic factors of fermentation include temperature, ingredients, and natural microflora.
• Factors that control fermentation include starter cultures, pre-growth conditions, fermentation conditions, and ingredients.
• End products determine application from Wine, beer, and cider (= ALCOHOLIC FERMENTATION) to bread, coffee, chocolate, soy sauce, and fermented dairy/meat/veggies (= NON-ALCOHOLIC FERMENTATION) and biofuels (= other applications)
• Microorganisms involved in fermented foods include lactic acid bacteria (LAB) such as Lactobacillus, Streptococcus, Pediococcus, Leuconostoc, Lactococcus and yeast.
• *Saccharomyces cerevisiae, Aspergillus, Penicillium, Propionibacterium, and Brevibacterium are microorganisms involved in fermented foods
• Yeasts are eukaryotic organisms in the kingdom fungi and genus Saccharomyces.
• Yeasts form colonies on solid medium.
• Yeast cells are 150-200 nm and are tolerant to acidic conditions (pH 3.5).
• Yeasts reproduce with budding (asexual (more common)) and sporulation (sexual) reproduction.
• Saccharomyces is a type of YEAST (not bacteria) used to ferment food.
• Yeasts are tolerant to acidic conditions (pH 3.5).
• This is significant/necessary because fermentation processes occur in acidic environments. If yeast could not tolerate acidic environments, they wouldn't be able to continue functioning and producing fermented products, leading to an uncomplete process.
• S. cerevisiae is a facultative anaerobe.
• Food products mostly associated with spoilage from yeasts include fruits, juices, purees, and concentrates.
• In yeasts, glucose turns into pyruvate, which turns into 2Co2 + 2 Ethanol + Energy
• The accumulation of ethanol stops fermentation.
• Byproducts are 2CO2 and Energy.
• Conditions required for it to occur = anaerobic conditions.
• Fructose is utilized slower than glucose.
• This is because glucose is a functional monosaccharide that is the primary input for glycolysis, while fructose needs to be converted into a usable form before it can contribute to glycolysis.
• Glycolysis is a set of reactions that turns glucose into pyruvate, before forming Ethanol, CO2, and energy.
• The Pasteur effect is what happens when an organism switches to a respiratory mode in the presence of oxygen and allows for higher rates of cell multiplication, while The Crabtree effect is when Acceleration of glycolysis/fermentation leads to the depletion of carbon reserves, and the accumulation of potentially.
• The Pasteur effect happens when there an organism switches to a respiratory mode in the presence of oxygen and Allows for higher rates of cell multiplication while The Crabtree effect is induced by lack of oxygen + excess of sugar, Aerobic production of ethanol happens by yeasts, allowing yeasts to grow at a high rate, producing ATP through an inefficient fermentation pathway
• Lactic acid bacteria (LAB) produce lactic acid from lactose.
• LABs are generally facultative anaerobes.
• Homolactic process are preferred for things requiring high lactic acid yield.
• Heterofermentative is preferred for things that benefit from additional byproducts.
• Homofermentative LAB differs from heterofermentative by Glucose yielding lactate.
• Mostly, in diary products, there is rapid development of lactic acid and reduced pH
• For Heterofermentative LAB Glucose yields lactate, CO2, ethanol, and other volatiles and is Rarely used on dairy.
• Conditions used to monitor fermentation include pH, CO2, biogenic amines, and nitrate reduction.
• Prebiotics are food for bacteria, which prebiotic substances that can only be metabolized by the gut bacteria, and does not affect the human host.
• Synbiotics are both pro and prebiotics.
• Probiotics are live bacteria and active bacterial cultures.
• Food products that serve as carriers for probiotics include fermented foods and cultured dairy products (contain up to 10^9 per mL more than other products).