Virtually all foods are derived from living cells of animal, plant, or, in some cases, microbial origin through biotechnological methods. Thus, foods are primarily composed of “edible biochemicals.” One of the most critical goals of food scientists is to ensure food safety, whether foods are consumed fresh or processed.
The judicious application of food processing, storage, and preservation methods helps prevent outbreaks of foodborne illness, defined as diseases or illnesses resulting from the consumption of contaminated food.
The processed food industry maintains an outstanding record in preventing such cases, considering that billions of cans, jars, packets, and pouches of processed and fresh food products are consumed annually. Occasionally, however, this excellent record is marred by limited outbreaks where individuals succumb to the effects of toxic foods.
Fundamentals of Food Preservation
Food preservation is an action or method designed to maintain foods at a desired level of quality. A number of new preservation techniques are being developed to meet current demands for economic preservation and consumer satisfaction in safety, nutritional, and sensory aspects (Potter and Hotchkiss, 1995).
Food preservation is necessary because foods are perishable or deteriorative by nature. Based on their mode of action, major food preservation techniques can be categorized as follows: slowing down or inhibiting chemical deterioration and microbial growth; directly inactivating bacteria, yeasts, molds, and enzymes; and avoiding recontamination before and after processing.
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Overview of Food Processing
Food processing is a segment of the manufacturing industry that transforms animal, plant, and marine materials into intermediate or finished value-added food products that are safer to consume. The aims of food processing, as outlined by Fellows (2009), are four-fold: (1) extending the period during which food remains wholesome (microbial and biochemical), (2) providing or supplementing nutrients required for health, (3) offering variety and convenience in diets, and (4) adding value.
The scope of food processing is broad, encompassing unit operations occurring after the harvest of raw materials until they are processed into food products, packaged, and shipped for retail. Typical processing operations include raw material handling, ingredient formulation, heating and cooling, cooking, freezing, shaping, and packaging.
These operations can be broadly categorized into primary and secondary processing. Primary processing involves post-harvest or post-slaughter activities to make food ready for consumption or use in other food products.
It ensures foods are easily transported and prepared for sale, consumption, or further processing (e.g., peeling and slicing an apple for fresh consumption or baking into a pie).
Secondary processing transforms primary-processed food or ingredients into other food products, ensuring versatility, extended shelf life, health, wholesomeness, and year-round availability (e.g., seasonal foods).
Need for Food Preservation
The preservation, processing, and storage of food are vital for a continuous supply during both seasons and off-seasons. A key distinction of agricultural processes from other industrial processes is their seasonal nature.
The main reasons for food processing and preservation include overcoming seasonal production in agriculture, producing value-added products, and providing variety in diets. People enjoy a wide variety of foods with diverse tastes, flavors, nutritional, dietary, and other characteristics.
Unfortunately, it is estimated that as many as 2 billion people lack sufficient food, and approximately 40,000 die daily from diseases related to inadequate diets, including insufficient food, protein, or specific nutrients.
In extreme cases, inadequate nutrition in children can lead to an advanced state of protein deficiency known as kwashiorkor or more widespread protein malnutrition.
Major causes of food deterioration are environmental factors such as temperature, humidity, oxygen, and light, which trigger reaction mechanisms that may render food either rejected by or harmful to consumers. Microbial effects are the leading cause of food deterioration and spoilage.
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Conventional Food Preservation Methods
1. Preservation by Heat Treatment
Heat is the most commonly used method of food preservation. Various degrees of preservation by heating determine the type of final product manufactured, with terms such as pasteurization and sterilization used to describe these processes.
To be effective, these processes must be conducted under strict temperature and time controls to ensure the elimination of pathogenic and non-pathogenic microorganisms. These same factors also cause thermal inactivation of food enzymes and some destruction of food constituents.
i. Heat Resistance of Microorganisms: Heat resistance is a fundamental topic in thermobacteriology, a critical aspect of food microbiology. The most heat-resistant pathogen found in foods, particularly those canned and held under anaerobic conditions, is Clostridium botulinum.
This spore-forming, proteolytic anaerobe can produce one of the most harmful known toxins, with amounts as small as 10⁻⁶ to 10⁻⁸ g capable of being lethal to humans.
However, non-pathogenic, spore-forming food spoilage bacteria, such as Clostridium sporogenes and Bacillus stearothermophilus, are more heat-resistant than C. botulinum spores. Thus, a heat treatment that inactivates these spoilage microorganisms will also eliminate C. botulinum and other pathogens.
ii. Kinetics of Heat Destruction of Microorganisms: The thermal death time is the duration of heating required to kill all vegetative cells of microorganisms, though theoretically, complete elimination is not possible; this term is used for practical purposes in thermobacteriology.
Microorganisms are killed by heat at a rate nearly proportional to the number of cells (expressed logarithmically) present in the system (food, laboratory nutritive medium, water, etc.) being heated, referred to as a logarithmic order of death.
A typical thermal death rate curve, as shown in Figure 1, provides data on the rate of destruction of specific microorganisms in specific media or food at a constant temperature.
2. Preservation by Low Water Activity (aw)
Water activity can be reduced by partial water removal (e.g., drying, reverse osmosis, concentration) or by adding substances that increase osmotic pressure, such as sugars, ethanol, glycerol, or salts.
Most microorganisms are sensitive to the water status in their immediate environment and remain metabolically active only within a narrow range of high water activities. Some organisms tolerate low water activity and high osmotic pressures.
The most water-activity-tolerant species can grow at osmotic pressures as high as 800 MPa and slowly below an aw of 0.62. The nature of the solute also affects growth potential, with ionic solutes like NaCl and KCl being more inhibitory than non-ionic solutes like sugars. Glycerol, unlike salts and sugars, rapidly permeates most bacteria but not yeasts, such as Saccharomyces rouxii and Debaryomyces hansenii.
For less water-activity-tolerant species, such as Staphylococcus aureus, this relationship is less straightforward. S. aureus is extremely salt-tolerant but more sensitive at higher water activity in glycerol than in sodium chloride.
Lowering water activity may also influence the rate of enzymatic and chemical changes in foods. While microbiological growth is completely halted below an aw of approximately 0.6, some enzymatic reactions causing spoilage continue, and reactions like lipid oxidation may accelerate at very low water activity values.
3. Preservation by Low pH and Organic Acids
i. Preservation by Low pH: Foods are classified by acidity as follows: non-acid (pH 7.0–5.3), low or medium acid (pH 5.3–4.6), acid I (pH 4.6–3.7), and acid II (pH 3.7 and lower). Microorganisms have characteristic pH ranges for growth.
Most bacteria have an optimum pH near 6.8 and may grow between pH 4.0 and 8.0, though some species can multiply at pH below 4.0 or above 8.0. Yeasts and molds can sometimes grow at pH values below 2.0. Generally, the growth rate decreases as pH drops below the optimum value.
ii. Preservation by Organic Acids: Some organic acids and their esters occur naturally in foods or result from microbial metabolism in fermented foods. Many foods are preserved by adding low concentrations of these compounds, which exhibit marked pH-dependent activity as preservatives. These compounds are primarily effective against yeasts and molds at low concentrations, but bacteria are also affected.
Lowering the pH increases the proportion of undissociated acid molecules, enhancing the antimicrobial effectiveness of organic acids. It is generally assumed that the antimicrobial activity of these acids is directly related to the concentration of their undissociated molecules.
Organic acids and esters used in food preservation include:
i. Acetic Acid: Has limited action as a preservative, primarily linked to its pH-reducing capacity.
ii. Propionic Acid: Sodium and calcium salts are used mainly against molds in cheese and bakery products at effective concentrations of 440 to 850 mM.
iii. Lactic Acid: Less effective than other organic acids, it is an excellent inhibitor of spore-forming bacteria at pH 5.0 but ineffective against yeasts and molds.
iv. Sorbic Acid: Used as such or as sodium, potassium, or calcium salts, most commonly as the potassium salt. It is more effective against molds and yeasts (25–500 ppm) than bacteria (50–10,000 ppm).
v. Benzoic Acid: Used as such or as its sodium salt, effective against yeasts (20–7,000 ppm), molds (20–10,000 ppm), and bacteria (50–1,800 ppm), with bacteria showing variable sensitivity.
4. Preservation by Carbon Dioxide, Sulphite, Nitrite, and Nitrate
i. Carbon dioxide (CO2): CO2 plays a significant role in modifying microbial growth. Modified atmospheres enriched with CO2, exceeding the normal air concentration of 0.03%, are a widespread natural means of extending the shelf life of non-sterile refrigerated foods.
Concentrations above 5% are particularly effective against psychrotrophic microorganisms that cause spoilage of chilled foods. Significant preservative effects have been demonstrated with fresh fermented meats, fish, fruits, and milk. The mechanisms of CO2 inhibition are not fully understood, but the most likely mode is the inhibition of decarboxylation reactions in living cells.
ii. Sulphur Dioxide (SO2): Sulphur dioxide, sulphite ([SO3]²⁻), bisulphite ([HSO3]⁻), and metabisulphite ([S2O4]²⁻) are used as preservatives in wine, fruit juices, sausages, and other foods. As antioxidants, they inhibit enzyme-catalyzed reactions, notably enzymatic and non-enzymatic browning.
Bisulphite accumulates in yeast at concentrations 50-fold greater at pH 3.6 than at higher pH, with the bisulphite ion showing greater inhibitory activity against bacteria and fungi than the sulphite ion.
iii. Nitrite and Nitrate: Nitrite and nitrate, as sodium and potassium salts, are widely used in the fermentation of meat products and the curing of pork for ham and bacon production. Originally added with sodium chloride, these compounds stabilize red meat color and inhibit the growth of pathogenic and spoilage microorganisms.
Many bacteria reduce nitrate to nitrite, which helps prevent microbial spoilage. The antibacterial effectiveness of nitrite increases as pH decreases. Nitrite inhibits Clostridium botulinum, reducing the risk in cured meats, and also helps prevent rancidity.
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