Methods and Effects of Food Preservation Techniques

Posted on Dec 9, 2024 in Geology

Food Preservation by Moisture Control

Dehydration, Drying, and Concentration of Food: Water is removed under natural conditions in the field, in cooking and baking food, and controlled processes of dehydration. Aimed to increase the shelf life of foods, reduce the weight and volume, reduce costs of transport, packaging, and storage, and to produce products with convenience of use.

Physical Principles Involved: Transmission of heat and removal of water are facilitated by increasing the surface of food, temperature, air speed, and lowering humidity and pressure.

Dehydration: Almost complete elimination of water at the cost of equipment. Examples include milk powder, instant coffee, and corn flakes.

Methods of Dehydration

1. Dehydration with Hot Air

Dryer or Tray Dryers: It is a camera that receives the trays, and the air is driven by a fan that goes through a heating system.
Tunnel Dryers: They are widely used in the dehydration of fruits and vegetables. Tunnels are 10 to 15 meters in length. They receive inside the mats with the raw material. The current of air can be natural or forced.
Atomization (Spray-Dryer): Consists of a cylindrical chamber where the fluid to be dried is introduced by spraying under pressure, resulting in droplets that receive a stream of heated air toward or against, drying up almost instantly. Drying is by spraying. It is used for liquids or pastes, which are transformed into products (e.g., milk powder, instant coffee). The spray operation is based on four phases:

Atomization of the liquid: Made by high-pressure pumps.
Atomized liquid contact with the hot air.
Evaporation of water: Made in the drying chamber (180-230°C).
Separation of the powder drying air: Occurs in cyclones.

Fluidized Bed Dryer: It is based on continuous drying where the material is placed in a perforated belt and receives hot air from the bottom, getting a drive similar to a boiling liquid.

Applications: Food and small particles, which can be fluidized without mechanical damage.

2. Dehydration by Direct Contact (Dryers and Rollers)

It is a dryer known by the name of dryer roll. Pasty food comes into contact with the hot surface, according to the number of drums. They allow high throughput and are economical. Moisture content is less than 5% and is useful for food that is heat resistant.

3. Freeze Drying

Also called cryodesiccation, it is a process of dehydration of products under pressure and temperature such that water, previously frozen, as is done in the absence of air, the chemical and organoleptic characteristics practically do not change. The first product was lyophilized rabies (1911).

Manufacture of Crystal Sugar

Purification: In this stage, the juice is heated and sent to the tabs (the process of separating), which absorb impurities, resulting in a clear broth.

Evaporation and Crystallization: This broth is subjected to an evaporation process, turning into syrup. The syrup is sent to another stage of concentration and cooking that results in a mixture of crystals with honey.

Centrifuging and Drying: The crystals extracted are sent to centrifuges that separate the honey. However, after separating, the sugar crystals still have a high level of humidity, which can only be eliminated with the action of dryers (hot air). Thus, sugar is produced.

Dissolution of Crystal Sugar and Purification: The first stage of manufacture of refined sugar is dissolved in water in the sugar crystal. This solution is subjected to a new purification process, generating a syrup.

Evaporation and Solidification: A spray generated in the purification process is heated to a set point. Then it is transferred to blenders, which makes it a mass of warm and moist sugar. At this stage, the crystals have a definite shape.

Drying and Cooling: To dry and cool, the sugar is sent to the dryers with the passage of hot and cold air.

Screening: The sugar is sifted to separate the clusters and to obtain uniform crystals. The thinnest part is extracted from sugar icing, and the rest, you get the sugar.

Effects of Dehydration on Food

Texture: The texture of loss of rehydrated foods (fruits and vegetables) is due to gelatinization of starch, crystallization of cellulose, and disruption of the cells, giving the appearance of food wrinkled and shrunken. There may be meat in the denaturation proteins, causing loss of ability to retain water, which leads to its hardening and solute concentration on the surface. The texture of the post is related to its bulk density and ease of rehydration.

Taste and Aroma: The heat vaporizes the water and also volatile. Dehydrated foods have less flavor than the original. The degree of loss depends on temperature, moisture content of food, vapor pressure, and solubility in water vapor of the compounds. Herbs and spices are dried at low temperatures. Food with a porous structure (open) allows access to O2 and also causes the oxidation of compounds and lipids during storage.

Color: In fruits and vegetables, the changes in the pigments chlorophyll and carotene are caused by heat and oxidation during drying and residual activity of PPO in storage.

Natural Drying

It is made naturally and has the advantage of being economical, plus dehydration and concentration. It is held in dry climates (Mediterranean, Norway, Canada, California, Northeast Brazil). Food can be placed on a shelf or hung on clotheslines. Operation rate (up to 10 days), drying in two stages: in the sun until the foods have lost 50 to 70% water, then in the shade so the products are not drying out and do not lose their natural flavor and aroma.

Food Concentration by Evaporation

Methods of dehydration. Form of preservation and can be used prior to dehydration. Reduces the water content (from 1/3 to 2/3). Economics of transportation, packaging, storage, etc. Some foods are preferred concentrated. It uses the evaporation process. Need other methods of conservation.

Receiving and Storage: The reception, due to the large amount of fruit, should have hydraulic lifts for unloading the trucks. The control operates by automatic weighing scales, weighing vehicles with and without fruit.

Cleaning: In the first stage, they are sprayed with water and a detergent solution and bactericide (chlorine), whose action is enhanced by the use of brushes. In the second round, fruits in all positions are rinsed with chlorinated water (some companies make use of chlorinated hot water), which removes the previous solution and drags the dirt while decreasing the microorganism contamination of the surface that could contaminate juice in the extraction step.

Selection: This step eliminates oranges that are cracked or bruised (contaminated by microorganisms which interfere with proper conservation). They also interfere with the extraction since the equipment is set up for a whole fruit. This selection is visual and manual, made by trained personnel.

Clarification: The newly extracted juice, regardless of the essential oil, is separated from the fragments of flesh or peel suspended by the work of a centrifuge. This operation does not eliminate the turbidity characteristic of citrus juice, removing only the coarsest suspension by a special centrifuge, continuous and automatic cleaning. It is also possible to use enzyme clarifiers, but for the concentrated juice, that is not necessary since the goal is not to clarify it but only to remove impurities.

Pasteurization: Pasteurization is to eliminate most of the micro-flora and inactivate the pectinesterase enzyme that destroys the stability of the turbidity, and the liquid is becoming clear in the upper portion, changing its natural appearance. It is made at a temperature close to 92°C for 45 seconds.

Cooling/Freezing: Juice concentrate is kept in motion for uniformity of temperature and composition. The cold, concentrated, and uniform composition is then passed on two heat exchangers, plate, on two stages, to cool the juice concentrate from 25°C to 7°C, and afterwards, to -7°C, and then it is packed.

Storage: The concentrate, packed in suitable containers, is stored at a cold temperature of -28°C.

Food Preservation by Cold

Cold Storage

Refrigeration is the reduction and maintenance of food temperature above its freezing point. Temperatures are usually between -8 and 1°C. It involves only changes of heat-sensitive products. The lower temperature slows the growth of microorganisms. Psychrophiles are what pose the greatest problems. It slows the metabolic activities of animal tissues after slaughter and vegetables after harvest, chemical and enzymatic reactions, and moisture loss. It has few adverse effects on flavor, texture, nutritional value, and color. The preservation time is short (days or weeks).

Factors to be Controlled During Storage at Refrigeration

Temperature: Every food has its optimum temperature. The temperature should remain stable throughout the storage. The oscillation in temperature should not exceed ±1°C of the recommended value. The frequency of door opening should be minimal.
Relative Humidity: High relative humidity causes water condensation on cold surfaces of the food and favors the growth of cracks in microorganisms and some fruits. Low relative humidity causes moisture loss of food, dehydration occurs, and their weight is reduced.
Purification and Air Circulation: Help heat distribution and composition of air. The air must be renewed to eliminate odors and purify the air to remove aromatic compounds resulting from a few foods. High flow causes dehydration.
Controlled Atmosphere Using Gas Mixture (composition, etc.): Used for storing large amounts of product (e.g., apples and pears).
Light: The chamber of refrigeration should be kept in the dark. Sometimes, light UV lamps are used to reduce the top growth of molds and bacteria.

Frozen Storage

An efficient method to preserve meats, vegetables, and fish, keeping them for longer.

Slow Freezing: It takes more than 3 hours to freeze the product at temperatures from -25°C, without air circulation. The first ice crystals are formed in the intercellular spaces, forcing the migration of water from the cell to the intercellular spaces, increasing the crystal size and causing disruption of some cell walls. Thawing: The food loses water and becomes limp, losing salts, water-soluble vitamins, and proteins.

Rapid Freezing: It takes less than 3 hours for freezing at temperatures from -25°C, with air circulation, or -40°C, with or without air circulation. Air circulation speeds up the exchange of heat. Water does not migrate; it freezes where you are. It is building a larger number of ice crystals distributed in order to cause the least damage to the cells, avoiding the disruption of membranes. Thawing should be slow for the food to reabsorb the liquid from the thawing salts, proteins, sugars, etc.

Types of Freezers

Air Freezers (Refrigerated Air Static): Heat transfer is low. Freezing speed is very slow (loss of product quality).

Freezers by Direct Contact: The food is distributed on parallel plates, and hollow inside circulates the coolant. Gentle pressure can be applied to improve food contact with cold surfaces of the plates (improves the coefficient of heat transfer and rate of freezing).

Frozen by Immersion: The food is transported by a mat for the watershed that flows countercurrent. Advantage: close contact of the food or packaging with the refrigerant, the fast processing of irregular parts, and maintenance of quality in foods susceptible to oxidation (avoid contact with air). Current use: freeze fish on board and citrus juices, concentrates, and packed.

Cryogenic Freezing:

The food is sprayed with liquid nitrogen, dramatically reducing the temperature of the surface quickly.
Then move on to another section to stabilize the temperature.
The last step is vitrification, which sprays water and takes advantage of the residual cold food. The final temperature is -18°C, and the surface of the food is protected from dehydration.

Frozen Food Packaging

Avoid dehydration during freezing by air or other systems. Avoid sunburn, which changes the color, texture, flavor, and nutritional value of foods. Preventing rust occurs and contamination of the atmosphere inside the chamber. Wood, metal, glass, and plastic are used successfully.

Food Additives

These are substances that are added to foods in order to maintain or change their flavor or improve their appearance. They are essential for us to eat foods that can be developed and maintained long-term. They maintain product consistency, improve nutritional value, keep the flavor, increase tenderness and pH, and improve the flavor and color.

They are classified by their origin and may be natural, obtained directly from raw materials, or artificial, produced synthetically. Sorted by occurrence, they may be intentional when the products are placed on purpose or incidental when they are appearing accidentally in food.

Types of Additives

Fatty: They make the flavors alive and serve as a preservative and antioxidant.
Acidity Regulators: They are used to modify or maintain the pH of foods.
Anti-Caking, Anti-Wetting: Avoid the agglomeration of particles of powdered.
Anti-Foaming Agents: Reduce or inhibit foaming in foods.
Antioxidants: Prevent food spoilage by oxidative mechanisms.
Bulking Agents: Incorporated to increase the volume of food without changing their characteristics.
Colors: Added to replace colors lost during preparation or to make food more attractive in order to change its color.
Fasteners Color: Preserve the original color of food.
Emulsifiers: Make the water and oil stay mixed.
Flavoring: Give particular flavors or aromas. May have artificial or natural.
Flavor Enhancers: Enhance the original flavor of food.
Moistening: Prevent food from drying.
Sweeteners: Added to reduce the energy provided by food and have a beneficial effect in cases of diseases.

Food Irradiation

Electromagnetic Radiation: The energy spread in space.

Irradiation: Direct exposure of a material to a radioactive source.

Ionizing Radiation: Radiation capable of converting atoms and molecules into ions. Carry enough energy to eject electrons from molecules.

Irradiation is an efficient technique in food preservation: eliminate or reduce the microbial population, inhibition of sprouting in bulbs and tubers, delayed ripening of fruits and vegetables, elimination of parasites, and disinfestation of grains, cereals, spices, and fruits.

Features in the Process of Irradiation in Food

Elevated temperature is virtually non-existent. Penetration through the packaging does not require post-treatment handling. It is very fast, not polluting the environment, economic, and leaves no residue.

Deleterious Effects on the Food

Odor, color, and flavor change adversely. Undesirable texture changes, loss of functional properties desired, reducing the content of some vitamins, production of radiolytic products, some of which may be toxic, water content, dosage, conditions, packaging, temperature, and atmosphere.

Effect of Ionizing Radiation in the Body

Effect on Microorganisms: Alteration of cell membrane effect on the enzyme system, effect on the chain of DNA and RNA, and metabolism change.
Resistance of Organisms: The effect is inversely proportional to the size of the molecule.
Dose of Inactivation.
Factors Affecting Radiosensitivity of Microorganisms: Composition of the medium as the content of the water components of the food itself, pH, and atmospheric composition.