Print bookPrint book

Methods of food preservation

Site: Plattform für Weiterbildung und Internationalisierung der Hochschule Weihenstephan-Triesdorf
Course: Entrepreneurship in Food
Book: Methods of food preservation
Printed by: Gast
Date: Monday, 30 March 2026, 12:28 PM

Description

Table of contents

1. Introduction

All methods of food preservation use the principle of modifying the atmosphere in such a way that 

a) microbes are either removed or killed, or can no longer multiply and spoil the food,

b) enzyme activity is controlled, and

c) oxidation and UV-light damage is prevented.

These methods are generally based on temperature control, pH control, moisture control, removal of air, or the use of chemical additives.


2. Refrigeration, freezing and freeze drying

This involves cooling food to low temperatures to slow down enzyme activity as well as the growth of microorganisms.

Refrigeration is cooling the environment artificially to bring the temperature below ambient temperature, usually ranging from 4 to 10°C. Foods commonly stored using cooling methods are usually highly perishable products, like meat, dairy, and fish, thus increasing their shelf-life.

Freezing means cooling food to temperatures of -18°C for longer storage periods. At temperatures below 0°C the water inside the food forms ice crystals, which means some foods cannot be frozen, because the melting ice will either cause structural breakdown of the food (like lettuce, cucumbers, soft herbs, dairy products, etc.) or be soaked up, leaving the food soogy, like crumb based cakes, etc.

Food type

Recommended freezing time frames (if your freezer runs at -18oC)

Uncooked meat  (roasts, steaks, chops etc)

4-12 months

Uncooked mince meat

3-4 months

Cooked meat

2-3 months

Cured meats (ham & bacon )

1-2 months

Uncooked poultry (chicken, turkey etc)

9-12 months

Cooked poultry

4 months

Fish-raw & cooked

2-4 months

Frozen dinners ( e.g. lasagne, pizza, shepherd’s pie)

3-4 months

Soups, stews and casseroles

2-3 months

Milk

Up to 1 month  

Butter

Up to 3 months

Grated cheese

Up to 4 months

Vegetables

Up to 12 months

Fruit

Up to 6 months

Eggs – either separate the yolk and white, or beat the eggs before freezing

Up to 6 months

Cakes and baked goods without icing

6-8 months

Bread

Up to 3 months

Raw pastry

6 months


Freeze drying is also known as lyophilization. During this process, the moisture in the food is removed (sublimated) under frozen and vacuum conditions, which causes a direct transition of water from its solid (ice) to vapor, omitting the liquid state, and then desorbing the water from the “dry” layer. The process is used for heat-sensitive foods.


3. Pasteurization

This involves heating food to a temperature high enough to kill any harmful microorganisms, but low enough to preserve the food’s flavor and texture.

Several methods are common:

Low-Temperature Long-Time (LTLT) pasteurization, also called “batch” pasteurization or “vat” pasteurization, is commonly used in small-scale and artisanal operations, where milk is heated to 63°C for 30 min.

High-temperature short time (HTST) pasteurizationis a continuous process accomplished by heating the product to 71.5°C and passing through a holding tube at a rate that ensures the required holding time of 15 sec.

Low-temperature short-time (LTST) is a method that uses heat lower than the usual pasteurizing temperature and low pressure to pasteurize a fluid milk product by spraying droplets in a chamber heated below the usual pasteurization temperatures, thereby increasing the overall quality of the product.

Ultra-high-temperature (UHT) pasteurization involves heating milk or cream to 138–150 °C for 1 or 2 sec.

4. Sterilization (aseptic processing)

This is a process of heating food to a temperature high enough to completely eliminate all microorganisms (usually food sterilization temperature ranges from 115 to 127 °C, depending on the type of food). This process is used to extend shelf-life for extended periods of time without the need for refrigeration.

Canning, which involves sealing food into cans or jars and heating it to a high temperature to kill any microorganisms and inactivate enzymes, uses sterilization as preservation method. Watch the video on the process of canning below:


Author: Rungarun Sasanatayart, @rungarunsasanatayart7852

5. Drying or dehydration

This technique involves removing water from food, to control microorganism growth. In countries with sufficient sunlight, solar drying is a viable option to preserve access produce. Dried mangoes, pineapples, plantain, zucchini, tomatoes, herbs, etc. are popular products in many markets. It is important to ensure good ventilation to enable drying within a day or maximum 2. See below a schematic for a highly efficient solar drier design:



The animation below takes you through the steps of drying mangoes.
 
 

If you are interested in the process of making sweetened dried mango follow the link to this video.

6. Smoking

To preserve highly perishable foods like fish and meat, they can be smoked. The preserving effect is a result of the drying of the fish or meat during smoking. The smoke particles that are absorbed by the meat or fish also have a slightly preserving effect as they inhibit bacterial growth, but their main role lies in the favourable effect it has on the taste and color of the food. Smoking is done by exposing food to smoke from burning wood or other materials to add flavor and preserve it. This is usually done by hanging or placing the food on racks in a chamber designed to contain and supply the smoke and heat of a natural sawdust fire. Hickory or other hardwood sawdust is the preferred fuel. Softwood sawdust, such as of conifers, spruce and pine contains pitch, which produces a film on the meat and imparts a bitter taste. Generally, smokehouse temperatures vary from 43°C to 71°C, and smoking periods vary from a few hours to several days, depending on the type of food and its moisture content. At the higher temperature rannge of above 65°C the smoked food gets cooked, but usually not dried.
Cold smoking at temperatures below 30°C produce and uncooked food, which needs to be kept cold after smoking and has the same short shelf-life as fresh fish or meat. Smoke-drying usually starts with smoldering of damp wood shavings at a temperature of around 45°C, then slowly raising the temperature to about 85°C (by increasing the oxygen flow), where it is cooked for 2-4 hours. Then, the temperature is reduced again to around 50°C and the food left to dry for several hours.


Fresh meat and fish are often brined before smoking. For this, a brine is prepared by boiling 400 - 450 g salt in 5 ltrs of water per each 10 kg of meat. The fish or pieces of meat are placed in a plastic container and topped with the iced or cooled brine. The pieces of fish or meat should freely float in the brine, not be packed tightly. A heavy stone (or similar) is placed on top of the meat or fish to ensure they stay submerged in the brine. Bayleaves, juniper berries and pepper corns, arlic and even sugar can be added to the brine. Leave the meat in the brine to cure for 4 - 5 days, depending on the thickness of the pieces. Thinner pieces get salty faster. During the entire curing time the meat/fish and brine have to be kept cold.

Traditional smoked foods are sausages, cured pork cuts, like bacon and belly, beef briskets, fish, or cheese. Even speciality smoked tea, spices, and beverages, etc. are traditionally produced.

7. Fermentation

This process involves the conversion of carbohydrates into alcohols and organic acids, which can preserve food for longer. In this process beneficial bacteria are used, which produce lactic or acetic acid, bacteriocins, and hydrogen peroxide, and act as biopreservatives. Many fermented foods are favoured for their health benefits due to the beneficial microbes that develop in lactic acid-fermented vegetables. However, in order to keep these microorganisms alive, these foods cannot be sterilized and need to be refrigerated. Typical examples are Sauerkraut and kimchi.

8. Pickling

This is the process of preserving food by packing it in an acidic brine or vinegar. Usually white types of vinegar are used to preserves a pleasing color of the pickled vegetables. To inhibit bacteria growth, the pH should be at least as low as 3.5, which means the vinegar has to have a concentration of at least 4%. As acids are corrosive, stainless steal utensils have to be used. The vinegar is usually mixed with sugar and sometimes herbs & spices according to the recipe. Pickled foods have a different taste and texture than the raw ingredient and are often eaten as condiments.

Three different methods of pickling are generally used:

  1. Vegetables are cleaned and cut into shape, then placed in a cold brine of 200g salt per liter of water, for several hours. They are next blanched in saltwater and then transferred into jars (without the blanching liquid). The jars are then filled with the hot vinegar solution so that all vegetables are covered, but about 1cm headspace remains below the rim of the jar. The jars are closed immediately.
  2. Vegetables are cleaned and cut into shape and raw packed into jars. The vinegar solution with added salt and sugar is poured over the vegetables until they are completely covered, leaving 1cm headspace. The jars are closed and sterilized in a boiling water bath or autoclave.
  3. The vegetables are cleaned and cut into shape and boiled for a few minutes in the vinegar solution that contains also sugar, salt, herbs and spices (as per recipe). The Vegetables together with the boiling hot brine are then filled into jars, and immediately closed.


8.1. Pickling example: Do Chua


9. Vacuum Sealing

This is the process of sealing food in an airtight container to preserve it. Under no-air storage conditions, microbes cannot grow and survive.

During vacuum packing, the foods are placed in a plastic bag, and a vacuum is created inside the bag, by sucking the air out of the bag. 

This method is also used for preserving nuts as it avoids oxidation reactions, such as rancidity, and thereby preserves the flavor.

Find more about vacuum packaging here.


10. Modern methods of food preservation

Irradiation

During this process foods are exposed to β-particles or γ-rays radiation to kill bacteria, molds, pests, and others. Although WHO and FAOn approved this process it must be noted, that the regulations for food irradiation vary greatly from country to country. In many countries of the European Union only dried herbs, spices, and seasonings can be processed with irradiation.

Chemical preservatives

Antimicrobial chemical agents are added for the long-term preservation of foods. These chemical agents are added in small quantities, as larger amounts can be toxic. Acids and benzoates are used to preserve acidic foods, such as jams, salad dressings, juices, pickles, carbonated drinks, and soy sauce. Sorbic acids and sorbates and sulphites are used to preserve cheese, and juices. Nitrates and nitrites are used to preserve meat products. For wine and dried fruits sulfur dioxide and sulfites are used, whereas propionic acid and propionates are used for baked goods.

Pascalization

In this process, foods like juices and meats are exposed to very high pressure ( ~5000kg/cm2) to destroy microbes. This process helps to retain the food’s original flavor, texture, and nutrients.

Hurdle technology

In hurdle technology, more than one approach is used to inactive the microbes. These combined preservation approaches act as multiple hurdles for microorganisms in order to prevent spoilage. Some of the hurdle approaches are high temperature, in combination with pressure, acidity, and adding biopreservatives. The selection of hurdles depends on the food’s nature and potential pathogens.

Nonthermal plasma

In this process, the food surface is exposed to a flame of ionized gas molecules, such as nitrogen or helium to kill the microbes on the food’s surface.

Modified atmosphere

In this process, the oxygen inside an airtight and sealed packaging or storage environment is reduced to 1% or 2%, and carbon dioxide (CO2) and/or nitrogen gas (N2) is increased to prevent the growth of bacteria, insects, molds, and to prevent oxidation reactions.