Introduction

We recently looked at preservation through cold storage, and now we’re going to delve into two techniques of food preservation using heat.

Preservation through the effect of heat

The application of heat is a method based on the use of high temperatures to kill bacteria and other microorganisms.

This is a very ancient technique when it comes to food drying. However, it wasn’t used as a method involving heat until Pasteur developed it to reduce the risk of microbial growth.

Principle

There is a decrease in the rate of chemical reactions due to the inactivation of enzymes, in a process known as denaturation.

Microorganisms are usually inhibited or destroyed.

Currently, companies use two main methods of applying heat as a means of food preservation. These are:

• Sterilisation
• Pasteurisation
• Smoking*
• Dehydration*

(* We will look at these in depth in another article)

Sterilisation is based on the destruction of bacteria and other microorganisms at high temperatures (> 100 ºC), and the food is generally packaged before the heat process is applied. This is usually carried out using an autoclave, similar to a pressure cooker.

Once opened, the container must be treated as fresh product.

A disadvantage is that it may cause physical and chemical changes in certain foods.

The number of microorganisms varies depending on the temperature and the duration of the treatment.

Sterilisation is used in canned products such as:

• Fish, to which liquids like marinades and fats are added.
• Vegetables and fruits where anti-fermenting agents and antioxidants are used, which act as bleaching and firming agents (such as metabisulphite and lime), green colourings (copper sulphate), red dyes (erythrosine), sugar (for fruits).

For storage, they must be kept in a cool place out of direct sunlight.

Poor sterilisation can result in transformations or alterations in the food, such as swelling of the container, bad odours, whistling sound when opened, changes in the added liquid, discolouration, and spoilage of the food.

Ultra-high temperature (UHT) treatment is a more modern sterilisation technique where high temperatures (140 ºC) are applied for 2 seconds. The food is sterilised, and the nutritional loss is usually less than with traditional sterilisation. No changes in colour or flavour occur. Once packaged, refrigeration is not necessary, but once opened, the product must be stored in the fridge (0–5 ºC) for a limited time.

In pasteurisation, temperatures not exceeding 90 ºC are applied for varying periods of time. This reduces the microbial load but does not destroy spores and inactivates enzymatic systems.

The downside of this technique is that pasteurised products have a limited shelf life and must always be kept refrigerated.

The advantages of this method are strongly related to the organoleptic qualities of the food, as it causes minimal changes in flavour, texture, and nutritional quality.

It is a technique very similar to blanching. This is a gentle heat treatment applied to vegetables and fruits before freezing. Its purpose is to inactivate enzymes that could cause food spoilage during frozen storage.

Have you realised you do this at home…?

Preservation using a water bath

This technique has always been used at home to preserve food. The food reaches temperatures of 70–90 ºC. Jars containing the food are hermetically sealed and submerged in a large pot with water covering the jars. They are boiled for 20 minutes to an hour, depending on the food being preserved. They are then cooled and stored in a cool, dark place until consumption.

There are microorganisms that thrive best between 40–70 ºC, so the cooking time is important.

This preservation method can keep food for several months.

It is important to note that home canning carries the risk of developing

Clostridium botulinum, so it is absolutely essential to follow strict hygiene measures during preparation.