Pasteurization

Cholera bacteria

The pasteurization or pasteurization is a thermal process that is carried out in liquids (generally food, such as milk) with the intention of reducing the presence of agents pathogens (such as certain bacteria, protozoa, molds, yeasts, etc.) that they may contain. Due to high temperatures (80 degrees) the vast majority of bacterial agents die. Process discovered by the French chemical scientist Louis Pasteur, together with Claude Bernard, on April 20, 1864.

One of the reasons for heat treatment is a method of controlling microorganisms in liquid foods, altering as little as possible their physical structure, their chemical components and their organoleptic properties. After the pasteurization operation, the treated products are rapidly cooled and hermetically sealed for food safety purposes; For this reason, knowledge of the mechanism of heat transfer in food is essential in pasteurization. Unlike sterilization, pasteurization does not completely destroy microorganism spores, nor does it kill all thermophilic microorganism cells.

Louis Pasteur improved the quality of life by making it possible for basic food products, such as milk, to be transported long distances without being affected by spoilage. In pasteurization, the primary objective is not the "complete elimination of pathogens" but the substantial reduction of their populations, reducing them to levels that do not cause food poisoning in humans (provided that the pasteurized product is kept refrigerated correctly and that it is consumed before the indicated expiration date).

First processes

Portrait of Louis Pasteurby Albert Edelfelt (Musée d'Orsay)

The first processes to sterilize food in closed containers have been historically attributed to the French inventor Nicholas Appert in his research carried out in the XVIII century . However, some research shows that attempts to sterilize food in sealed containers had been attempted before. Toward the end of the century XIX, German chemists transferred this procedure to raw milk, and already at that time (before Pasteur) it began to "suspect" that heat treatments were effective in destroying the bacteria present in milk. In this way, not only an important conservation method was created, but also a fundamental hygiene measure to protect people's health and preserve the quality of food. These works laid the foundations for what Pasteur would later discover and explain scientifically.

Some of Pasteur's contemporaries, including the eminent German chemist Justus von Liebig, insisted that fermentation was a purely chemical process and did not require the intervention of any living organism at all. In 1864, at the behest of Emperor Napoleon III, Pasteur investigated why wine and beer turned sour over time, causing great economic losses to French companies due to the perishability of these merchandise. Pasteur returned to the village of his childhood, Arbois, with the aim of finally solving the problem. There he studied the problem that affected the vineyards. With the help of a microscope, he discovered that two types of organisms were actually involved — a yeast and a bacterium from the acetobacter family — that were the key to the fermentation process. One produced alcohol and the other acetic acid, which soured the wine producing vinegar.

Pasteur used a new method to eliminate microorganisms that could degrade wine or beer: after storing the liquid in well-sealed vats, its temperature was raised to 44 °C for a short period of time. He verified experimentally that the populations of bacteria of the genus Acetobacter were extremely reduced until the food was "almost sterilized". Despite the industry's initial horror at the idea of heating wine, a controlled experiment with batches of heated and unheated wine conclusively demonstrated the effectiveness of the procedure. Subsequently, Charles North successfully applied the same Pasteur method to milk in 1907. Pasteur took the first step in what would be this new method, later called "pasteurization" in his honor, and was applying it to other liquid food. This process is applied today as a hygiene standard in many basic processes in the food industry and effectively guarantees the safety of many food products throughout the world.

The history of food sterilization was reviewed by Harold Burton (1988). Sterilizers were patented and built to heat milk to temperatures ranging from 54.4 °C to 60 °C before the < century span style="font-variant:small-caps;text-transform:lowercase">XIX, curiously before its benefits were fully understood. Sterilized milk was developed industrially in 1921, and the steam injection process was developed in 1927 by G. Grindrod in the United States. However, the most relevant initiatives that led to the commercialization of the UHT method began to take place. develop in the late 1940s, due to the technique developed in concentric tube sterilizers and upgrade steam in milk production systems. It must be understood that the efforts at that time were very great in the industry to achieve aseptic packaging of milk, until it was finally successfully achieved in 1961.

Pasteurization processes

Pasteurization is a chemical thermal process carried out on food: thermal processes can be carried out with the intention of reducing pathogenic populations of microorganisms or to deactivate enzymes that modify the flavors of certain foods. However, in pasteurization temperatures below the boiling point are generally used (in any type of food), since in most cases temperatures above this value irreversibly affect certain physical and chemical characteristics of the food product. Thus, for example, if the boiling point is exceeded in milk, the casein micelles irreversibly "coagulate" (or in other words, the milk "curdles"). The heating process of pasteurization, if done at low temperatures, also has the function of stopping enzymatic processes. Today, pasteurization is carried out on food in a continuous industrial process applied to viscous foods, with the intention of to use energy efficiently and thus also reduce production costs.

There are three distinct types of processes: VAT or slow pasteurization, pasteurization at high temperatures for a short period (HTST, High Ttemperature/Short Ttime) and process at high temperatures (UHT, U b>ltra-Hhigh Ttemperature).

VAT process

From English "vat" = tub, jar, to be made in large containers. Also called slow pasteurization. It was the first pasteurization method, although the food industry has been renewing it with other, more efficient systems. The process consists of heating large quantities of milk in a container at 63 °C for 30 minutes, then allowing it to cool slowly. It takes a long time to continue the packaging process of the product, sometimes more than 24 hours.

HTST process

This method is used in bulk liquids, such as milk, fruit juices, beer, etc. As a general rule, it is the most practical, since it exposes the food to high temperatures for a short period of time and, furthermore, little industrial equipment is needed to carry it out, thus reducing equipment maintenance costs. Among the disadvantages of the process is the need to have highly qualified personnel to carry out this work, which requires strict controls throughout the production process.

There are two different methods under the category of HTST pasteurization: batch (batch) and "continuous flow". For both methods the temperature is the same (72 °C for 15 seconds).

• In the process batch a large amount of milk is heated in a container (industrial autoclave). It is a method used today especially by small producers because it is a simpler process.
• In the "continuous flow" process, the food is made circular between two metal plates, also called plate heat exchanger or tubular form (PHE). This method is the most applied by the large-scale food industry, as it allows the pasteurization of large amounts of food in relatively little time.

Scheme of a plate heat exchanger, common in heating and cooling of food liquids

UHT process

The UHT process is continuous flow and maintains the milk at a higher top temperature than that used in the HTST process, and can be around 138 °C for a period of at least two seconds. Due to this very brief period of exposure, minimal degradation of the food occurs. Milk when labeled as "pasteurized" has generally been treated with the HTST process, while milk labeled as "ultrapasteurized" or simply UHT, should be understood to have been treated by the UHT method.

The technological challenge of the XXI century is to be able to reduce as much as possible the period of exposure to high temperatures of food, making the transition from high to low temperatures as quickly as possible, reducing the impact on the degradation of the organoleptic properties of food; For this reason, microwave-based technology is being investigated, which allows this type of effect (it is even used in meats). This method is very suitable for slightly acidic liquid foods (acidity is measured by pH), such as such as fruit juices and vegetable juices (such as gazpacho), since it allows conservation periods of 10 to 45 days if stored refrigerated at 10 °C.

Regulatory bodies of the standard

Pasteurization methods correspond to a series of methods standardized by the food managers of each country and are controlled by the agencies in charge of monitoring the quality of food (some examples are the USDA in the United States and the Food Standards Agency in the UK) through the implementation of a specific food right. These agencies require and monitor that, for example, HTST-pasteurized dairy products carry the appropriate food label. As a general rule, there are different standards depending on the dairy products to be processed. The main factor to take into account is the fat content of the product. In this way, the cream pasteurization parameters differ from the parameters used for skimmed milk, and the parameters for pasteurizing cheese are designed and implemented in such a way that the enzymes that process phosphates, useful for maintaining the properties, are not destroyed. cut and texture of the cheeses.

Standard HTST pasteurization methods have been designed to achieve an extension of shelf life of about 5 days (ie 0.00001 times the original period) by reducing the number of microorganisms in milk and other foods. This method is considered adequate for the reduction of vegetative cell populations of almost all pathogenic bacteria, including those bacteria resistant to high temperatures (particularly the species Mycobacterium tuberculosis, which causes tuberculosis, and Coxiella burnetii, which causes Q fever in milk). The HTST pasteurization process does not eliminate bacterial spores because the process uses a temperature-time regime of 75 °C for 15 seconds, which is insufficient for the reduction of bacterial spores, due to its high resistance to heat, normally requiring temperatures higher than 100 °C so that the exposure time is relatively short and to avoid damage to the nutritional and sensory components of the food. However, the process is designed in such a way that the products are heated evenly, avoiding that while some parts are subjected to excessive temperatures for too long, others do not reach the necessary parameters.

Dynamics of pasteurization

Pasteurization is a process that follows first-order chemical kinetics. We denominate N the number of microorganisms alive at a given exposure temperature T, and N_o the population of microorganisms initially. If K_d is the kinetic constant of death due to temperature (death rate of microorganisms), the decrease in the population (culture) depends on the following exponential formula:)

${displaystyle N=N_{o}e^{-K_{d}T}}}$

This formula is essential to determine the evolution of a crop as a function of temperature. It can be seen in it a great dependence on the exposure temperature T. The formula is also the foundation of the so-called "survival diagrams" in the food industry, where log(N/N_o) is the exposure time at a fixed temperature T. Typically, heat survival graphs for microorganisms appear as straight lines on a semi-logarithmic scale. The existing correlation between the speed (or ratio) of death of microorganisms and the temperature fulfills the Arrhenius equation.

An important factor assigned to each microorganism is the so-called «decimal reduction time» or also «D value» of a microorganism, and is defined as the time necessary for its population to be reduced by 90% at a given temperature in the treated product. It is an expression of the resistance of a microorganism to the effect of temperature. His expression is:

${displaystyle D_{T}={frac {Delta t}{log N_{o}-log} N}}}}$

Where ${displaystyle Delta t}$ is the period to which the sample is exposed, N_or is the initial population and N the final population. Different D values can be obtained for a given microorganism, or for a particular food process, determining survivors at different temperatures. High D values indicate that the microorganism is more resistant than others with a lower value. There are other values such as constant thermal resistanceoften known as value z, which is defined as the difference in temperatures necessary to cause a reduction of 90% in the D value. This pasteurization eliminates 98 % of bacteria like Vibrio cholerae, Shigella or E. coli.^{[chuckles]required]}.

Factors affecting the process

Acidity of the food

Acidity has a lot of influence on the degree of survival of each bacterial organism. The main parameter to characterize acidity is pH. In general, most foods are considered acidic or slightly acidic. It must be considered that most toxic bacteria such as those of the species Clostridium botulinum are no longer active below a pH value of 4.5 (meaning that a simple lemon juice deactivates them). Foods can be considered acidic if they are below this pH value. Most carbohydrates are in this range, especially monosaccharides. For foods with a higher pH, a heat treatment of 121 °C for three minutes (or an equivalent process) is necessary as minimal processing (i.e. milk, vegetables, meats, fish, etc.). However, many of these foods become acidic when vinegar, lemon juice, etc. are added to them, or they simply ferment, changing their acidity value. The cause of this effect resides in the deactivation of the microbial activity due to the simple influence that the acidity value, indicated by the pH, has on the life condition of these microorganisms.

Resistant organisms

Some organisms and bacteria grown in food are resistant to pasteurization, such as the bacilli of the species Bacillus cereus (cultures of these can even thrive at low temperatures), and Geobacillus stearothermophilus. However, the resistance to thermal elimination depends to a large extent on the pH, water activity, or simply the chemical composition of the food, the ease or probability of being contaminated again (in what is called postprocessing contamination in English). , or PPC)

Physical form of food

Mentioning the shape as a factor to take into account in the pasteurization of food is equivalent to saying that what influences is the outer surface of the food. It is possible to think that the main objective of the pasteurization process is the increase in the ratio between the cooling capacity and the surface of the same. In this way, the worst ratio corresponds to foods similar to a sphere. In the case of liquid foods, efforts are made to have optimal shapes so that the temperature variation, both in heating and cooling, can obtain an optimal ratio.

Thermal properties of food

Some thermal properties of food indirectly affect the final performance of pasteurization on it, such as heat capacity (the amount of energy that must be "injected" per unit mass of food to raise the temperature), thermal conductivity (guarantees the homogeneity of the process in the food), thermal inertia (foods with less thermal inertia are more likely to be pasteurized than those with greater inertia).

Pasteurization of milk

Since its origins, pasteurization has been associated with milk. The first researcher who suggested this process for the dairy product was the German agricultural chemist Franz von Soxhlet in 1886, with Charles North applying this method to milk for the first time in 1907. Microorganisms activate their populations by growing from optimally in the temperature range 25 °C to 37 °C. For this reason, during the manufacturing and packaging process of the dairy industry, the temperature of the milk is avoided from being in this range after pasteurization. Milk is generally a slightly acid medium with a pH less than 7 (6.7). Cow's milk pasteurized by the HTST method and that has been properly refrigerated has an extended expiration period that can reach two or three weeks, while ultra-pasteurized milk can have an extended life that ranges from two to three months. Longer shelf lives (even without refrigeration) can be achieved when UHT pasteurization is combined with proper handling and sterilized packaging technologies. At the same time that the colonies are reduced, the most heat-sensitive microorganisms, such as coliforms, are also eliminated from the milk, inactivating alkaline phosphatase (the level of this enzyme defines the degree of efficiency applied to milk pasteurization; see test of the phosphatase). Despite applying pasteurization, the treated milk still contains some microbial activity, as a general rule lactic acid bacteria (not pathogenic, although capable of fermenting the milk) and refrigeration is necessary.

Several types of milk packed in a supermarket in Portugal. However, none of them is pasteurized because they are not kept refrigerated.

Diseases it prevents

Consuming raw, unpasteurized animal milk exposes you to certain risks of contact with disease-causing organisms and bacteria. In some countries its sale has even been prohibited. Some of the diseases prevented by pasteurizing milk are tuberculosis (Mycobacterium tuberculosis), diphtheria, polio, salmonellosis, scarlet fever, brucellosis, and typhoid fever. Today, many of these diseases are not very relevant due to the widespread use of pasteurization processes in the early stages of milk handling.

Organisms affected

Species of organisms whose populations can be greatly reduced by pasteurizing milk include:

Are current pasteurization methods adequate?

The pasteurization of milk has gradually been the subject of increasing controversy. On the one hand, it has been discovered that some pathogenic organisms have developed a resistance to the decrease in population with temperature, managing to survive pasteurization in significant amounts. Pasteurization under certain incorrect conditions has been found to destroy vitamin A and vitamin B but these conditions do not occur in current pasteurization processes.

Pasteurization of juices

Orange juice

Packaged juices (and even nectars) undergo two different types of pasteurization processes: on the one hand, there are unprocessed juices (raw); on the other, ultra-pasteurized juices or sterile juices.

Juice producers are familiar with pasteurization processes and with both methods: the VAT or batch process (used in small-scale producers) and the UHT (used in small-scale producers). higher production). The HTST method is accepted in the industry as it does not produce appreciable flavor degeneration. Pasteurization is very effective in juices because they are acid media and prevent the proliferation of sporulated microorganisms, the most resistant to high temperatures. In many countries, such as the United States, 95% of marketed juices are pasteurized. On some occasions, the organisms in charge of food surveillance and hygiene are required to indicate to the consumer that they are drinking a “raw juice”. Juices are usually heat treated by the pasteurization method at 70 °C for 30 minutes, but the ideal temperature based on pH is currently under investigation.

Frequent microorganisms in juices

Depending on their origin, juices contain various microorganisms and it is necessary to reduce the total concentration of their populations through pasteurization. Thus, it is known that apple juice may contain the species Salmonella typhimurium, Cryptosporidium and Escherichia coli. In orange juice it is common to find the species Bacillus cereus, Salmonella typhi and Salmonella hartford. In some vegetable juices, generally low-acid juices, such as carrot juice, there is a particular risk of the species Clostridium botulinum remaining.

Effects of pasteurization on juices

Juices can change their color and tend to brown due to enzymatic deterioration of polyphenol oxidase. This is partly due to the presence of oxygen in the liquid. For this reason, juices and nectars usually have the air removed before beginning the pasteurization process. In the same way, the loss of vitamin C and carotene is reduced by prior deaeration. Since these nutrients are lost in the process, in many cases they are usually reincorporated artificially (enriched juices).

Recent Research

Certain populations of the species Mycobacterium avium (belonging to the subspecies M. avium paratuberculosis), which cause Johne's disease in slaughter animals, have been found to —and suspected Crohn's disease in humans as well—have survived pasteurizations of certain dairy foods in the United States, the United Kingdom, Greece, and the Czech Republic.^{[citation needed]} In view of the survival of certain species in addition to the above, the UK authorities in charge of monitoring the quality of food decided to reassess pasteurization standards.

A current method is flash or instant pasteurization, which uses shorter exposure times at high temperatures and seems to be an adequate method to preserve the organoleptic properties of food, since it better preserves their flavor and texture. Cold pasteurization is a term sometimes used synonymously with ionizing radiation (see food irradiation) or other meanings (eg, chemicals) to reduce bacterial populations in food. Food irradiation is also sometimes called "electronic pasteurization." The possibility of extending pasteurization to non-fluid foods, such as beef, has been investigated. An advance in non-intrusive pasteurization that solves many problems in the canning industry is the so-called electromagnetic pasteurization of liquid foods, which uses microwaves at 2.45 GHz frequency to activate thermal processes. This method has proven its efficiency in pasteurizing water.

There are studies oriented to the Third World in which it is possible to carry out what is called solar pasteurization. The idea is based on solar cooking and the fact that it is not necessary to bring liquids to a boil to achieve pasteurization, being able to pasteurize with this method with temperatures above 56 °C. With this measure, an attempt is made to prevent the cause of diseases caused by the ingestion of contaminated water. The method is known as "water pasteurization", in which certain elements capable of indicating the state of pasteurization of the water and its possibility have been developed. safe intake. One of the most widely used is the water pasteurization indicator (WAPI). Solar pasteurization requires exposing water in containers for six hours. The program that was applied in certain regions of Africa was called SODIS (short for solardis). infection).

Pasteurized foods

Apart from milk and juices, other foods are pasteurized by the food industry; As a general rule, they are those that have a liquid or semi-liquid structure. Some of the most mentioned are the following: