Introduction: How to Master the Pasteurization Process

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It was in 1856 when the French wine producer M. Bigo asked Louis Pasteur for a solution to avoid wine deterioration during long maritime expeditions. Pasteur's observations on the microscope showed that in addition to the normal yeast residues, the spoilt wine contained oblong (elongated) cells that, instead of experimenting alcoholic fermentation, went through lactic fermentation and caused the deterioration of wine. As a consequence, Pasteur suggested to heat the wine up to 57ºC and maintain it at that temperature for several minutes to eliminate all living organisms (bacteria, fungi) before the lactic fermentation happens.

This process has been later used to preserve other products adjusting the temperatures and pasteurization times to the content.

Step 1: The Goal

To be sure that all living organisms are eliminated, the product inside the jar MUST reach a temperature of 60-80°C and maintain it for 10-20 minutes. Although heating the product to higher temperatures would enhance the anti-bacterial effect, it is not recommended because the product inside would lose its organoleptic properties and, although it will be well preserved, it will not taste like one freshly made. The best temperature for the pasteurization process is around 80ºC.

After the pasteurization process, the product must be sealed tightly to isolate it from the outer environment that contains the unwanted fungi, yeasts and bacteria.

In case of preparing food for people with a weak immune system, a tyndallization will also be done as final step.

This whole process will be ilustrated with two examples: how to make Black-plum compote and how to make peaches in syrup.

Step 2: Food Preparation

In this instructable two recipes are presented to show the two different method explained in the following steps.

Peaches in syrup

Ingredients: Unripe peaches and syrup

  • Peel the peaches with a knife
  • Cut them open and remove the pit
  • Cook them in a pot with syrup for 15-20 min. You can add a couple of cloves for a better taste
  • Put them in a jar with enough syrup to cover them

Black-plum compote

Ingredients: Ripe black plums and syrup

  • In order to peel the plums, start by making an X cut at the bottom and boil them for 2-3 minutes.
  • Take them out of the pot and remove the peel.
  • Now cut them open and remove the pit completely. This is very important because otherwise you'll get a bitter taste in the compote.
  • Put them in the jar with enough syrup to cover them

Syrup

Ingredients: 0.5kg sugar per 1 litre water. You can use more sugar but 0.5kg is enough.

  • Put water and sugar in a pot and bring them to boil
  • Keep them at high temperature and stir the mix so that the sugar at the bottom does not get burnt
  • Once the mix is homogeneous and has thickened up, remove the pot from the stove

Step 3: Eliminating Bacteria

How Do I Reach the Right Temperature?

The most common method is to heat the filled jars in a pot with boiling water. The boiling temperature, depending on its purity and the atmospheric pressure, will reach a value of about 90-98ºC, which will make the product inside the jar heat up to 70-80ºC (see pictures above). Make sure you put a cloth between the jars and the pot so that it doens't break due to thermal shock.

The other possibility is to heat up the jars in the oven. This option is less messy because there is no water around the jars and we won’t have to deal with water drops jumping into the jar or a wet surface when you take them out. However, finding the right temperature for the oven is a bit tricky. The ovens we have at home have the temperature sensor close to the metallic sheets that form the main compartment, so the sensor is affected by their radiation and it doesn’t measure the temperature of the air in the oven but something close to the one of the sheets. If we think that setting the oven at 90ºC would give us the perfect pasteurization, we are wrong. Each oven is different and you’ll have to learn it to hit the right temperature but, just as orientation, I set it at 120ºC, the air in the oven reaches 110ºC, the glass of the jar goes up to 90ºC and the content up to 70ºC (see pictures above).

One must understand that the transmission of heat can happen in 3 different ways:

  • Conduction: Heat travels through materials by direct contact between two elements, e.g. touching a radiator with your hand.
  • Radiation: Infrared radiation reaching an element, e.g. putting your wet shoes to dry below the radiator.
  • Convection: Heat transported by a flow that reaches an element, e.g. putting your hand above the radiator and feeling warm air coming upwards due to its lower density.

Therefore, while heating the jars in boiling water is done by conduction (hot water touching directly the jar), the oven surrounds them with a convective flow or warm air and radiates them from the resistances. The heat conductivity of water is much higher than the one of air (air chambers are used to insulate houses) and so, the transmission of heat is more efficient when using water.

How Much Should I Fill the Jars?

During the pasteurization process, the water and air inside the jar experience thermal expansion as a response to the increase in temperature. Therefore:

  • If the content is cold or room temperature when you put it in the jars, leave about 3-4cm free.
  • If the content is hot when you put it in the jars (right after cooking it, around 60-70ºC), it would have almost reached full expansion, so leave about 2cm free.
  • Do never fill the jars to the top because the content will spill out of them during pasteurization and leave traces on the rubber or the gasket of the lid, which will make the jar not be airtight when you close it.
  • The parts of the jar that will be in contact with the lid must be clean and completely free of solid particles to allow good adhesion of the lid to the jar.It is highly recommended to provide 1-2 cm of air space above the product in the jar (after heating) so that the elastic properties of air can regulate the pressures in the jar when cooling.

Step 4: Sealing the Jar

In order to get the suction effect, ONE MUST ALLOW THE SURPLUS EXPANDED AIR TO LEAVE THE JAR. The two most common kinds of jars are the TWIST and the WECK. Depending on the type of jar used, the placement of the lid during the pasteurization process varies a little.

TWIST JARS

  • While heating: Put the lid on top of the jar and turn it a little bit so that it grabs the beginning of the thread. The intention here is to secure the stability of the lid during the heating process while leaving it loose for the air to scape.
  • When closing: Tighten the lid right after removing the jar from the water or the oven. Be careful not to do it so tightly that the lid or rubber are damaged. These deformations may leave gaps through which air will be able to get inside. This first tightening has a temporal character as it is the contraction of the content of the jar what will create a suction force that will guarantee the sealing.

WECK JARS

  • While heating: Put the lid and rubber on the jars (don’t put the closing springs) so that hot air can escape from the jar.
  • When closing: Place the springs immediately after removing the jars from the water or oven.

After sealing them with the lids and once the content has cooled down, a negative pressure (respect to 1 atm) will be created inside the jar and will keep the lid pressed against the jar preventing the entry of contaminated air from outside. If you close the jars tightly before heating them, the surplus air won’t leave the jars and you won’t get the aid of negative pressure for sealing them.

After one or two hours (depending on the ambient temperature), the jars would have cooled down and should be tightly closed. When it comes to TWIST jars, the test consists of tapping the middle of the lid. If the tone is high, it means it’s well closed (the lid has the meniscus pushed downwards) and if the tone is low (hollow), the jar is not closed properly.

Step 5: Science & Popular Knowledge

SCIENCE

Let’s apply some basic physics to get the order of magnitude of the suction that the contraction of the content of the jar will create. First, a couple of simplifications will be done. As the content is mainly formed by water, we’ll assume that the elastic properties of the product inside will be defined by water. The air inside, will be considered to be an ideal gas. Therefore, for the calculations we will need the help of the ideal gas equation (PV=nRT) for the air and the thermal expansion equation for the water and glass.

Taking the following values,

  • Bulk coefficient water: 2.15 GPa
  • Volume expansion coefficient water: 207·e-6 K^-1
  • Volume expansion coefficient glass: 9.9·e-6 K^-1

We will calculate the suction of water once it cools down to room temperature. First, we calculate the expansion of the materials with the expression dV=Vo·ExpCoeff·dT. In the oven, the temperature of the glass will be 90ºC and water will be 70ºC. We assume that the content we put inside is at room temperature (20ºC).

Then, the volume of water that would be spilled out of the jar if it had been filled to the top is calculated by the difference of the expansions. For a jar of 400ml:

dV water = Vo · ExpCoeff Water · dT water = 0.4 · 0.000207 · 50 = 0.00414 l

dV glass = Vo · ExpCoeff Glass · dT glass = 0.4 · 0.0000099 · 70 = 0.00027 l

Vspill = dV water - dV glass = 0.00386 l

By closing the jars when the content is at high temperature, we restrict any changes in volume and they would have to be compensated with a sub-pressure. These 0.00386 l is the amount to be compensated. Using the expression P=Bulk·Vspill/Vo , we get to a result of 20,762,550 Pa , which is about 205 atmospheres. If the jar had a lid of 8cm of diameter, it would be the equivalent to having 10.3 tons resting on top of the jar, i.e. an adult great African elephant on top of it. In practice, this value is so high that it could break the lid and the jar, so I guess that for such magnitude some air will still get in the jar until an affordable value for pressure is achieved.

In the case of air, the pressure due to its contraction has been calculated with the ideal gas equation, giving a value of 0.854 atm, so the suction pressure will be the difference respect to the exterior (1 atm). Therefore, air creates a suction of 0.146 atm, which is the equivalent to 14770.41 Pa, much less than water... why?

Well, it is important to note the physical characteristics of air and water. Air is a compressible fluid and water is in practice considered most times incompressible. This is probably the key of the sealing. The little volume of air you leave in the jar is going to respond to the suction of water by expanding itself. This expansion of air will reduce the stresses in water because it allows water to contract and equilibrate the pressures inside the jar.

The tensions caused by changes in temperature (dilation/contraction) are always quite surprising and often hard to believe, but just think of what happens when you leave a glass bottle full of liquid in the freezer for too long… it breaks!

POPULAR KNOWLEDGE

Some people put the jars upside down while cooling. I haven’t found any physical reason that suggested this is beneficial for sealing them. If anything, I’d say that the hydrostatic pressure may prevent outside air from entering the container. However, the contraction of the content when cooling has such magnitude that the outside air will enter despite this water column. If we take the food industry as reference, I have seen no food factory where they put the jars upside down after pasteuzation.

Step 6: Tyndallization

The development of bacteria or fungal spores is relatively slow, so a single pasteurization is only sufficient if the consumption takes place within a period of less than one year, without endangering healthy people with possible infections.

For preserved foods to be consumed by people with low defenses (children, sick, chronically ill, elderly) triple pasteurization, called tyndallization, is very convenient (mandatory). This process consists of three pasteurizations with one-day intervals to destroy the spores and fungi that survived the first pasteurization (they are very resistant).

The tyndallization process is about the elimination of the remaining bacteria from the content of the jar. It has nothing to do with the sealing of the jar. Actually, during the second and third pasteurizations you MUST NOT LOOSEN THE LID when putting the jar in boiling water or in the oven. The jars must be completely closed.

In the first pasteurization process, the internal air expanded and its excess has already left the jar. So, in the next two pasteurizations (tyndallization), the volume increase of water and air inside the jar will only temporally reduce the suction effect. In addition, the air inside the jar after the first pasteurization is already sterile and if you loosen the lid, contaminated air may enter the jar.

Step 7: Storage

Label the jars leaving clear the content and the date it was made.

It should be noted that many types of spores grow thanks to photosynthesis, so pasteurized or tyndallized jars should be stored in dark places where light cannot reach them.

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