The Smoking, Curing and Preserving processes demystified

[Wade]

To begin the Smokevember month here is an explanation of the roles of the different processes used in curing and preserving. Everyone who cures, smokes or preserves food will use a combination of these methods, often without realising why.
When you first begin curing food it can be quite stressful to know whether, what you have just created, is safe for you or your family to eat. However, with even a basic understanding of what you are doing will help you to eat with confidence.
The saying "If in doubt chuck it out" is generally good advice, however this can often lead to food that is perfectly safe to eat  being thrown away. Fitstly, the more you understand about the processes the less doubt you will have and therefore the less that will be "chucked". Secondly, there are a lot of people who cure and preserve food regularly who are here and can offer you advice. I would therefore recommend that the advice be updated to "If in doubt ASK before you chuck it out"

What is curing when applied to food?
Curing is a method of preserving food to prevent spoilage. It refers to various preservation and flavouring processes, especially of meat or fish.

Spoilage can be prevented (or delayed) by using several processes. Most curing processes involve applying a combination of these to achieve the desired taste, texture and storage time. Sometimes the extended storage time is only measured in days (as in traditional smoked salmon), weeks (as in dry cure bacon) or months/years (as in jerky, salamis and Palma/Country style hams)

The different processes that can be used in combination:

Reducing available water – including dehydration

The growth of bacteria and bacterial spores is significantly affected by the availability of water in their surroundings. Water is also required by the natural enzymes in foods that cause autolysis – the self-destruction of cells by their own enzymes. If the water is removed or made unavailable, the storage life of most foods will be prolonged.
Although we could totally remove the water from the food to have the preservative effect, it only really needs it  be made unavailable to the spoilage organisms/enzymes. This can be done though several different processes.

• Removing the water – This is often achieved by warming the food in a relatively dry environment which enables the water in the food to evaporate. The smoking process is a good example of this, where the air that includes the smoke passes across the surface of the food causing the water to evaporate and pass out of the chamber with the smoke. This is why is us usually important to maintain good air flow through your smoker when cold smoking.
• Changing the state of the water – For bacteria to utilise water it must be in liquid form. One of the effects of placing food into a freezer is that it changes most of the water into ice and therefore make is unusable by bacteria. Freezing, however, does not remove liquid water completely as even when frozen some of the ice will be transitioning from solid to liquid and then back to solid. The lower the temperature in the freezer the less water there will be in liquid form at any point in time.
• Removing water from the bacteria cells – This is done by increasing the amount of salt and/or sugar around the bacteria, causing the water within their cells to be drawn out by a process called Osmosis. The removal of water from the cells prevents the bacteria from metabolising and growing and often results in their cell walls rupturing and the bacteria being destroyed.
• Binding the water – Various substances, including sugar, will chemically bind large quantities of water to their molecules and will therefore make it unavailable to the bacteria. This process is one of the important steps when making a BBQ sauce or ketchup that is not going to be eaten immediately. There is still water within the sauce so that it remains pourable, however much of it is bound to the sugars that are also present and which become more concentrated as the volume of the sauce is reduced through simmering.

As you can see, you do not need to physically remove the water from the food to have a preservative effect - You just need to make it unavailable for any spoilage organisms to use.

Jargon alert:  The available water in food is known as Water Activity (aw) and to totally inhibit most spoilage bacteria and moulds an aw of 0.8 or below is required. Many commercial food processers will use an (aw) meter to check that their products are safe for long storage, however these are very expensive and are outside the pockets of most home curers. Even though most home curing processes will not achieve an (aw) of 0.8 or below, there will still be a preservative effect, though more limited, at higher (aw) levels.

Increased Salinity – adding salt

Salt is usually added in one of three ways – applying directly to the outside (Dry Brining), applying as a liquid solution (immersion brining or injecting), adding salt as an ingredient. There are several ways in which salt will inhibit microbial growth. The most notable is through osmosis, or dehydration (see above). Salt’s other antimicrobial mechanisms include interference with a microbe's enzyme activity and weakening the molecular structure of its DNA.
When used as an effective bacterial inhibitor it requires salt solutions of ~10% to make most food spoilage organisms inactive, however salt at this concentration in our food would be unpalatable. Lower salt concentrations will also reduce bacterial growth but to a lesser extent. Many bacteria are very hardy organisms though and as salt concentrations fall the activity of many will again begin increase.

The ways of applying salt through the differing curing methods will affect the storage time of the final product. A good example is the difference between Dry cured and Immersion cured bacon. Both methods can be used to produce an end product that contains 2.5% salt, however the resulting shelf life of both methods is very different. Unsliced Dry cure bacon can be safely stored refrigerated for up to 6 weeks whereas the equivalent safe storage life for immersion cured bacon is only 2 weeks. One of the reasons for this is the levels of salt that the bacteria are exposed to during the curing process.

• With Immersion cure bacon the levels of salt in curing solution will depend on the volume of immersion cure being used and the weight of the meat however it will usually not exceed twice the desired end concentration – therefore will rarely exceed 5% (and will frequently be lower when using larger volumes of cure solution). Although this will inhibit the growth of most bacteria it will not inhibit it completely.
• When Dry curing bacon you are applying crystalline salt directly onto the surface of the meat – where most of the bacteria will be present. As this salt dissolves in the water from within the meat it results in the surface bacteria being exposed to a saline solution of 100%. As the salt diffuses towards the middle of the meat this concentration will reduce however during normal curing times the surface brine concentration is unlikely to fall below 10% until the salt concentration in the meat is fully equilibrated.

Another difference between the two curing methods is that the process of immersion curing will result in an increase in the water content of the meat by about 10%, whereas the process of dry curing will decrease the meat water content. 

Commercially the levels of salt in brining solution will be measured using a saline meter. These are fairly inexpensive and are often affordable by the home preserver. The electronic salt meters are best as they measure the salt through its ionic properties. Another common saline meter is an optical refractometer. These should ONLY be used for saline solutions that do not also contain sugars, as the presence of sugar will make the resulting "salt" readings highly inaccurate.

Increasing sugar
Sugar will have a similar osmotic effect on the bacteria cells as salt, causing them to dehydrate, however sugar molecules will also bind water molecules lowering the (aw) and making them unavailable for the bacteria to use. Sugar may also provide an indirect form of preservation by serving to accelerate accumulation of antimicrobial compounds from the growth of certain other organisms. Examples include the conversion of sugar to ethanol in wine by fermentative yeasts or the conversion of sugar to organic acids in sauerkraut by lactic acid bacteria.

Increasing acidity (decreasing pH)
Increasing the acidity of foods, either through fermentation or the addition of weak acids, has been used as a preservation method since ancient times. In their natural state, most foods such as meat, fish, and vegetables are slightly acidic while most fruits are moderately acidic. Only a few foods such as egg white are alkaline. Generally most food spoilage bacteria thrive within a narrow pH range and as you move away from this range they become increasingly inhibited. The effect differs though between organism. For example, a pH of 4.6 is sufficient to control most spore forming organisms (e.g. C. Botulinum) however a pH of 4.2 is required to control other vegetative pathogens (e.g. Salmonella).
Increasing the acidity (reducing the pH) can be achieved in several different ways. The adding of an acid directly (e.g. Vinegar in sauces, pickles and chutneys), adding acidic fruit or juices (e.g. orange juice pH 4; lemon/lime juice pH 2; tomatoes pH 4.2) or adding a bacteria that increases the acidity of its environment as it grows (e.g. the use of a lactobacillus in salamis)

Commercially the acidity levels are monitored by using a pH meter and these are inexpensive and can often be afforded by the home curer.

Addition of Nitrite (and sometimes Nitrate)
When added to foods such as cured meats, nitrite has at least three functions. Firstly, it contributes to the flavour - the nitrite is responsible for imparting the characteristic “bacon” flavour. Secondly, it reacts with myoglobin in the meat which gives the characteristic pink colour of cured meat. Thirdly, it inhibits the growth of food spoilage bacteria, most importantly Clostridium botulinum.
The chemistry of nitrite when curing meat is quite complex and the exact mechanism by which it inhibits the growth of C. Botulinum is still a matter of scientific discussion. Nitrite is known to be effective at levels of less than 50mg per Kg of meat. Without laboratory analysis it is not possible to know with certainty how much of the nitrite remains in the meat following the curing process, as within the meat it will undergo a number of different reactions where it can remain free, be bound up byother molecules or cell components, it can be metabolised and broken down. Because of this the use of Nitrite in curing is actually calculated using “ingoing” amounts of Nitrite. Until a few years ago permitted Nitrite levels were ~350 mg/Kg (Ppm) however recently the EU, FSA and USDA have reduced the limits of ingoing nitrite to 150 mg/Kg (Ppm) for uncooked cured meat (and 100 mg/Kg for cooked meat). There are some exceptions for some traditional regional products where higher levels of Nitrite are permitted, however these exceptions limit the amount of residual nitrite in the meat which would have to be established through laboratory testing.

As nitrite is gradually broken down in the food it will lose its effectiveness if time for controlling C. Botulinum. For foods that are stored chilled and have a short shelf life this is not a problem however for longer term storage (e.g. Parma/Country style hams and salami style sausages) Potassium Nitrate is also added. The nitrate does not play a direct role in the biological control but over time it is slowly broken down to form Nitrite, replacing the nitrite levels as they are lost.
In the USA the use of Nitrate in bacon is no longer permitted as, when heated to high temperatures (e.g. when bacon is fried), the nitrates are converted to nitrosamines which have been linked to cancer in rats. In the UK the situation is less clear however the FSA stipulate that “The use of potassium and sodium nitrate is permitted only in non-heat-treated meat products, to a maximum amount added of 150 mg/kg”. Frying bacon before eating would be considered a heat treatment.

Unfortunately, some of the manufacturers of ready-to-use cures (e.g. Supracure) have yet to change their usage instructions to comply with current regulations. When used at the recommended 5%, Supracure will result not only in an ingoing Nitrite level of >300 mg/Kg and will result in Nitrate being included in the bacon cure, but it will also produce a salt level of 5% which most people will find unpalatable.

Exclusion of oxygen
Preventing oxygen from reaching most bacteria will stop them from growing and increase the shelf life. This can be simply achieved by vacuum packing but can also be achieved by canning. The use of set sugar can also be used to form an oxygen barrier, as in jam.
Care needs to be taken when excluding oxygen, as spore forming bacteria (e.g. C. Botulinum) require an oxygen free environment to produce their toxins. It is therefore important that food that is going to be stored this way should also have additional methods of spore control. This could be the use of added nitrite, by reducing the pH through pickling, or by suitable heat treatment.

Heat treatment
The only way to ensure that your food is effectively free of food spoilage organisms is to heat treat it, although this requires specialist equipment as some bacteria spores (e.g. C. Botulinum) are not killed by boiling water. At 120 C in a pressure cooker/canner (retort) it will take between 10 and 30 minutes kill the C. Botulinum depending on the acidity of the food.

Chilling or Freezing
The storage life of most foods can be extended by chilling or freezing. Although freezing would appear to be just an extension of chilling they actually work in different ways.

The process of chilling food results in the slowing down of the metabolism of food spoilage organisms so that their effect is delayed. Even when chilled to 4 C they will still remain active and will eventually result in the food becoming unsafe to eat. The chilling effect is on both vegetative and spore forming bacteria. When stored at 4 C or below it will take C. Botulinum in excess of 10 days to produce levels of toxin that could begin to become hazardous to vulnerable individuals.

The process of freezing works in three ways. Firstly, it continues to reduce the metabolism of the bacteria. Secondly it changes most of the available water to ice which cannot be used by the bacteria. Thirdly it causes ice crystals to form within the bacteria cells which can cause them to rupture – although only some of the bacterial cells will be destroyed.
Freezing can help with the removal of water. Fish or meat that has been frozen will undergo some internal cell rupturing (though this usually does not affect the end product) and will result in approximately a 3% loss of water upon thawing.
Freezing can also be used to control parasites and flukes in fish that is to be eaten raw. Flukes require to remain frozen at -20 C for 7 days (or -35 C for 15 hours) in order to be killed.

Smoking
Smoke adds flavour and is both a mild antimicrobial and antioxidant, but since it does not actually penetrate far into meat or fish is insufficient alone for preserving food. When smoking food the smoke is really there for flavouring, however the smoking process can play an important role in the removal of water.

The process and effects of curing

Examples of different foods and the usual cure methods

Type of food	Cure/preserving methods	Expected shelf life
Traditional Smoked Salmon	Dehydration Salt Smoke	10 days at 4 C
Immersion cured bacon	Salt Nitrite Smoke (opt)	14 days at 4 C (unsliced)	1 week when sliced
Dry cure bacon	Dehydration Salt Nitrite Smoke (opt)	60 days at 4 C (unsliced)	1 week when sliced
Hard/Dry Sausage	Dehydration Salt Nitrite Nitrate Smoke (opt) Surface mould	Whole, 6 weeks in pantry; indefinitely in refrigerator.	3 weeks when cut
Salami style sausage	Dehydration Salt Nitrite Nitrate Lactobacillus Surface mould	Whole, 6 weeks in pantry; indefinitely in refrigerator.	3 weeks when cut
Meat or fish Jerky	Dehydration Salt Heat Nitrite (opt)	Home produced – 1 to 2 months	Commercially packaged - 12 months
Country (Parma) style ham	Dehydration Salt Nitrite Nitrate	Whole, uncut ham can be stored safely at room temperature for up to 1 year.	The ham is safe after 1 year, but the quality may suffer.
Chutney	Reducing water Reducing pH Excluding oxygen	Unopened, up to 2 years at room temperature	4 weeks once opened
Jam	Reducing water Sugar Reducing pH Excluding oxygen	Unopened, up to 2 years at room temperature	Up to 3 months once opened and refrigerated
Fermented vegetables	Salt Reducing pH (by fermentation) Heat Excluding oxygen	4 months if not boiled, 18 months if boiled.