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Preservation Basics

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Preservation Basics

Watch for Jack's follow-up course on preservation which will appear in early November!

Author: Jack Lang (jackal10)

This course is intended to cover some of the basic methods of food preservation. It does not purport to be a complete treatise, but should give you an introduction to the basics and hopefully stimulate further study. The course chapters are:

1. General Principles

2. Bottling/Canning

3. Pickling

4. Drying

5. Jam and Jelly Making

1. General Principles

Until recently, the need to preserve the summer’s bounty was a matter of survival through the winter. Preserving food often changes its taste or texture, and people came to like the taste. Today, although fresh food is easily available throughout the year, we preserve some foods, (bacon for example), primarily for the taste.

Some of us have gardens that overproduce in summer, and we cannot abide to waste the food, or we want to take advantage of a market glut. Having a larder full of delicious things widens the choice of what to eat and can act as a handy back-up for inclement weather or when the power fails.

To understand how to preserve foods, we need to first understand what causes them to spoil.

A. Biological spoilage

The environment, and raw food, are chock full of little critters (micro-organisms). Many of these are benign, but a few have the capacity to make us ill or they excrete toxins that can be extremely poisonous if eaten in any quantity. Preserving doesn’t make bad food good again, and although it might kill the bugs, the excreted toxins (and bad tastes) are left. So it is important to start with food that does not already have significant spoilage and to process it quickly so that it does not spoil in the process.

The main groups of micro-organisms are:

Bacteria. such as Salmonella, Staphylococcus, Shigella, and Clostridium.

Cl. botulinum. (Botulism) excretes botulinum toxin, an extremely potent muscle-paralysing agent. It is used in minute quantities in plastic surgery as Botox to paralyse the muscles that cause wrinkles. Cl. botulinum is a spore-forming organism: in adverse conditions it forms viable spores, which are extremely heat tolerant. Fortunately it cannot stand an acid environment, and needs a pH of 4.6 or higher to grow, so most pickles are safe. (pH is a measure of how acid things are; 7 is neutral, lower is acid, higher is alkaline. Lemon juice is about pH 4, vinegar about 3, baking soda about 8).

Yeasts and moulds. These are more likely to produce off-flavours than to be actively dangerous if eaten. Some yeasts can live on the surface of jams despite the high sugar concentration. The yeast can just be scraped off, with some of the jam underneath. Some moulds grow, albeit slowly, even at freezer temperatures.

B. Enzymatic spoilage.

If you leave a cut apple in the open, it goes brown. This is not burning (oxidation), nor is it the same mechanism that browns food while cooking – the complex Maillard reaction, but is the natural enzymes in the food released when you cut the surface. Enzyme activity can be desirable, for example tenderising meat when it is hung, but can lead to discolouration and off-flavours. Though it is not actually poisonous, it is not good to eat. Enzyme activity is slowed by deep freeze temperatures, and most enzymes are destroyed by heat or acid.

C. Oxidation

Oxidation happens slowly, even at deep-freeze temperatures, if the food is exposed to oxygen, such as in the air. If the food is in a sealed container, oxydation will cease once the oxygen in the sealed air is used up, that is it until the container is unsealed.

The defence against spoilage is to use one of the following:

Heat. Most organisms are destroyed by heat. The trick is to heat the food long enough and to a high enough temperature to destroy the micro-organisms without destroying the food.

Cold. Cold doesn’t destroy bacteria (deep freeze temperatures may destroy some yeasts). It does, however, slow the biological activity. At deep freeze temperatures the activity is effectively stopped. However, freezing creates ice crystals in the frozen food, and in some foods these crystals can puncture the cell walls, causing the food to go mushy when thawed. In addition, the environment in a freezer is very dry indeed since all the water is frozen out of the air. So, if the food is not sealed well, the surface can dehydrate, causing “freezer burn”.

Dehydration. All life needs water to multiply. Drying stops the multiplication, and in some cases kills the micro-organisms, and has the side effect of concentrating flavours.

Acid. Pickling in A strong enough acid kills most bugs. It also flavours and transforms the food.

Salt. Salt both kills the bugs and draws out water, drying the food.

Once the agents of spoilage have been eliminated, it is important to keep new agents away by sealing the food in a sterile environment.

2. Bottling/Canning

In the USA the term "canning" applies to sealing the food in any container and then sterilising it. In the UK and Europe "canning" only refers to preserving in a metal can,. The term “bottling” means sealing in a preserving jar, usually with a rubber ring and lid, and then sterilising the filled jars.

I will discuss only preserving in jars. If you are lucky enough to have a metal canning machine and a supply of metal cans, then the principles are the same, but follow the instructions that came with your device.

Preserving jars can either have a separate lid and screw-on ring, or a glass lid held on by a wire clip. Both are sealed with a rubber ring. Its important to ensure that the top of the jar is not chipped, and the rubber ring is in good condition for a proper seal. If the design is one with a thin rubber ring bonded to a metal cap, make sure you use new caps each time. It is good practice, when you have made the preserve and it has cooled to check the vacuum and the seal. You should be able lift the jar by its lid without it coming open.

In the US preserving jars are often called Mason Jars. Ball and Kerr are well known brands. In the UK you may be able to find Kilner jars or the French “LeParfait” brand, which is stocked by Lakeland. The ones used here are “Le Parfait”.

Disclaimer: Although Kilner and "LeParfait" type jars have been used for decades, the United States Dept. of Agriculture no longer considers them "safe". Only jars whose seals can confirm the presence of a vacuum, are approved. The jar that meets their standards has the rubber bonded to the lid and "pops" when properly sealed.

Gage Plums

I will discuss the bottling of plums, but the principles can be applied to any fruit not needing prior preparation. For amounts refer to this handy reference. A quart jar will need about 2lb- 2 ½ lb of fruit and 1 pint of syrup.


Plums on the tree.


Plums picked.

Wash them, and discard any doubtful ones.

Wash and dry the jars.


Check the rubber rings.


Check the tops of the jars for chips.


Fill the jars as full as they will go.


Oh dear, there are a few left over that I will have to eat fresh.

Fill with hot syrup (140F/60C) (1 sugar: 2 water, by volume). A wide neck funnel may be useful.


Put the jars into a pan with hot water to cover the tops by at least 1 inch. Note that they are standing on a plate to stop the bottom of the jars getting too hot and overcooking. You can use a proper rack, bits of wood or any other suitable material to keep the bottom of the jars away from the pan bottom.


Cover the pan and simmer for the process time, about 20 minutes. Process times are given in the table below. The compromise is between getting the centre of the jars hot enough to kill the bugs, without overcooking the fruit.


Cool quickly under a tap and lift out the jars. (The metal tool is a jar lifter for handling hot jars.)

The fruit will rise a little in the jars as it cooks. To minimise this, make sure the jars are tightly packed with fruit, and cool quickly so that they don’t overcook – leave them in cold water until they are quite cold.


Store in a dark, cool cupboard.


Bottling peaches is a little more complex, since they need peeling and preparing. You need to work quickly to prevent their discolouration.


To peel the peaches, dip them for one minute in boiling water, then into ice water to stop them cooking.


A teaspoon of Vitamin C powder (Ascorbic Acid) in the ice water will help stop discolouration.

Wash and dry the jars, check the rings and tops to ensure a good seal.


Pack the jars and fill with hot (140F/60C) syrup. Peaches can use a heavier syrup, maybe equal quantities by volume of water and sugar. Spiced peaches (cloves, cinnamon) are excellent, and make good gifts.


Close, and like the plums above, simmer for the process time, 20 mins.


Cool, label and store.



This is essentially a bottled refrigerator pickle. I’ve adapted melkor’s recipe posted in the Gardening thread. I upped the vinegar and reduced the salt. Fabulous!



1 cup (240ml) white vinegar

2 cups (480ml) water

4 Tbs coarse sea salt (or kosher salt)

until the salt is dissolved.

place in the bottom of each quart jar:

1/2 tsp mustard seed

1/2 tsp whole black peppercorns

1/2 tsp red pepper flakes (optional)

2 tbs chopped fresh dill (or more)

2 tbs chopped garlic (or more, lots more)

4 cucumbers scrubbed and sliced lengthwise into quarters and lengths to fit the jar.


Pack the cucumbers in the jars and cover with the liquid mixture leaving ¼ inch (6mm) of head space.


For new green refrigerator pickles, that’s it. Just put them in the refrigerator and in three days they will be perfect. You can refill the pot with fresh cucumbers when you have eaten some, which will not be long because they are so delicious. You may need to top up the liquid with a little more vinegar.

To keep them without refrigeration, they need to be sterilized. To keep the pickles crisp, you can only heat them for a short time, but because of the acid environment this will not be a problem.

Once all the jars are filled place them in a large pot and fill with enough cold water to cover the jars with an inch of water. Bring the water to a boil and boil for five minutes.


Remove from heat and add cold tap water to overflow the pot. Keep running the tap water until the jars are cool. These can in theory be stored in a cupboard but I keep mine in the fridge. They are ready in a day or two and will continue to improve for a few weeks.


Tomato sauce

When I have a glut of tomatoes I make tomato sauce. Great for instant pasta sauces.


Wash, remove the stalks and any moldy or doubtful fruits.

Classically, for 10lb (5Kg) of raw tomatoes, start by frying 5oz (140g) of salt pork or unsmoked bacon (can be omitted: substitute extra virgin olive oil or butter) until the fat runs. Then add a mirepoix of 6oz (170g) of carrots and 6oz (170g) of onion cut into fine cubes and fry until they begin to brown. Add the tomatoes and put the pan over gentle heat. Add a bay leaf and a small sprig of thyme, ½ (14g) oz of salt, 1oz (28g) of sugar and a pinch of pepper.

Alternatively one can make a puree. A puree in some ways is more flexible in that it consists of tomatoes and more tomatoes. Simply omit the bacon, mirepoix and seasonings.


Simmer until all the tomatoes soften and burst. Puree. For large volumes a stick or immersion blender is by far the easiest option. Reduce the puree to your desired thickness – tomatoes contain a lot of water.

Cool so that you can handle it easily, and sieve out the pips, skin etc.


Put into jars and process for 10 minutes if the puree is very hot, or 40 mins if starting from cold or warm.


Processing times

There are many sets of processing tables published on the web and in books, for example http://www.ext.vt.edu/pubs/foods/348-594/348-594.html

If you have a canner, use the instructions that came with it. The method presented here is the quick water-bath method. Other methods include using a pressure cooker, the oven, or adding cold syrup or other liquid and processing for longer.

The quick water bath method: Fill jars with hot (140F/60C) syrup or water, process starting with warm (100F/38C) water, raise to 190F/88C within 25-30 min. Maintain as below:

Soft fruit, normal pack, in water or syrup.

Process for 2 minutes.

Blackberries: If packing with apples, scald the apples as for solid pack apples.

Raspberries, Loganberries, Mulberries: Remove unsound fruit, leaves, stalks.

Strawberries: Not usually satisfactory.

Red or black currants: Remove stalks, rinse in cold water.

Rhubarb, unsoaked: Cut into short lengths. Use soft water, as hard water may leave a harmless white deposit.

Gooseberries: Top and tail.

Apple slices: Dip into lemon juice, or vitamic C (ascorbic acid) to prevent discolouring.

Soft fruit, tight packs: minimum space between the fruit.

Process for 10 minutes.

Stewed gooseberries,

Soaked rhubarb: Cut into short lengths. Pour hot syrup over the fruit and leave for 8-12 hours. Syrup can be concentrated to the original volume before filling.

Stone fruit, tightly packed.

Process for 10 minutes.

Remove stalks and rinse in cold water.

Apricots, Cherries, Damsons, Plums, Gages.

Solid packs: no space for liquid.

Process for 20 minutes.

Apples: Dip slices in boiling water for 2-3 min or steam for 3-5min until softened.

Apricots, halved.

Nectarines, halved.

Peaches, halved.

Plums, halved.

Process for 40 minutes.

Tomatoes, whole in brine.

Pears in syrup.

Quinces in syrup.

Process for 50 minutes.

Tomatoes solid pack.

Tomato puree.

4. Pickling

Pickled shallots are more delicate than pickled onions, but the same method can be used with small onions. Shallots are easy to grow as well. For about 4lbs (2Kg) of shallots you will need:

3 pints (1.4L) of brine made with 6 oz (168g) of salt and

2.5 pints (1.2L) of vinegar. (I prefer brown malt for these).

2 tsp whole coriander seed

2 tsp black peppercorns

4 bay leaves

10 cloves

4 birds-eye hot chillies ( less if you don’t like them, more if you are a chilli head)

4 oz (112g) white sugar

16 thin slices fresh ginger root


Freshly harvested shallots.

Peel and trim the shallots. This is the most tedious part of the whole process.

As with onion it helps to do this under or near running water, as the eye-watering fumes will then dissolve in the water rather than making you weep.

Soak them for 24 hours in the brine. This makes them crisper.


In a non-corrosive pan bring to the boil the vinegar, the spices (except the ginger and the chillis) and the sugar.


When it boils, turn off the heat and let the spices infuse while it cools to room temperature. Strain out the spices.

Pack the jars tightly with the onions. Add the ginger slices and the chillis if you are a chilli-head. I like a bay leaf as well.

Pour the cooled vinegar over the onions.


Seal, label and store in a cool, dark place. Because of the acid in the vinegar they do not need to be sterilised further. Leave for at least a month, preferably two if you can. They will keep for at least 6 months, and probably a year if allowed to do so.

4. Drying

Without water there is no life. Drying kills or inhibits most bugs.

If the climate is appropriate you can dry in the sun. Home drying kits are also sold, but you can do a lot with just an oven and some patience.

Tomato crisps

Delicious tomato crisps can be made easily.

Slice tomatoes with a very sharp knife (or a mandolin, if you have one) about ¼ inch/5mm thick. Put onto non-stick baking parchment or a silpat.


Put into a very low (65C/150F) oven for 24 hours. For domestic ovens set it as low as it can go, and prop the door open an inch or so. Check every now and then to ensure the tomatoes are not burning. A good oven digital thermometer makes this much easier.


Peel them carefully off the non-stick surface, and store in a jar, if you can resist eating them then and there.

Keep them in a closed jar in a dry dark place. The refrigerator is a good place to keep them as it is a dry dark place, but they do not need to be kept cold. They will keep for a year or so, if allowed to do so without being eaten, but eventually they will discolour and oxidise.


Mi-cuit tomatos in oil

Mi-cuit means half-cooked. These are even easier and even more delicious than tomato crisps. Here oil acts as an air-tight seal, keeping the bugs out. This works best for cherry tomatoes.

Slice the cherry tomatoes in half and put on a non-stick parchment or baking sheet.


One or two larger tomatoes crept in there, but that is OK. The small yellow variety is Sungold, the small reds are Gardeners Delight, and the larger ones Fireworks and Rose de Berne. Brush with extra virgin olive oil, sprinkle with salt and crushed garlic. Bake in a low (150C/300F) oven for an hour.


Pack into jars, and cover with good olive oil. The tomatoes are sufficiently acid for botulism not to be a worry. If kept in the fridge the olive oil may cloud, and should be allowed to warm to room temperature before use.


Dried Herbs

Herbs can easily be dried in a very low oven (65C/150F) for 8-12 hours or overnight. Traditionally they were dried in the residual heat in a bakers' oven. First the overnight stews would be cooked, then the meringues and finally, the remaining heat would dry herbs. The dry brick and gentle heat ensured rapid drying. You can dry herbs in the sun in dry climates, or hang bunches of woody herbs, like rosemary or lavender, upside down in a warm dry place.


Fresh herbs.


Fresh herbs.


12 hours in the oven.

Basil does not dry well, and much of the volatiles escape, but enough remain to be useful.

Crumble, and remove the stalks.

Dried herb mix ready to enliven forcemeat stuffing in the winter with reminders of summer.


5. Jams

Jams set because the pectin in the fruit will form a jell in the presence of sugar and acid. As long as there is a sufficient concentration of pectin and sugar in an acid environment, the jam will set. The usual reason for a jam not setting is not enough sugar or acid, rather than too little pectin.

The normal process of making jam is to boil the fruit with water to extract the pectin, reduce somewhat to concentrate it, then add the sugar and if need be some acid such as lemon juice or citric acid. This mixture is then boiled and concentrated until the setting point is reached. Sugar increases the boiling point, and the pectin is destroyed slowly at the elevated temperature at which the sugary jam boils, so the boiling time, once the sugar has been added, should be minimised.

The setting point can be determined by a number of methods:

a. By observation: Strongly setting jam will jell on the spoon and fall off in flakes rather than drips. This method can be reliable for an experienced jam maker.


If some jam is put on a cold plate it will set, and wrinkle when pushed with a finger.


b. More reliable methods measure an approximation of the sugar concentration. The ideal sugar concentration is 65% by weight, of which 60% is the added sugar and 5% the sugar from the fruit. Hence a standard recipe yields 10lbs (5Kg) of jam for 6lbs (3Kg) of sugar.

Using electronic scales makes this very easy. Tare the scale (see The Kitchen Scale Manifesto) to include the weight of the pan and spoon. If you are making the jam with 6lbs of sugar, reduce the starting weight of fruit by boiling to 4lbs (2Kg) before adding the sugar, bring to the boil again, and check that the final weight is 10lbs (5Kg).

c. By volume: Fill your pan with the water from 10 x 1lb (500g) jam jars. Mark this depth clearly on a stick. When you want to test the jam, take it off the heat, let the bubbles subside, then check with your stick. When the jam has been reduced to the marked level the setting point should have been reached.

d. By temperature. Use a special jam thermometer or your trusty digital thermometer.

When the jam reaches 222F/106C setting point should have been reached.

Redcurrant jelly

Redcurrants (like blackcurrants, whitecurrants, gooseberries) are naturally high in pectin, and consequently are easy ingredients for jam or jelly.

For jelly don’t bother to strip the currants from the stalks. Dump the lot in a pan, add 1pt/500ml water and heat until the juice runs.


Tip into a jelly bag or sieve lined with muslin, kitchen towel or a coffee filter and let the juice run through into a basin. Do not be tempted to squeeze or stir or you will get cloudy jelly. If you don’t think enough has been extracted, reheat with another pint/500ml of water and filter again.


Measure the amount of juice.


For each pint of juice allow 1lb (500g) of sugar. Put the juice in a pan with the sugar.


Stir to dissolve the sugar and bring to the boil. Boil rapidly for 4 minutes. Add a knob of butter to disperse the scum, or skim it off. If you like you can add a glass 1/4cup/60 ml of port or brandy now (and drink another). Adding it earlier would let too much flavour evaporate.

Bottle while hot into jars that either have a lid that seals, or seal some other way. Traditionally a paper disc dipped in brandy covered the jam, and the top was tied down with greaseproof paper and a gingham cover, but the food police no longer consider this adequate. I have always just scraped any mould that occurs off the top of the jam from a poor seal, but others do not recommend this.


Strawberry Jam


Strawberry jam is more difficult, because strawberries are low in pectin, the gelling agent in jam. Raspberries and blackberries are also low pectin. You have three options:

a. Mix the fruit with a high pectin fruit, such as redcurrants. Other combinations are blackberry or elderberry and apple, rose petals and apple.

b. Use commercial pectin.

c. Follow this recipe, but accept a softer set. You can always add liquid pectin or use Jam Sugar that has pectin in it should you need a firmer jam, albeit at a slight dilution of flavour.

Pectin is mostly contained in the cell walls of the fruit. The problem is that breaking the cell down to allow the pectin to dissolve also breaks up the fruit. In strawberry jam people prefer the whole fruit. The trick is to divide the quantity of fruit into two halves. The first half is stewed to release the pectin, and the second half then added as whole fruit.

To help prevent the fruit rising to the top of the jam, the whole fruit is marinated in the sugar overnight.

For 10lbs (5Kg) of jam start with 7lbs (3.5Kg) of hulled strawberries. The easy way to hull the strawberries is to simply cut the stalk off with a knife. Put 3lbs (1.5Kg) of the berries in a basin with 6lbs (3Kg) of sugar and leave in the fridge overnight. The next day, put the other 4lbs (2Kg) in a pan, with the juice of two lemons and reduce over gentle heat by half (or more if you can) without burning.


Add the sugar and the remaining strawberries and boil rapidly to setting point.


Note the digital thermometer probe.

Remove from the heat, stir in a knob of butter or a spoonful of oil to disperse the foam (or skim it, and have it on toast next day as cook’s perks). Stir to disperse the fruit and bottle into sealable jars. Label and store in a cool dark place.


Further reading

The Joy of Pickling - Linda Ziedrich

Blue Ribbon Preserves - Linda Amendt

Complete Book of Year-Round Small-Batch Preserving - Topp, Ellie and Howard

Home Preservation of Fruit and Vegetables - R.N. Crosset

Links to food safety sites




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