The science and magic of jam-making.

The science and magic of jam-making | Andy Connelly | Science | The Guardian:
"The rule is, jam tomorrow and jam yesterday – but never jam today.
/Lewis Carroll/"
The recipe:
Jam recipes mostly comprise equal weights of fruit and sugar.
You can play with this 1:1 ratio as much as you want, but too much fruit and you may lose the preserving effects of the sugar; too much sugar and it may crystallise during storage.

The choice of fruit for jam-making is almost endless.
I always try to use seasonal fruit to get the best flavour for my jam.
Slightly unripe or "just ripe" fruit will form a jam more easily than very ripe fruit as it contains more pectin and is more acidic.
2 tablespoons of lemon juice is the equivalent to the juice of one lemon.

1kg fruit
1kg granulated sugar
Lemon juice and/or pectin (depending on the fruit you use)
* Some fruits are naturally high in acid but others are low so we need to compensate for this by adding acid at the start before we start cooking.

Start by removing any leaves and twigs, wash the fruit if you feel it necessary, and remove any stones.
Add the fruit to a pan big enough to ensure the fruit does not reach more than halfway up the side.

- Heating
Place your pan on a low heat.
As the fruit heats through, a glorious fresh, warm smell will fill the air.
Prolong this by heating slowly until a very gentle boil is reached.
Cook until tender – any longer and the fruit will lose its shape.
No sugar is added at this stage because a high sugar concentration can cause water to be removed through osmosis and result in hard, unappetising fruit.
You might need to add a little water though if your fruit is very dry.

Boiling is key to jam-making because it releases a long fibrous compound known as pectin.
Even though pectin only makes up 0.5-1% of the jam, you will have to learn to play it like a snake charmer or you will add your tears to your mixture.

The first handling of a jam the morning after making is full of trepidation.
The jam maker's nightmare is to find a wet, sloppy strawberry sauce, not the semi-rigid, elastic substance that chemists describe as a "gel": a liquid dispersed in a solid.
Pectin forms the solid that holds the liquid together.
Some fruits, including apples, blackberries and grapes, can do this alone as they contain sufficient pectin.
Some fruits are low in pectin, however, and so need a little more help, for example apricots, rhubarb and strawberries.

You can add commercial pectin, which is extracted from the white inner skin (the pith or "albedo") of citrus fruits or from apples.
You can also buy special jam sugars with added pectin.
But jam makers of yore discovered through trial and error that if they mixed low-pectin fruits with high-pectin fruit (often apple) they could create the perfect consistency.
Personally, I like to mix high and low pectin fruits to keep it "in the garden", for example I might add a cooking apple to my blackberry jam.

We add sugar, which binds to the water molecules and frees up the pectin chains to form their network.
The negative charges are reduced by acid naturally found in the fruit or added to the mixture.
The acid reduces the electrical charge on the pectin branches and so allows them to bond.
To increase acidity lemon juice can be added.
But be careful: if your mixture is too acidic, this will damage the pectin.

As a rough guide, the juice of a whole lemon (30-40ml) will be needed for very low acid fruit, whereas half a lemon will be enough for medium acid fruit, and you won't need any for the high acid fruits.
In general, fruit with high pectin will also have high acidity and vice versa.

- Adding the sugar:
To warm the sugar, put into a cool oven, Gas Mark 1 (140°C/275°F), for a few minutes before adding to the fruit.
Add the sugar and stand back as it starts to foam up the sides of the pan.
A sentimentality-inducing childhood smell of sweet fruit fills the air.
Allow the sugar to dissolve over a low heat then bring rapidly to the boil.
Avoid stirring at this point as you may break up the fruit or cause crystallisation.
A foamy scum may form on the surface of the jam; this is normal and can be removed by adding a little butter (about 20g) to break the surface tension or by skimming it off with a spoon while your mixture is cooling.

This is the exciting bit: the smell of jam fills the air and you're desperate to get it into jars and on to some toast, but patience is required.
However, you will normally have to wait around 5–20 minutes for the pectin network to form.
The time varies depending on the type of fruit, the type of pan etc.
A wide-mouthed pan is ideal as it allows water to escape, helping to bring our precious pectin molecules closer together.

- Time to pour:
There are many ways of telling when your pectin network has formed and you are ready to pour the jam out.
It normally forms at around 104-105C, when the sugar content is high enough to allow the pectin branches to join.
Unfortunately, temperature is not a reliable signal because it varies according to acidity, amount of pectin, etc.
My preferred method is direct measurement.
Pour a little blob of jam on to a cooled saucer, let the jam cool in the fridge and then push against the side of it with your finger.
If the surface wrinkles it means the pectin network has solidified, setting point has been reached, and you should take the mixture off the heat.
If you don't boil it long enough the pectin network will not form properly.
Boil it too long you risk not only losing the fresh flavour and colour of the jam but having a jam with the texture of set honey.

- Cooling and decanting into jars:
This is my favourite part, but I allow the jam to cool and thicken for about 10 minutes before pouring it into jars, to prevent the fruit from floating to the top.
Try not to leave the jam too long, however, as lukewarm jam is a great breeding ground for mildew spores which are present in the air.

To keep you busy while you are waiting, get your pre-sterilised jars ready.
You will need five or six of them.
My preferred method of sterilisation is to wash them in soap and hot water, rinse them with clean water to remove any detergent, and dry them in the oven at about 160C.

Jams can remind us of summers past, even summers several years gone.
It is the sugar and acid that makes this possible.
Jams usually contain about 60% sugar, which is enough to stop most microorganisms growing.
The high acidity also makes it an unpleasant place to breed.
However, some moulds can grow even in these harsh conditions and so it is important to take care when preparing and sterilising your jars.

The satisfying gurgle of jam being poured is music to the ears.
Each jar should be topped up to just less than a centimetre below the surface.

- Capping and storage:
I remember being puzzled why my parents always put a waxed paper disc on the surface of their homemade jam.
I now know that it prevents the condensation of water on the jam's surface.
Condensed water would dissolve sugar, producing an area of low sugar concentration and allowing mould growth.
I must confess that jam never sits in my cupboard long enough to worry about this.

Now that our jam-making is at an end, there is only one stage to go: eating.
I always struggle not to get overexcited and try my jam straight away before it has developed its "quiver".
I am torn; I feel I should wait until the autumn, when I can close my eyes and relive summer.
But I'm realistic.
I wait till the next day to spread the noble jam thickly on a delicious chunk of simple bread and butter.

- Jam and Jelly Making – Fruit, Acid, Sugar and Water:
Some fruits are naturally high in acid but others are low so we need to compensate for this by adding acid at the start before we start cooking.

- Thane Prince: 'There’s something quiet and proper about jam-making' - Telegraph:

- How to make perfect strawberry jam | Life and style | The Guardian:
Makes 4 x 200ml jars

2kg small ripe strawberries
1.7kg jam sugar
Juice of 2 lemons

1. Hull the strawberries and discard any rotten ones.
Set aside about 10 of the smallest berries, and then mash the rest up into a rough pulp.
Put into a wide, thick-bottomed pan, add the sugar and the lemon juice, and bring to the boil.
Add the remaining strawberries to the pan, and put a saucer in the freezer.

2. Boil the jam for about 15 minutes, stirring regularly checking the setting point every minute or so during the last 5 minutes.
To do this, take the cold saucer out of the freezer, put a little jam on it, and put it back in to cool for a minute.
If it wrinkles when you push it with your finger, then it's done.
Strawberry jam is unlikely to set very solid though, so don't expect the same results as you would with a marmalade.

3. Take off the heat and skim off the pink scum.
Pour into sterilised jars and cover with a disc of waxed paper, seal and store.

- Strawberry jam | BBC Good Food:
1kg hulled strawberry
750g jam sugar
juice 1 lemon
small knob of butter (optional)

- Diana Henry: jam making, a guide:
There are purists who will continue to make it the old-fashioned way but Pam's loose-set raspberry jam contains 1.5kg fruit and only 750g sugar.
Plain apple jelly can be flavoured with lavender, thyme, even the scent of Earl Grey tea.
Lemon juice can be added to low-acid fruits to help the release of pectin and ‘brighten’ the flavour;
I often add it after the setting point is reached and before potting, to give a jam freshness.
I prefer a soft set and a fresher flavour.
Use granulated sugar, as large crystals dissolve quickly.
I often use sugar with added pectin, usually called jam sugar (not the same as preserving sugar) to help set jams with lower sugar or pectin.
Never use it with high-pectin fruit, or you'll get a very hard jam.
Setting point for jam is 104.5C/220F, though high-pectin fruits can set a couple of degrees lower.
In Sweden I found what I call ‘nearly’ jam: they just boiled fruit with lemon juice and sugar - lower in sugar than usual.
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