Good fruit, acid, pectin, sugar, a wide saucepan and a whole lot of stirring: here's the science behind the homeliest of culinary arts.
The beauty of home-made jam lies in its purity. It should be the colour of the original fruit and, most importantly, should taste like a sweeter, more intense version of it. The primary flavour can be enhanced with a flourish of something extra (a splash of booze, a drop of fragrant flower water or a hint of spice), but this shouldn't overpower the main player.
Choose the best quality fruit you can, and wash it well. You want firm, slightly under-ripe, yet fragrant fruit. It's at this point the fruit's pectin level is at its peak. Pectin is key to producing a well-set jam: this component of the fruit's cell wall interacts with acid and sugar to cause jam to gel. Pectin levels vary among different types of fruit. Those naturally high in pectin include apples, oranges, plums, lemons and quinces. Others, such as pears, strawberries and cherries, have very low pectin levels and need additional pectin for the jam to set. Some recipes using these fruits call for a small amount of another high-pectin fruit to assist the set. If the recipe calls for lemon juice, you can increase the pectin content further by using the seeds as well. Tie them in a small piece of muslin, then add them to the pan when you add the sugar. Remove the seeds before bottling. Alternatively, you can add commercial pectin, available from the baking section of some supermarkets and health food stores.
Acid is required to act with the pectin - no acid, no set. Lemon juice is the most common additive. As well as reacting with the pectin, it counteracts the sweetness and preserves a bright colour. Some recipes use citric or tartaric acid, but lemon juice can be substituted.
Choose a wide, heavy-based saucepan. A wider surface area allows for rapid evaporation, resulting in a fresher-tasting jam. A copper jam pan is an investment piece if you plan on making a lot of jam.
In many jam recipes, the fruit is cooked before the sugar is added. This breaks down the cell walls and releases the fruit's acids, which also aids setting. Some recipes call for the sugar to be warmed so the temperature of the hot fruit mixture is maintained. Spread the sugar 3cm deep in a baking dish and warm at 120C for 10 minutes. Once you've added the sugar, stir well to ensure it's dissolved and avoid crystallisation.
To skim or not to skim? It's all down to the quality of the sugar and fruit. There's no need to skim any foam that forms; you really only need to skim scum and impurities.
Stir frequently during the early part of cooking, then vigilantly as the jam thickens. It's only a short step from almost-there to irretrievably scorched. What you're looking for is 'setting point'. As the jam cooks, it will reduce and start to coat the back of a spoon with a slick of gel. Remove the pan from the heat, spoon a little jam onto a well-chilled saucer and place it in the freezer for 20-30 seconds. Push the edge of the jam - if it wrinkles, it's ready. If not, return it to the heat for a few more minutes, then test again. You can also use a sugar thermometer to test for the setting point. When the jam reaches 103-105C, it's done.
Have your sterile jars ready. That way it'll be hot jam going into warm jars, the best way to ensure a sterile environment. Run jars and lids on a hot rinse cycle in a dishwasher or wash in hot soapy water and rinse thoroughly. To dry, place jars on a baking tray in a cold oven, turn on to 120C and leave for 30 minutes. Remove from oven and cover with a tea-towel until required.
Use a ladle or jug to fill jars to 5mm below rims, wipe jars clean of any spills and seal firmly while jam is hot. Let the jam cool, stash it in the fridge or a cool pantry, and when you want to remind yourself what summer tastes like, tuck in.