Sunday, 10 July 2011

Marmalade

I made marmalade today, something I’ve never done before.  I used mandarins from Johanna Knox’s garden.  Marmalade is citrus jam and the word marmalade comes to us from the Greek melimelon (a quince jam made with honey; the Greeks didn’t have cane sugar) via Portuguese.
Clockwise from top left: mandarin, navel orange, lime, lemon.
Citrus is a genus in the family Rutaceae, and the particular kind of fruit they have is a hesperidium.  Slice it cross-wise and you’ll see radiating walls like the spokes of a wheel; these are the walls of the individual carpels that make up the ovary and fruit.  Outside the carpels is the rind, made up from the ovary wall.  The rind is rich in flavorsome oils, which will burn dramatically if you squeeze lemon peel next to a lighted match. 
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The inside of each locule (cavity) is packed with juice-filled hairs, and a few seeds along the central core of the fruit.  Each of those juice-filled bags within a carpel contains lots of juice-filled cells, and the cell walls have a special role in the making of marmalade.
Sliced lemon, showing 8 carpels, pith, and rind.
I’m indebted to a wonderful book for much of what follows: On food and cooking, by Harold McGee.  I have the 2004 edition.  I can’t recommend this book highly enough: food, botany, and chemistry—who could ask for more!
Plant cells are enclosed in relatively rigid cell walls made of cellulose.  Each cell secretes its own cell wall, by transporting vesicles of cellulose to be extruded through the cell membrane. The individual cells are stuck together by a thin layer of pectin, the middle lamella.  Pectin is an essential ingredient for marmalade, because it makes the gel that gives solidity to the jam. 
Pectin is a polysaccharide, a long chain of sugar molecules joined end-to-end.  The pectin chains are attracted to each other and form the glue that binds the cells tightly together.  But when fruit pulp is boiled in water, the pectin strands separate and they can’t come back together to find each other among all that water.  So the first step in making marmalade is to boil up the fruit in water.  I boiled mine for 2 hours.
Once the pectin is boiled out of the middle lamellae into solution, the jam-maker has to reassemble the pectin gel to set the jam.  Three properties of pectin are useful here.  First, adding sugar to the solution gives the pectin molecules something to coalesce around; they’re attracted to the sugar molecules.  Secondly, an acid environment helps the pectin gel to form.  I beefed up the acidity by adding the juice of a couple of lemons.  Thirdly some of the water has boiled off by now, and more will boil off from the sugar solution because adding sugar raises the boiling point to about 110C.  When these conditions are right, the pectin forms a gel as the solution cools, trapping water in the network of pectin strands: marmalade.
Four and a half jars of Theobrominated marmalade.

My marmalade turned out pretty cloudy, because I got lazy and simply put all the fruit through the food processor to make a pulp; then I sieved a lot, but by no means all, of the solids out of it. The left-over pulp will be great stuff for cleaning the glass of the shower; citrus juice is so acid that it’ll dissolve the scale (calcium carbonate) that forms as water dries on the glass.   The marmalade also set pretty solid; perhaps I over-sugared it.  

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