Vol. XIII · Keeping Food Plate 6 of 10jam, preserves, sugar, set point, 104 °C, water activity, pectin, candied fruit, low-sugar jam, marmaladeMS

Jam and Candied Fruit.

The jam thermometer reads 104.5 °C at the set point because that is where the syrup is two-thirds sugar.

Traditional jam is fruit cooked with about its own weight of sugar and boiled until much of the water has gone. The sugar does three jobs: with the fruit's pectin and acid it sets the gel (X.9); it sweetens and preserves colour; and it lowers the water activity to about 0.80–0.85, where bacteria cannot grow and most yeasts and moulds grow only slowly. A sealed jar of such jam keeps for a year; once opened, the surface can still grow mould, so it goes in the fridge.

The boiling point tracks the concentration (VII.6). A sugar syrup boils at 104–105 °C at sea level when it is about two-thirds sugar, which is why that is the "set point" on a jam thermometer — lower at altitude, by the same amount water's boiling point falls. Low-sugar jams, made with less sugar and low-methoxyl pectin (X.9), have a much higher water activity: they taste of fruit, but they must be refrigerated or processed by a tested method. Candied fruit takes the idea further, soaking fruit in ever stronger syrups until its water is replaced by sugar. The workshop makes jam.

Below: the sugar concentration, water activity and yield of jam boiled to the temperature you choose (water bench: syrup boiling point and sucrose water activity), with the growth floors of microbes marked.

What you will learn
Workshop
Water bench: the syrup concentration whose boiling point equals the temperature you choose (Norrish water activity up to 80 %); fruit taken as 10 % sugars and 5 % other solids; the other solids are counted in the yield but not the syrup. Floors: most bacteria 0.91, most yeasts 0.88, most moulds 0.80 (ICMSF).

Two-thirds sugar

Boiled to 104.5 °C at sea level, the jam's syrup is about 67 % sugar with a water activity of 0.85: below the floor of bacteria and most yeasts, close to that of moulds. Stopped at 102 °C, it is only 52 % sugar with a water activity of 0.93 — runny, and a home for yeasts and moulds. At 2 000 m the set point falls to about 97.8 °C; take the boiling point of water where you are and add about 4.5 °C.

Practical jam-making. Use a wide pan so water leaves fast and the fruit keeps its colour; add lemon juice to low-acid fruit for the pectin set; boil hard to the set point and confirm with the wrinkle test on a cold saucer (X.9). Pour into hot, clean jars and seal. For long storage at room temperature, process sealed jars in a boiling-water bath for the time a tested recipe gives (XIII.8); refrigerator jam and low-sugar jam keep for weeks in the fridge. Throw away jam with mould, rather than scraping it off: moulds can spread invisibly and some make toxins.

At the stove

Make a small batch of jam with a thermometer. Note the temperature each time you do the wrinkle test: the first wrinkle usually comes within a degree of 104.5 °C at sea level.

Candied (glacé) fruit, made by soaking fruit in syrups strengthened day by day for weeks, was a luxury of medieval Arab and Venetian confectioners; Chinese 蜜饯 preserve fruit the same way.

Terms

Set point
The boiling point at which jam reaches setting concentration, about 104–105 °C at sea level.
Low-sugar jam
Jam with too little sugar to preserve itself; kept cold or processed.
Wrinkle test
A drop of jam on a cold saucer wrinkles when pushed at the set point.
蜜饯
Chinese candied and preserved fruits.
Jam keeps because it is about two-thirds sugar, water activity near 0.80–0.85. Boil to about 104.5 °C (less at altitude); low-sugar jam needs the fridge.

Sources

  1. R. S. Norrish, "An equation for the activity coefficients and equilibrium relative humidities of water in confectionery syrups", J. Food Technol. 1, 1966 (K = −6.47 for sucrose).
  2. ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens, Blackie, 1996 — growth limits (minimum temperature, pH and water activity) and typical heat resistance; the course rounds them.
  3. USDA National Institute of Food and Agriculture, Complete Guide to Home Canning, 2015 revision; National Center for Home Food Preservation.