food preservation heat treatments © pdst home economics
TRANSCRIPT
FOOD PRESERVATION
HEAT TREATMENTS
© PDST Home Economics
HEAT TREATMENTS
Fruit and vegetables can be preserved at home using high temperatures.
Methods used are: jam - making, chutney - making and bottling.
JAM MAKING
Jam is cooked fruit boiled with 65% sugar.
HOW SUGAR PRESERVES FOOD
Sugar dissolved in food forms a solution with a low water concentration.
Microbial cells have a high water concentration and the cells are enclosed by a semi-permeable membrane.
The water is drawn out of the microbial cells by osmosis and the cells are dehydrated and destroyed.
PRINCIPLES OF JAM - MAKING
High temperatures are used to destroy enzymes and microbes in the fruit.
Jam is sealed in pots to prevent re-entry of microbes.
65% sugar prevents microbes growing in jam once opened.
Pectin, acid and sugar make jam set.
WHAT IS NEEDED FOR GOOD JAM
Fruit
Sugar
Acid
Pectin
FRUIT
Just ripe. Good quality. Acidic. High in pectin. If low in acid and pectin,
combine with a fruit high in these e.g. apples or use jam sugar.
SUGAR Preservative. Helps setting. Too much causes
crystallisation. Too little causes
fermentation and jam won’t set.
Jam sugar / Sure-set sugar is sugar crystals coated with pectin and acid.
Using Jam sugar means a shorter boiling time and as it is purer, less scum is produced.
ACID Draws out pectin from
cell walls of fruit. Prevents
crystallisation. Improves colour and
flavour of jam. If there is too little
acid in fruit, add lemon juice, citric acid or use jam sugar
PECTIN
Polysaccharide. In cell walls of ripe
fruit. Fruit with a low
pectin content can be combined with a high pectin fruit. Alternatively, pectin can be added or jam sugar can be used.
CHARACTERISTICS OF A GOOD JAM
Clear
Well set
Keeps for a year
Bright colour
Fruity taste
EQUIPMENT
Heavy based stainless steel saucepan or preserving pan.
Wooden spoon. Sugar thermometer. Measuring jug. Jam jars. Jam pot covers- wax and cellophane. Elastic bands and labels.
STAGES IN JAM - MAKING Weigh fruit and sugar accurately. Jars - wash and sterilise in oven- no chips or
cracks. Prepare fruit- wash, peel, remove stones &
chop. Simmer fruit & water according to recipe. Add pre-warmed sugar & dissolve. Boil rapidly. Test for setting - ( temperature
105ºC , plate/wrinkle test or flake test) Skim jam & fill jars to 5mm. Cover immediately with waxed discs &
cellophane. Clean jar & label - name & date. Store in a cool, dark, dry, well – ventilated area.
CHUTNEY - MAKING
Chutney is fruit, vegetables or a mixture of both boiled with sugar, vinegar and spices until the mixture is thickened.
Chutney is served with meat, cheese, sandwiches, salads, burgers or as an accompaniment to curry.
PRINCIPLES
High temperature used to destroy enzymes and microbes.
Sealing prevents recontamination.
Sugar preserves by dehydrating microbial cells.
Vinegar lowers pH preventing growth of microbes.
MAKING CHUTNEY
Same equipment as for jam. Plastic – lined, metal jam pot lids or fabric circles steeped in melted wax are used instead of cellophane covers. These prevent evaporation.
Follow recipe and method carefully.
Chutney is kept for a few months before eating to let the flavour mellow.