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Freezing and Blanching

Medium term storage

Why Freeze Food?

  •  Quick, easy, convenient
  • Preserves food colour, flavour and texture
  • Retains much of the original food value
  • Makes it possible to have great variety of seasonal foods year round

How Freezing Preserves Foods

  1. Freezing greatly reduces the growth of yeasts, bacteria and molds that can grow on food surfaces.  Growth is almost stopped at -18°C or lower; however, most microorganisms continue to grow again as temperatures rise (ie. thawing).
  2. Freezing slows the activity of the enzymes that cause ripening in fruit and vegetables.

 How to Minimize Quality Loss in Frozen Foods

  • Control enzyme activity (blanch or add natural preservatives)
  • Control oxidation (store in airtight containers)
  • Control ice crystal size (keep freezer at constant temperature (-18°C or lower))
  • Control freezer burn (package in proper containers)

What is Blanching?

Vegetables often require further control of enzyme activity then meat and some fruits.  The process of blanching inactivates the naturally present enzymes that cause undesirable changes in food flavour, colour and texture.  Almost all vegetables require blanching prior to freezing, with a few exceptions.

To blanch you will need a large kettle with a cover (4 L water to about 500 g vegetables or 8 L for leafy greens).  Place vegetables in wire basket or tie loosely in cheesecloth bag leaving length of string for ease of handling.  Lower vegetables into vigorously boiling water, cover and immediately start counting blanching time. Keep heat on high so water quickly returns to a boil. Once blanching time is up, chill vegetables immediately in cold running water. If water is not cold enough then add ice. Leave vegetables in cooling water for same time used in blanching but no longer than necessary. Drain cooled vegetables thoroughly and pack in freezer containers (label and date) and place in freezer.

Make sure to follow exact blanching times for each type of vegetables as under-blanching does not destroy all enzymes while over-blanching overcooks and reduces flavour and nutrient content.

Adding Natural Preservatives

Some fruits brown during frozen storage (ie. apples, peaches, apricots).  This can be avoided by adding natural preservatives such as salt, sugar, or lemon juice/ascorbic acid.  Fruit can also be packed in sugar syrup to prevent browning.

Table 1: Measurements for different concentrations of sugar syrup

Type of syrup

Sugar

Water

Yield

Thin

250 ml

500 ml

About 650 ml

Moderately thin

250 ml

400 ml

About 550 ml

Medium

250 ml

250 ml

About 375 ml

Heavy

250 ml

200 ml

About 325 ml

7 Steps to Successful Freezing

  1. Select fresh, high quality foods.
  2. Prepare foods quickly but carefully (cleaning, cutting, blanching, packing).
  3. Package in odorless, tasteless, moisture proof, vapour proof packaging to prevent dehydration or absorbing flavours.
  4. Freeze at -18°C or lower ASAP (if delay place package in fridge as soon as sealed).
  5. Store at -18°C or lower with as little temperature fluctuation as possible.
  6. Thaw carefully and use soon after thawing.
  7. Cook according to directions. Do not overcook frozen vegetables.

Packaging

  • Plastic freezer bags – Press out as much air as possible, twist top several times and fold over in loop. Secure with metal closure.
  • Freezer wrappings (aluminum foil, freezer paper, plastic film) – Fold over edges forming tight seal and leaving as little air space as possible. Seal with masking tape.
  • Freezer containers (plastic, aluminum foil, cardboard) – Square/rectangle containers store more compactly then round.  Select either flexible or rigid plastic with tight fitting lids or cardboard with thick plastic coating or lined with plastic to prevent drying out.  When packing liquids leave 1-2cm headspace.

 Freezing Fruits 

  • Select fruits at ideal maturity (ripe but firm)(not bruised, overripe or beginning to spoil).
  • Berries, sour cherries and rhubarb freeze the best (pears, sweet cherries become too soft on thawing).
  • Peaches, apples and apricots require ascorbic acid treatment to prevent discoloration.
  • Wash fruit thoroughly and then pack unsweetened or use either a dry sugar pack, dry pack with ascorbic acid, syrup pack or syrup pack with ascorbic acid.
  • Blueberries, cranberries, currants, gooseberries, rhubarb and saskatoons freeze well without sugar or syrup. Freeze in premeasured quantities.
  • Fruits may be stored for 1 year at -18°C

Table 2: Preparing and packing fruits

Fruit

Preparing

Packing

Apples

Pies & puddings: peel, core, slice

Applesauce

Pack in dry sugar using 50 ml sugar to 1 L fruit

Sweeten if desired and pack cold

Apricots

Pudding or as fruit: halve and pit

Pies: cut in quarters and pit

Pack in moderately thin syrup

Pack in dry sugar using 175 ml sugar to 1 L fruit

Blueberries

Stem

Pack without sugar

Cantaloupe

Halve, remove seeds and soft fiber.  Cut in 2 cm cubes or balls

Pack in dry sugar using 125 ml sugar to 1 L fruit

Cherries (sour)

Pit

Pack in dry sugar using 250 ml sugar to 1 L fruit; or pack in heavy syrup to cover

Currants, Cranberries, Saskatoons

Stem

Pack without sugar or syrup

Fruit juices

Extract juice for jelly making

Pack without sugar

Fruit salad/fruit cocktail

Prepare as individual fruit

Pack in thin syrup to cover

Gooseberries

Remove stems and blossom ends

Pack without sugar or syrup

Peaches

Dip in boiling water ½ - 1 min, then in cold water. Remove skins and pits. Slice

Pack in moderately thin syrup to cover; or pack in dry sugar using 175 ml sugar to 1 L fruit

Plums

Halve and pit

Pack in dry sugar using 175 ml sugar to 1 L fruit; or pack in thin syrup to cover

Raspberries

Leave whole

Pack in dry sugar using 175 ml sugar to 1 L fruit; or without sugar

rhubarb

Cut stalks in 3 cm lengths

Make into sauce

Pack without sugar

Sweeten to taste and pack cold

Strawberries

Leave whole, cut in quarters or slice

Leave whole

Pack in dry sugar using 125 ml sugar to 1 L whole berries, or 175 ml sugar to 1 L quartered or sliced berries

Pack without sugar.

Vegetables

  • Select high quality, fresh, young, tender, ideal maturation (not tough, starchy or woody)
  • Asparagus, beans, peas, spinach, whole kernel corn and mashed squash freeze well
  • Corn on cob can develop off flavours unless blanched for correct time.  Celery, lettuce, tomatoes and cucumbers become limp on thawing – unless tomatoes pureed for sauce or cooked slightly for use in soups and stews. Eggplant darkens and becomes watery.
  • Carrots and parsnips can be frozen but best to store in cool room.
  • Cooked beets and raw onions are best frozen sliced or diced as texture will become rubbery and limp on thawing.
  • Wash, blanch and pack.
  • Vegetables may be stored for 1 year at -18°C

Table 3: Preparing and packing vegetables

Vegetable

Preparing

Blanching

Packing

Asparagus

Remove tough ends. Cut in uniform lengths

Med – 3 min

Lrg – 4 min

Chill, drain and pack

Beans (green/wax)

Trim ends. Leave whole or cut in 3 cm pieces

Cut – 3 min

Whole – 4 min

Chill, drain and pack

Beets

Leave root ends on; cut off tops leaving 3 cm stems. Cook in boiling water until tender. Cool. Peel and slice

None

Pack

Broccoli

Remove woody stems and trim. Cut through stalks so that pieces are not more than 3 cm across

Med – 3 min

Lrg – 4 min

Chill, drain and pack

Brussel Sprouts

Trim stems and outer leaves

Sm – 3 min

Med – 4 min

Lrg – 5 min

Chill, drain and pack

Cabbage

Trim outer leaves and core. Cut in wedges or shred coarsely

Wedges – 2 min

Shredded – 1 min

Chill, drain and pack

Carrots

Remove tops and scrape or peel. Leave small carrots whole. Cut larger carrots in 1 cm slices or dice or cut lengthwise in fingers

Cut – 3 min

Whole – 5 min

Chill, drain and pack

Cauliflower

Break heads into small flowerets about 3 cm in diameter

3 min

Chill, drain and pack

Corn – whole kernel

- on cob

Remove husks and silk

Remover husks, trim cobs to even lengths

4 min

Sm – 7 min

Med – 9 min

Lrg – 11 min

Chill, cut kernels from cob. Pack

Chill, drain and pack

Herbs

Chop

None

Freeze on tray before packing

Mushrooms

Slice. Sauté 500 ml in 30 ml butter 4 min

None

Drain and pack

Onions

Remove outer skin, root and stem ends. Chop

None

Pack or freeze on tray before packing

Parsnips

Remove stem and root ends. Peel. Cut in 2 cm fingers or 1 cm slices

Fingers – 1 min

Slices – 1 min

Chill, drain and pack

Peas

Shell

2 min

Chill, drain and pack

Peppers

Leave whole or remove seeds and half stems, cut in half or chop

None

Pack dry or freeze chopped pepper on tray before packing

Pumpkin

Cut or break apart, remove seeds and fibers. Cut in chunks. Steam, boil or bake until tender. Cool and scoop from rind. Mash or sieve

None

Pack

Spinach, Chard

Wash thoroughly. Cut chard in 3 cm pieces or separate leafy parts from stalks and cut stalks in 10 cm lengths

2 min

Chill, drain and pack

Summer squash

 Cut in 1 cm slices

2 min

Chill and drain. Freeze on tray before packing

Winter squash

Cut in pieces and steam, boil or bake until tender. Cool, scoop from rind and mash

Peel and dice (butternut)

 

None

2 min

Pack

Chill, drain and pack

Tomatoes

Peel by dipping in boiling water for 30 – 60 seconds, cooling in cold water and gently slipping off skin. Cut in quarters. Add 5 ml salt, dash pepper and 5 ml sugar to 1 kg tomatoes and cook gently until tender (5-6 mins)

None

Pack

Turnip

Peel, dice and boil until tender. Mash

Peel, dice and blanch

None

2 min

Pack

Chill, drain and pack

Ref: Ontario Ministry of Agriculture and Food. (1990). Freezing Foods. Food Advisory Services. Canada Department of Agriculture.

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