FRUIT AND VEGETABLES: VEGETABLE PRESERVATION
Diagram
Preserved by heat, or preserved by cold
Canned vegetables
Sealed in a can and heated to sterilise. Shelf stable at room temperature, but soft from the heat.
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Frozen vegetables
Blanched and frozen fast. Kept frozen, firmer and brighter than canned, but need a cold chain.
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The key point
One uses heat to sterilise, the other uses cold to hold the vegetable still. This pair is distinct from the built article on canning versus bottling, which compares two packing methods, not canning against freezing.
Figure 1: Canned vegetables are heat sterilised in a sealed can; frozen vegetables are blanched and kept frozen. Drawn in our own style from Sinha et al., Handbook of Vegetables and Vegetable Processing, chapters on refrigeration and freezing preservation and on principles of vegetable canning, and Mallett, Frozen Food Technology. This pair is distinct from the built article on canning versus bottling.
Introduction
Canned and frozen vegetables both keep vegetables for months, but they use opposite forces. Canned vegetables are sealed in a can and heated until the food inside is sterile, so the can keeps at room temperature. Frozen vegetables are blanched to stop enzymes, then frozen fast and kept frozen, so cold holds the vegetable still. The heat route gives a soft vegetable that needs no cold chain, while the cold route gives a firmer, brighter vegetable that needs a freezer all the way to the kitchen.
Sinha et al. cover both refrigeration and freezing preservation and the principles of vegetable canning, and Mallett covers frozen food technology. This article follows those books.
Quick comparison
Table 1: Canned vegetables compared with frozen vegetables
| Aspect | Canned vegetables | Frozen vegetables |
|---|---|---|
| Preservation | Heat sterilisation in a sealed can | Blanching, then fast freezing and frozen storage |
| Storage | Room temperature, shelf stable | Kept frozen, well below 0 C |
| Cold chain | Not needed | Needed all the way from factory to kitchen |
| Texture | Soft, because the sterilising heat breaks down the structure | Firmer, but softer than fresh, because ice crystals damage cells |
| Colour | Duller, because heat changes pigments | Brighter, because blanching stops the enzymes that dull colour |
| Flavour | Cooked flavour from the heat | Closer to fresh, because little heat is used |
| Nutrients | Some loss from heat, especially heat sensitive vitamins | Largely kept, with some loss in blanching |
| Microbes | Killed by sterilisation, including spores | Held back by freezing, but not killed |
| Shelf life | Long, often a year or more at room temperature | Long, for months, while kept frozen |
| Typical vegetables | Beans, peas, carrots, sweetcorn, tomatoes | Peas, beans, broccoli, spinach, mixed vegetables |
Scientific explanation
Definition
Canned vegetables are vegetables sealed in a can and heated until the contents are sterile, so the sealed can keeps at room temperature. Frozen vegetables are vegetables blanched to stop enzymes, then frozen fast and kept frozen, so cold holds the vegetable still. One is preserved by heat, the other by cold, and Sinha et al. cover both methods in the Handbook of Vegetables and Vegetable Processing.
Working principle
In canning, the sealed can is heated in a retort until the coldest point reaches the sterilising temperature and holds it long enough to kill spores, so the food inside is sterile and keeps at room temperature. The heat also softens the vegetable and dulls its colour. In freezing, blanching first stops the enzymes that would spoil flavour and colour in the freezer, and then fast freezing turns the water to small ice crystals. While frozen, microbes cannot grow and enzymes are stopped, so the vegetable keeps for months, but it must stay frozen, because thawing lets microbes grow again.
Apparatus
The books used here name no laboratory apparatus for telling the two apart, so no apparatus list is given.
Measurement procedure
The books used here name no single standard laboratory procedure for telling canned from frozen vegetables. The distinction is made from the pack and the storage: a sealed can kept at room temperature is canned, and a pack kept frozen is frozen. The books give no worked food example with numbers for this comparison, so none is given.
Factors affecting readings
For canning, the can size and the vegetable thickness change the heating time, because heat must reach the centre. For freezing, blanching time and freezing speed decide texture and nutrient loss. Storage temperature decides how long a frozen vegetable keeps, and a break in the cold chain spoils it.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Canned vegetables are used where no freezer is available and a long shelf life is wanted.
- Frozen vegetables are used in cooking, soups and ready meals, where a firmer texture is wanted.
- Canned beans and sweetcorn are used in salads and sandwiches straight from the can.
- Frozen peas and broccoli are used where the fresh season is short.
- Both are used in catering to cut preparation time and waste.
Advantages and limitations
- Canned vegetables: advantage: shelf stable at room temperature, with no cold chain needed. Limitation: soft texture and cooked flavour from the sterilising heat.
- Frozen vegetables: advantage: firmer texture, brighter colour and flavour closer to fresh. Limitation: need a freezer all the way to the kitchen, and thawing softens them further.
- Canned vegetables: advantage: sterile, so they keep for a year or more. Limitation: some nutrients are lost to the heat.
FAQ
Which is more nutritious, canned or frozen vegetables?
Frozen vegetables usually keep more nutrients, because they meet little heat, while canning uses sterilising heat. Both lose some nutrients in preparation.
Do canned vegetables need refrigeration?
No, until opened. The sealed can is sterile and keeps at room temperature.
Why do frozen vegetables need a cold chain?
Because freezing only holds microbes still, it does not kill them, so thawing lets them grow again.
Is this the same as canning versus bottling?
No. That built article compares two packing methods. This article compares canning against freezing.
References and further reading
- N. K. Sinha et al. (eds.), Handbook of Vegetables and Vegetable Processing, Wiley, 2010 (Google Books page read).
- C. P. Mallett, Frozen Food Technology, Springer, 1993 (Google Books page read).
