I bought too much broccoli a couple of weeks ago. Not wanting to waste it, I boiled and froze it. But when I came to defrost and eat it, it wasn鈥檛 as nice as the pre-frozen broccoli from a supermarket. Even though I hadn鈥檛 overcooked it, it was soft and a bit mushy, and the florets had all stuck together 鈥� whereas pre-frozen ones are separate in their bag. What do the producers do to avoid my problems?
鈥� The fundamental problem with freezing anything for later use is that when water freezes it expands and forms jagged ice crystals that slice cell walls open. When defrosted, you end up with mush.
The faster you freeze something and the colder it becomes, the smaller the ice crystals are. When water is below freezing point, crystallisation starts at random nucleation points, usually around impurities, that grow larger and larger (think of an icicle) until everything is frozen. But the colder it is, the more likely it is that any given point will form an ice crystal nucleus 鈥� and the more crystals there are, the less room any given crystal has to grow.
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Almost all commercial flash freezing schemes involve circulating very cold air at temperatures as low as -196 掳C, because otherwise an insulating boundary forms at each surface of the food. To avoid florets sticking together, a fluidised bed freezer blows freezing air up from underneath so strongly that the individual chunks are suspended in the air and don鈥檛 stick together while freezing. And once frozen, they won鈥檛 stick together unless partially defrosted.
The best you can probably do at home is blanch the broccoli, then put it on a baking tray in the freezer with the pieces separated to expose their maximum surface area and to stop them from freezing together. Then put them in an airtight bag. But let鈥檚 face it, who has a freezer with that much spare room?
Ron Dippold, San Diego, California, US
鈥� There are a number of reasons why the frozen broccoli did not turn out well. Commercially produced frozen vegetables are generally more palatable than home-frozen vegetables because they have been prepared soon after harvesting and will not have been packed, transported, warehoused and at some point exposed to room temperature. The main reason, however, is that factory freezing is done at a far lower temperature than that reached by the average domestic freezer, which is generally set somewhere between -18 掳C and -30 掳C. There are various methods in use commercially; cryogenic freezing with liquid nitrogen can produce temperatures as low as -196 掳C.
鈥淔reezing continues the breakdown of the broccoli鈥檚 cells started by the boiling process鈥�
The small crystals that occur with fast freezing at ultra-low temperatures do minimal damage. The pioneer of commercial freezing, Clarence Birdseye, noted that fish caught in the Canadian Arctic froze almost instantly in the ultra-cold air after being removed from the water and remained in good condition. His techniques have been refined up to the present day.
Your correspondent also boiled the broccoli. Freezing would then have continued the breakdown of the cells started by the boiling. Blanching the florets is generally recommended for home freezing. This involves brief immersion in boiling water followed by chilling in cold water, so the vegetable is not cooked through but natural enzyme activity in the plant tissues is slowed. If this doesn鈥檛 happen the plant鈥檚 enzymes remain active and can break down the tissues, even below 0 掳C.
Chris Warman, Hinderwell, North Yorkshire, UK
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This article appeared in print under the headline 鈥淕reen machine鈥�