【英语时差8,16】急速冷冻（在线收听）

Have you ever noticed how frozen vegetables bought at a supermarket tend to cook up like fresh ones; retaining much of their firmness, while cooked home frozen veggies tend to be softer and mushier? Well, this difference is due to the different freezing processes the veggies undergo at home, and before arriving at your local supermarket. Vegetables contain a lot of water, for example about 95 percent of your average tomato is water, and when we say that a vegetable is frozen what we really mean is that all the water it contains is frozen. Frozen water itself is made up of countless minute crystals. But as minute as they are, their size differs greatly depending on how fast the water is frozen; the faster the freeze, the smaller the crystals. It takes at least thirty minutes to freeze a fresh vegetable in your home freezer at about zero degrees Fahrenheit, and so the crystals in the frozen water your veggie carries are quite large. Those large crystals form in or near individual cells in the vegetable, and often rupture or completely destroy them. When the veggie is thawed, and then cooked, it has lost much of its structure the cells provided, and so, too, much of its firmness. On the other hand, a frozen veggie you buy from the supermarket is initially frozen very quickly, in about three minutes and at a temperature of at most minus four degrees Fahrenheit. The crystals which form in the water it carries are small enough not to damage too many of the veggie’s cells, and so the cooked veggie retains much of its structure and firmness.