Q. I am accustomed to seeing commercials for chicken. But it occurred to me that brand-name birds are a recent phenomenon and that "generic" chicken is disappearing. What are the reasons for this trend?

A. The change is due to the fact that nowadays many producers take the process of raising the birds from hatching baby chicks all the way through to cutting, packaging and even pricing. In production circles, it is referred to as "vertical integration" of the industry. The practice now covers 40 percent of broilers nationwide, and much of it is concentrated among a handful of producers. In 1983, the four largest vertically integrated producers claimed 34 percent of the total broiler market.

Brand-name chickens are more common in some places than in others. In New York, for example, over 90 percent of broilers were sold by brand in 1982. Producers spent more than $4 million advertising their products in that state and $30 million nationwide, with the emphasis in advertising on quality and color of the bird, as well as on the fact that they are high in protein and low in fat.

The concept of vertical integration has assured a more uniform bird to sell, one which producers feel is superior. That confidence has inspired them to advertise. In one variation on vertical integration in the broiler industry, large supermarket chains contract with several smaller processors to produce a "house" brand which meets standards they have set.

Q. In a column on hot soups, you said that the major nutritional benefits were to be gained by reconstituting them with milk. Is that true?

A. Yes. If you examine the ingredients list on a typical can of condensed cream soup, you will note that it usually includes water, the vegetable or other food from which it got its name, vegetable oil, flour, cream, salt and a host of other additives for flavor and texture. In short, it rather closely resembles the ingredients in cream sauce before the milk is added.

Thus a cup of cream soup reconstituted with water contains on average 90 calories, although it can range anywhere from a low of 75 to a high of 155. It may provide anywhere from 1 to 5 grams of protein and from 2 to more than 10 grams of fat, or a little over 2 teaspoonsful.

Depending on the variety you choose, it will contain differing amounts of several other nutrients. For example, a serving of cream of mushroom soup would supply 45 percent of the day's riboflavin and 2 percent of the U.S. RDA for niacin, while cream of celery would yield 4 percent of the day's vitamin A and 2 percent of the day's calcium. Tomato soup is a particularly good source of vitamin C and a better source of vitamin A than most.

And cheese soup, in many ways the exception to the generalization, provides 14 percent of the day's calcium, more than 20 percent of the vitamin A, 8 percent of the riboflavin, and other B vitamins. But it is also highest in calories (155 per cup) and fat.

While the nutritional contributions of soup are generally small and uneven, the use of nonfat milk can change the nutritional profile. Reconstituting soup with vitamin-A-fortified skim milk means just 45 more calories, but along with it comes 15 percent of the day's calcium, 10 percent of both the vitamin A and the riboflavin, and 4 grams of protein. And milk is rich in tryptophan, which the body converts to niacin. Especially if you use nonfat dried milk, this is indeed a nutritional bargain.

Q. What allows instant puddings to gel without heat?

A. One of the useful properties of starches is their ability to gel when they are heated. In formulating instant-pudding mix, the starch is treated to "pregelatinize" it. The dry mixture contains phosphates or other alkalizing agents. When cold milk is added to the alkaline environment and the mixture is refrigerated, the calcium ions cause the milk protein to gel with this specially treated starch.