This section is from the book "Practical Cooking And Serving", by Janet McKenzie Hill. Also available from Amazon: Practical Cooking and Serving: A Complete Manual of How to Select, Prepare, and Serve Food [1919].
The object to be desired in croquette mixtures, besides flavor, is a consistency of such firmness as will allow the shaping of the mixture easily when cold, and yet will insure a soft and creamy mass when heated. This condition may be best secured by using veal or chicken stock in the sauce which forms the foundation of all these dishes. Stock made from veal or chicken is rich in gelatine, and, thus, a liquid when hot, it becomes a firm jelly easy to handle when cold. The sauce for these mixtures is to be prepared according to the directions given under division I. To the sauce is added the materials to be used, one or more in kind, chopped or cut small, not crushed. Yolks of eggs and cream are added for richness or additional nutriment, but most delicious croquettes may be made without either in the body of the mixture; eggs, however, are needed to make a fat-proof covering. After the meat or vegetable has been stirred into the sauce, set aside in a shallow dish to become cold. To shape, egg-and-bread-crumb and fry.
 
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