This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
If there is an empty space in the jar it may be caused by any one of these conditions:
1. The jar may have been packed too loosely.
2. The product may not have been blanched enough to cause necessary shrinkage.
3. The pressure may have fluctuated instead of being kept steady.
4. The pet-cock of the pressure cooker may have opened before the pressure dropped to zero.
To be sure, the empty space will not affect the quality of the food, but it does detract from the appearance and wastes space.
Lack of liquid in the jar may also be caused by uneven temperature, and when the jar is packed closely there is little space left when the water is added. The product will keep in this condition, but the flavor seems to be better if the food is practically covered with liquid.
This condition is not always apparent until the jar is opened. The appearance may be unchanged as to color and texture, but when the jar is opened the taste and odor are bad and the product must be destroyed. Occasionally neither odor nor taste is detected until the food is heated. Sometimes the presence of flat sour is indicated by the slightly clouded color of the liquid and a general change in the color of the product itself.
This condition seems to develop at different stages along the route from the garden to the canner. It is imperative then to use only fresh sound vegetables, since changes are said to take place in vegetables when they are allowed to stand for a time in a warm place. Hence it is also necessary to prepare only a limited number of jars at one time, so that the product can be handled rapidly.