Hi jond1960, There is a book Diet for a Small Planet that describes amino acids quite well. It may not be in print any longer, though and/or there may be something published more recently. Because there is more to it than what most of us know.
In a nutshell, there are all kinds of amino acids. Some are manufactured by the body (and thus dubbed non-essential amino acids) and some must be obtained from food sources (essential amino acids). So, it is the essential amino acids that are of concern to us.
The eight essential amino acids needed by adults must be eaten in certain proportions in order for the body to be able to assimilate them and use them to build proteins for the body. (It used to be believed that the "eight" needed to be eaten at the same time in order for the body to use any of them, but it is now generally agreed that if you eat the "eight" throughout the day you body will be able to use them.)
It is not entirely accurate to say that some foods are "missing" essential amino acids. Actually, most foods that contain protein do contain all the essential amino acids, but there is so little of one or more of them that the body cannot assimilate most of the protein. (This is the case with most plant foods.) A more accurate way to look at it would be to list the percentage of the protein that can be assimilated from any particularly food. The highest is chicken eggs - healthy humans can assimilate about 98 percent of the protein in eggs because the proportion of the amino acids allows most to be used by the human body. I cannot find any info online offhand that lists more percentages, but the protein even in meat, which is considered a "complete" protein, is in proportions so that only about 60-something percent is used. (I believe anything above about 60 is considered a "complete" protein.) So, even the so-called "complete" proteins are not completely available to the human body.
With plant foods, while one or more amino acid may be in small quantity, another plant food may have excess of that(those) amino acids. So if two foods are eaten together, more of the protein can be assimilated. Funny enough
, many traditional food combinations accomplish exactly this; for example, corn and beans, rice and beans, peanut butter and crackers (is that traditional?) and others.
I have never noticed amino acids on nutritional labels (though I have not paid attention), so I cannot comment about that. But my understanding is that the amount of protein listed on nutritional labels does NOT take into consideration that all the protein in that one food might not be assimilated completely because the balance of essential amino acids is not correct.
