Actually there are 9 essential amino acids.
It used to be thought that people need to compliment plant proteins to get all 9 essential amino acids in the right proportions
to make protein synthesis possible.
According to the American Dietetic Association's Position Paper On Vegetarianism ( http://www.eatright.org/cps/rde/xchg/ad ... U_HTML.htm
This isn't necessary to get adequate
amounts of all 9 essential amino acids. If you eschew junk food, compliment your proteins over the course o f a day, and eat enough calories to maintain your weight.
However, doing these things is not enough to get full use out of your amino acids. Hot dog buns come in packages of 8, hot dogs in packages of 6. You will get six "complete" hot dogs from the two, but you will have two buns which will not go to a complete hot dog.
Likewise with soy and "complete proteins" ( all 9 essential amino acids ). Soy does have all 9 essential amino acids, making it superior to other beans and a staple food. However, just because it has all 9 doesn't mean it has all 9 in proportions so that all of the amino acids are usable......buns and hot dogs.
To make sure you optimize your protein intake you can still compliment your proteins and you can still do so at ever meal. For vegans it is easy as there are only two combination to remember:
1. Legumes and WHOLE
3. Legume and seeds
Note, these rules say nothing about amounts. I'm not an expert but I doubt if you are getting 14 grams of protein from a cup of lentils and 5 grams of protein from a cup of rice if all of the amino acids from the lentils are getting paired up.
Again, I am not an expert, but what I do for this "suspicion" is I use a grain based protein supplement since most of the protein I get from food comes from legumes. I figure not only do I get the protein from the brown rice protein powder but that protein powder also makes the protein I get have a higher biological value.
Note, the current standard for measuring protein quality is "biological value". You want to google and read up on that.
Two of the coauthors of the ADA Paper On Vegetarianism wrote one of the best books on vegan nutrition called "Becoming Vegan".
Note, tofu is not a whole food. It is made from only part of the soy bean and lacks all of the nutrition of the whole soy bean. Organic ( non-GMO ) soy beans are much cheaper than other soy foods, are a lot less gassy, and have a LOT more nutrition.
If you could only do one rule for optimizing nutrition that one rule would be variety
In other words, don't let yourself settle into using one food as a source of anything.