I do not have an answer but an educated guess... since grains are seeds that must store energy for germination and to sustain the seedling until it can extract energy from the soil and perform photosynthesis, I would say that the answer to your question has to be yes. Other compounds are not efficient enough to store energy.
Gaber, That was nt the suggestion as I see it. Maize may well have oils in their seed that can serve as sources of energy that the seed can use until it is able to photosynthesize.