Adam Clarke on Deuteronomy 32:14

Fat of kidneys of wheat - Almost every person knows that the kidney is enveloped in a coat of the purest fat in the body of the animal, for which several anatomical reasons might be given. As the kidney itself is to the abundantly surrounding fat, so is the germ of the grain to the lobes or farinaceous parts. The expression here may be considered as a very strong and peculiarly happy figure to point out the finest wheat, containing the healthiest and most vigorous germ, growing in a very large and nutritive grain; and consequently the whole figure points out to us a species of wheat, equally excellent both for seed and bread. This beautiful metaphor seems to have escaped the notice of every commentator.

Pure blood of the grape - Red wine, or the pure juice of whatever color, expressed from the grapes, without any adulteration or mixture with water: blood here is synonymous with juice. This intimates that their vines should be of the best kind, and their wine in abundance, and of the most delicious flavour.