Musar zu Bamidbar 18:8

The reason that the wine saved from גן עדן is called משומר, is that its dregs, its yeast has been removed. When Eve squeezed the grapes and gave her husband to drink she had not removed those dregs first. The Torah (Numbers 18,8) writes of Aaron, who was the rehabilitation of Adam, that he was to be the recipient of what G–d calls משמרת תרומתי, "The residue of My gifts." It is also said of Abraham (Genesis 26,5) וישמר משמרתי. Some of our sages interpret the repetition of the expression שמר in this verse as meaning that Abraham introduced "fences," preventive measures around G–d's laws so as to ensure that the laws themselves could not easily be violated. This would be similar to the Mishnah in Shabbat 20,1 permitting the suspension of a sieve i.e. meshameret on the holiday in order to catch the shemarim, sediments, yeast, of the wine.

The mystical dimension of all this is that ever since the pollutant of the serpent, i.e. the negative fall-out of the sin, infected man it has become necessary to build "fences" around G–d's laws to make it almost impossible to transgress an actual Biblical injunction. When someone is guilty of being פורץ גדר, demolishing such a fence, the serpent will bite him. In other words, if someone does not construct a משמרת he is liable to transgress the essence of G–ds commandments. In the future, when G–d will remove the pollutant of the serpent there will no longer be a need for these fences; this need stems from the power of the Sig, dross, introduced into our bodies by the serpent. It is significant that in the parlance of our sages the fence is not called גדר, but rather Se\yag, i.e. the same word as Sig. Our Rabbis in Avot 1,1 wanted to make sure that we appreciate the reason for the need to construct such "fences." Aaron came close to G–d in lieu of Adam. He was the Meshameret. This is the meaning of G–d saying in Numbers 18,8 that he would be the משמרת תרומתי.