New Wine into Old Wineskins

I never quite understood what the chemical process involved in this parable "new wine into old wineskins". In today's gospel reading, which is found in Luke 5:33-39, Jesus was answering a number of objections by the Pharisees and their scribes, who were complaining to his disciples: (1) “Why do you eat and drink with tax collectors and sinners?”; (2) “John’s disciples, like the disciples of the Pharisees, frequently fast and pray, but your disciples eat and drink.”; (3) “Why are you doing what is not lawful on the sabbath?” - this is because Jesus' disciples plucked some heads of grain, rubbed them in their hands, and ate them on a sabbath day (Luke 6:1-11 and a parallel passage found in Mark 2) - work prohibited under the old Jewish law. This is similar to an "offense" Jesus did on healing a man with a withered right hand (Luke 6:6).

So Jesus told these Pharisees and Scribes two parables: (1) sewing a piece of new garment material on an old garment; and (2) pouring new wine into old wineskins. I think I understand the first parable. It has to do with sewing a piece of new garment that has not been pre-shrunk to an old garment. Next time the old garment is washed the patch of new garment will shrink pulling itself off the old garment and ruin the entire garment. His audience understood that. What about the new wine into old wineskins? Well, it has to do with the fermentation process! This explanation is probably the best I have read: 
"In ancient Israel, the grapes were pressed in the winepress and left in the collection vats for a few days. Fermentation starts immediately on pressing, and this allows the first "tumultuous" (gassy) phase to pass. Then the must (fermenting juice) was put in clay jars to be stored, or into wineskins if it was to be transported some distance.

The wineskins were partially tanned goat skins, sewn at the holes where the leg and tail had been. The skins were filled with must (partially fermented wine) in the opening at the neck and then tied it off.

If one puts freshly pressed must directly into the skin and close it off, the tumultuous stage of fermentation would burst the wineskins, but after this stage, the skins have enough stretchiness to handle the rest of the fermentation process. However, skins that have already been used and stretched out ("old wineskins") cannot be used again since they cannot stretch again. If they are used again for holding wine that is still in the process of fermenting ("new wine"), they will burst."

Some take the meaning of Jesus' parables to be a reason to pull away from all traditions and start new churches (see Home Church Network Association's article). Surely St Paul didn't mean all traditions are bad when he said "So then, brothers and sisters, stand firm and hold fast to the traditions that you were taught by us, either by word of mouth or by our letter." (2 Thessalonians 2:15 and 2 Thessalonians 3:6). Surely Jesus commanded His disciples to "Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything that I have commanded you." in the form of oral traditions and writings known as the Great Commission (Matthew 28:18-20).

This homily in the Catholic Online archive is the best. The key point is this: "Through spiritual blindness, the "wineskins" of the Old Covenant had been rendered incapable of receiving the Good News of salvation.  A total transformation was necessary." And the application:
"Without proper care, our souls, too, can become like old, thin wineskins, weakened by sin and spiritual sloth.  Maintaining the suppleness of our souls - their capacity to hear, receive, and act upon the Word of God - requires vigilance and care.  But the Lord is always at hand, offering us his grace through the sacraments to repair what is broken, strengthen what is weak, and fortify what is healthy."