If the part of the dough offered as firstfruits is holy, then the whole batch is holy; (Romans 11:17a)
Sour dough bread from an artisan bakery tastes heavenly! It is made from mother dough, a mixture of ordinary ingredients, unbleached flour and water. In 12 hours, the mixture of flour and water bubbles and produces bacteria with a rotten smell. Then, and every 12 hours for another 6 to 8 days until the mixture begins to double in size, half of the mixture is discarded and replaced with additional flour and water. At this point the dough has a pleasant sweet yeasty smell, and the process of discarding half of the dough and replacing it with fresh flour and water needs to be repeated only weekly. The mother dough never goes bad! The Boudin bakery in San Francisco has been using their mother dough for over 150 years! If you want to make mother bread or wild yeast go to http://heoyeahyum.com/2012/10/sourdough-boule/
In Paul’s time all bread was made from mother bread using that process; there were no commercial yeast packets. Paul uses this process to explain that Abraham was the firstfruit (an offering of the first sheaf of grain to God to give Glory to God and thank Him for the harvest. See Leviticus 23:14). Abraham was an ordinary man, just as flour and water are ordinary ingredients. But God can take an ordinary man, just as he can take ordinary ingredients, and make something extraordinary for His glory. He chose Abraham to be the beginning of all of His children. He gave Abraham The Promise.
Abraham was rotten in the beginning as was the mother dough – he worshipped another god. At first Abraham required a lot of maintenance to change his ways, but as he developed faith, he required less maintenance (just like the mother dough). God made Abraham holy. As Abraham and his descendants have died, as part of the dough is discarded, part of Abraham remains through DNA. Paul uses this analogy to tell us that all the descendants of Abraham, through the line of Jacob, are holy because they are from Abraham’s batch and will be saved.