Can 7 year old casing become sour? 1st attempt at sausage

[glee006]

So my friend and recently gave my family and I 5 lb. of Chinese sausage. My family said that this was the best Chinese sausage they have ever tasted and i agreed with them. If someone wants to try it let me know i'm willing to vacuum seal and send it over. Rich in alcohol, flavoring and texture. I asked him to teach me his recipe process and methods, he denied and stated i would never be able to make this. I bet him 200 dollars i would make it in 6 months. Being a huge fan of good Chinese sausage and a very competitive person by nature I am now welcoming this task. My whole family doesn't think i can do it. Except my grandma who has aided me in my efforts by providing me with 7 year old collagen casings given by her friend.

What i used:
Sugar
Salt
MSg
Nitrate
Pork Picnic
Pork Backfat
Alcohol
Here are processes and procedure i used in my efforts.

Day one: Mixed most the ingredients (sugar, msg, salt, nitrite, spices, soy sauce) in a metal bowl and stuck it in the freezer because my refrigerator is packed to the brim.
Day two: cut up the 1 lb. fresh Pork picnic into cubes and added 25% back fat. stuck the fat in boiling hot water because i heard it helps the fat become harder. Grind it up and remembered i forgot to remove the mixed ingredients from the freezer. So i stuck the ingredients on the stove for 10 secs. (i fear this may have caramelizing the sugar, if so i dont know how this would affect the outcome). It melted so i added the alcohol and pour this mixture on to the pork picnic. Mixed for about 5 minutes. Stuck the boil aside. out in the open. For 24 hours, came back notice it browned slightly. I was worried but i mixed the meat around it was pink under the browning . Two hours later i came back and notice it had browned again. I was confused but i continued to stuff the 32mm casing(7 year old casing) into the half diameter sausage feeder and began to fill the sausages. It seemed very slow as the half inch sausage feeder was very small, and i felt it took work to stuff it tightly. stuck it in the 120 degree oven for 12 hours.
Day three: removed from oven,

Results:

Good coloring,
Good Taste
Slight sours i dont know from where.
Texture kind of soft not as firm as i wanted it to be.

Questions:
How do I create a firm sausage?
How do i make sure my fat is firm and pops when i chew.?
What stage in my process may have caused a slight souring?
(Picnic used was freshly killed and butchered 2 days prior. And was frozen.
How are my temperatures and cook time?
Anyone with a good Pallet want to try my friends sausage?

Feel free to ridicule as you wish. I love criticism.

[Wunderdave]

stop heating everything up, for the first thing. Everything should be very cold, from cutting to grinding to mixing. This will certainly affect your texture.

The sourness is probably not from the casings, but instead because the sausages fermented after you added the sugars, left them at room temperature for 24 hours, and then in your warm oven overnight. This is an intended flavor, chinese lap chong (sp?) is typically fermented.

Lap chong is semi-dried, so hanging your sausages to dry after you stuff and ferment, for a few weeks could get you to the firmness you desire, if you have a dry curing environment (or maybe without one if they're small enough casings).