You might want to switch to a one piece and wash your bag using a couple of drops of antibacterial dish soap and water while on the toilet.
You can burp the bag by dumping and pressing down a little to let extra air out but the main cause of gas is diet and not killing the digestive enzymes bacteria that are eating your food still while in the bag and producing gas.
The antibacterial dish soap wash solves that problem mostly but drinking carbonated drinks, certain foods etc. still produce gas.
Eating Guide
Partially copied from my nutritionists guide. Ileostomy Nutrition Therapy from the Academy of Nutrition and Dietetics. (This handout may be duplicated for client education.)
My recommendation is to see a nutritionist for the full guide as not all can be pasted here
I've added my own observations to further clarify
Chew all foods well to the consistency of paste. Spit out any hard stuff or can't chew. Important!
Foods That May Cause Blockage (very bad avoid!)
Apples, unpeeled (peeled and applesauce are excellent thickeners, don't eat the skins or the core, chew well)
Bean sprouts, Cabbage
Casing on sausage (inside fine if no tough parts, avoid swallowing if a chunk is felt) Also sausage like meats like pepperoni and salami contain indigestible bits.
Chinese vegetables (stir fried crunchy, too hard, need very soft veggies with no skins)
Coconut, Coleslaw, Celery (avoid)
Corn (grits also, corn anything doesn't seem to dissolve in stomach)
Cucumbers (skins bad, inside chewed up good seems okay, no seeds)
Dried fruit, raisins
Grapes, blueberries etc. (skins and seeds the problem.)
Green peppers (red, yellow also) pureed or very very small and soft seems okay in small amounts
Mushrooms (doesn't dissolve in stomach) pureed or very very small seems okay in small amounts
Nuts (totally avoid)
Peas (mushy interior okay, like pea soup, skins a problem)
Pickles (skins a problem, inside if chewed up good seems okay, seeds not)
Pineapple (liquid okay, mushy parts fine, hard parts not, rather risky as parts of mushy may not digest)
Popcorn (totally avoid)
Potato skins, inside (no hard parts) fine.
Relishes and olives
Salad greens (diarrhea nightmare too)
Seeds and nuts (avoid) Watermelon seeds bad
Spinach (doesn't digest, pureed perhaps)
Tough, fibrous meats (for example, steak on grill, well done, tough parts especially, over grilled chicken or dry pork chops. Juicy grilled or pan fried chicken, shredded pork in juices, medium rare steaks, hamburger better. Nothing deep fried ever, dirrarea city.
Vegetable and fruit skins, (any avoid)
Whole grains (no grits, rolled oats oatmeal seems to be okay, oat cereal is an excellent thickener)
Foods That May Cause Gas or Odor
Alcohol, Apples, Asparagus (stink), Bananas, Beer, Broccoli (clog hazard)
Brussels sprouts (clog hazard)
Cabbage (clog hazard)
Carbonated beverages
Cauliflower, Cheese, some types like any cheese food is bad. Real cheese may be okay. Provolone, Swiss, Cheddar, not American white or yellow
Corn (clog hazard), Cucumber skins and seeds bad.
Dairy products (toot city, small amounts okay)
Dried beans and peas (clog hazard)
Eggs (some don't digest in stomach like scrambled or omelets, but over easy seems fine for some reason)
Fatty foods, Grapes
Fish (cooked unseasoned salmon in small portions may not, nor stink, same for raw tuna seems fine)
Green pepper (red and yellow also, clog hazzard unless very small and very soft in small quantities)
Melons, Onions (clog hazard)
Peanuts (clog hazard) Prunes
Radishes, Turnips
Soda and sipping using straws, (drink from glass instead)
Seafood (smell rotten stink!)
If your having vacuum caused pancaking issue a small amount of something above (like having a soda during a meal) could introduce some air into your diet and thus into the bag. Also by avoiding too much thickening foods by themselves which often contribute to pancaking of the output.
Foods That May Help Relieve Gas and Odor
Buttermilk, Cranberry juice, Parsley
Yogurt with active cultures (Greek yogurt)
Foods That May Cause Diarrhea (looser or more frequent stool)
Alcohol (including beer)
Apricots (and stone fruits)
Beans, baked or legumes (clog hazard)
Bran, Broccoli, Brussels sprouts
Cabbage, Caffeinated drinks
Chocolate
Corn (in my experience corn anything doesn't digest in stomach, just passed through and out, kernels/popcorn will clog)
Deep Fried meats, fish, poultry (KFC, supermarket rotisserie chicken very bad, and anything soaked in brine)
Fruit juice: apple, grape, orange (small amount okay)
Fruit: fresh, canned, or dried (small amount skinless okay)
Glucose-free foods containing mannitol or, sorbitol
Gum, sugar free, Licorice
High-fat foods, High-sugar foods
High salt foods (KFC, supermarket rotisserie chicken)
High seasoned foods (like blackened)
Milk and dairy foods (small amounts okay)
Nuts or seeds (bad, clog hazard)
Peaches (stone fruit, one skinless a day okay)
Peas, Plums (stone fruit).
Pork (chops, fatty)
Prune juice or prunes
Soup, Spicy foods
Sugar-free substitutes
Tomatoes, doesn't digest (soup okay) Turnip greens/green leafy
Vegetables, raw
Wheat/whole grains, Wine
Rice (very soft 1/4 cup in mixed in food okay, like in soup), refried beans (1/4 cup okay) pasta (one cup max).
Any foods soaked in brine or deep fried (rotisserie chicken, Kentucky fried chicken etc)
Solent nutritional meals
Foods That May Help Thicken Stool
Applesauce (peeled apples, no cores)
Bananas, Barley (when OK to have fiber)
Cheese (processed cheese food a clog problem, use real cheese)
Dried Chinese Crispy Noodles (severe thickening, mix with other food that causes diarrhea)
Marshmallows
Oatmeal (when OK to have fiber)
Pasta (sauces may increases symptoms) (in my experience more than a cup of pasta causes diarrhea)
Peanut butter, creamy only, nuts clog (may make skin greasy)
Potatoes, no skin (skin clogs, more than a cup may cause diarrhea)
Pretzels (salty may cause more flow water consumption)
Metamucil, mix or crackers (mix in cup of water and drink before it gells up)