Here's a basic home recipe for keeping cut flowers fresh
1 quart water
2 tablespoons lemon juice
1 tablespoon sugar
1/2 teaspoon bleach.
The sugar is the food part and the bleach will help to slow fungi and bacteria growth.
Another trick for keeping long-lasting cut flowers without making flower food is to add a few drops of regular bleach to the water (extra drops work best for large vases of water). The bleach kills any bacteria so the uptake of fresh water is better.
Note: remember to trim off leaves that are below the water line. They are the culprit of most of the bacteria. And clean your scissors or snips with alcohol before each use so they are nice and clean too.