For fresh corn, use about 18 medium ears to get 9 cups of kernels: husk them, blanch for 5 minutes, cool in an ice water bath, then cut off the kernels and proceed as written. If using frozen corn, thaw and drain it first. Use white vinegar with 5% acidity and do not reduce the vinegar or sugar — this is a tested canning recipe. At altitudes above 1,000 feet, extend the processing time per USDA guidelines.