If you have some commercial lemon juice, you can add that instead of the citric acid. Some tomatoes have enough acid with anything, but unless you grew them, you may not know what variety you have.

There is no expiration date for citric acid IF it has been stored well and kept dry. If it has clumped or solidified toss it. If it is still in its normal dry powdered form it is fine to use.

Proper storing is defined as in an air tight dark container (not glass or plastic), kept dry and not exposed to heat.

Dave

Since our air tends to the humid, with citric acid and pickle crisp, at the end of the season I vacuum-seal the jar as double insurance. Other than that it's kind of like the honey in Egyptian tombs - good indefinitely.

Carol