• Wash jars, rims and lids according to manufacturer's instructions. Sterilize clean jars in a large stock pot or professional water canner. Place jars in pot and and cover with water, boil jars for 15 minutes. Leave in pot over medium heat until ready to use.
• Heat a separate water canner with enough water to cover filled jars by 1 -2 inches. Heat water to 180 degrees F. by the time jars are ready to be added for processing. Use food thermometer to monitor heat.

CAUTION: THIS IS A VERY IMPORTANT STEP WHEN CANNING LEMON CURD

• Do not heat the water in the canner to more than 180 degrees F. before jars are added. If the water in the canner is too hot when the jars are added, the process time will not be long enough. The time it takes for the canner to reach boiling after the jars are added in expected to be 25-30 minutes for processing lemon curd. Process time starts after the water in the canner comes to a full boil over the tops of the filled jars.
• If using lemon zest: Combine lemon zest and sugar and set aside for about 30 minutes to allow flavors to meld
• Heat water in the bottom of a double boiler until a gentle boil. The water should not boil vigorously or touch the bottom of the top of the double boiler pan in which the lemon curd is to be cooked. Hot steam is sufficient for the cooking process to occur.
• In the top of the double boiler, on the counter away from the heat, whisk egg yolks and whole eggs together until thoroughly mixed. Slowly whisk in the sugar and zest, blending until smooth. Blend in the lemon juice and then add the butter pieces to the mixture.
• Place the top of the double boiler over boiling water in the bottom of the pan. Stir gently but continuously with a silicone spatula or cooking spoon, to prevent mixture from sticking to the bottom of the pan. Continue cooking until the mixture reaches 170 degrees F. Use food thermometer to monitor heat
• Remove double boiler pan from heat and place on a heat protected surface. Continue to gently stir until curd thickens (about 5 minutes). Strain curd through a mess strainer into a clean stainless steel or glass bow; discard zest
• Fill hot curd into cleaned half-pint jars, leaving ½ inch headspace. Remove air bubbles and adjust headspace if needed. Wipe rims of jars with a clean damp cloth or paper towel; apply two-piece metal canning lids.
• Place filled jars into 180 degree water bath, be sure water is over the tops of jars, when water has come to a full rolling boil, process jars for 15 minutes.
• Remove jars from canner and allow to cool on a dry kitchen towel, store in a cool dry place for up to 3 months