Why is my lemon curd metallic? How should I fix it?

Question

I used this recipe and followed it to the letter. I used organic/unsprayed lemons (and was careful not to zest the pith) and free-range eggs. I did a quick Google search, but the only cause I could find was using a reactive saucepan, which couldn't be the case because both the whisk, the strainer, the bowls, and the pan were nonreactive, stainless steel. Yet the lemon curd ended up with a metallic aftertaste, that got stronger as it chilled.

Why was my lemon curd metallic, and is there any way to fix it? How?

3 large eggs, or 1 large egg plus 3 large egg yolks
Zest of 1 medium lemon
1/2 cup strained fresh lemon juice (from about 3 medium lemons)
1/2 cup sugar
6 tablespoons unsalted butter, cut into chunks

1. Set the strainer over a medium bowl.
2. Whisk the eggs (or egg and yolks) in a small nonreactive saucepan to blend. Whisk in the lemon zest, juice, and sugar. Add the butter. Whisk over medium heat, reaching into the corners and scraping the sides and bottom of the pan, until the butter is melted and the mixture is thickened and beginning to simmer around the edges, then continue to whisk for about 10 seconds longer.
3. Remove from the heat and scrape into the strainer, pressing gently on the solids. Scrape any lemon curd clinging to the underside of the strainer into the bowl.
4. Refrigerate until chilled before using. Lemon curd keeps in a covered container in the refrigerator for up to 1 week.

EDIT: I made a new batch, using 1 part lemon juice to 1 part sugar plus the zest from the lemons, and the same amount of butter. I omitted the egg yolks and used cornstarch instead. The lemon curd came out perfect, so it must have been the yolks. Why?

Answer 1

From your Edit, it looks like you have narrowed the problem down to the egg yolks.

Did anyone else taste your original batch of lemon curd? If you still have original batch, you may have a close friend or relative see if they can taste difference. For years (side effect of medication) I would perceive either "off or metallic taste" which my husband or anyone else could not taste. Pregnant women will also experience metallic taste (dysgeusia):

https://www.whattoexpect.com/pregnancy/symptoms-and-solutions/metallic-taste.aspx

Off taste from yolks are usually associated with "old eggs." It is usually described as tasting like sulphur.

What chickens eat also affect the taste of the eggs:

https://grist.org/urban-agriculture/2011-11-1-peebottle-farms-what-to-feed-your-chickens/ archived at: http://archive.is/WccwR

http://homesteadingguide.com/115/news/best-tasting-eggs-what-to-feed-your-chickens/ archived at: http://archive.is/rt2RF

The following post has made me shy away from any "natural feeds" which contain kelp: http://www.maryjanesfarm.org/snitz/topic.asp?TOPIC_ID=64661 archived at: http://archive.is/EUac3

Answer 2

It might be that an acid react with metal to form salt and hydrogen. If you can change from metal to a glass bowl, that might help.

Answer 3

Try cutting back on the amount of lemon. I have noticed a metallic taste in some Greek lemon soup (Avgolemono) when the amount of lemon seemed to be greater. Another possibility is to try to use the ripest lemons available. I realize you are using sugar, but I have also read reference to the use of powdered sugar (when less fresh) apparently contributing to the taste.