Blood orange and brewing

Since it's the weekend, I'm writing this while enjoying some solo yakitori and a glass of mead. Suddenly, I remembered how, when I was studying abroad in China, I inexplicably used "Maido!" as a greeting—perhaps because I wanted to show off my Kansai roots? Feeling a mix of embarrassment and an uncontrollable urge to keep eating popcorn and drinking alcohol, I decided to write this post.

Setting that aside, I’ve recently been looking into blood oranges (BO) since I just started a batch with them.

I hadn't actually finished my previous story about vanilla, but I got so fired up while researching blood oranges that I’ve decided to run with this topic instead.

Why are blood oranges red?

The most distinctive feature of a blood orange is, of course, its color. So, why are they red? Because they are packed with anthocyanins!

Well, ending it there would be boring, wouldn't it? I did some proper digging.

While searching through Italian academic literature, I found a paper titled "The Origin of Blood Oranges." According to this, the red color is caused by a gene called "Ruby." Isn't that cute?

The research also noted that cold temperatures are necessary for the production of anthocyanins. Wait, cold temperatures? What does that mean?

Digging a little further, I arrived at "autumn foliage." Sharp readers might have already guessed. That's right: the reason blood oranges turn red is the same principle behind why leaves change color and apples turn red.

With autumn leaves, when it gets cold, the sugars produced by photosynthesis don't move into the branches but instead accumulate in the leaves. When sunlight hits them, the chlorophyll breaks down, and a chemical reaction involving those sugars produces anthocyanins.

For foliage to turn color, nighttime temperatures apparently need to drop below 5°C. This is because if the night is too warm, the tree consumes the sugars produced during the day through photosynthesis.

In other words, cold temperatures are required to produce anthocyanins, and cool nights are essential for blood oranges to turn red.

The band-aid aroma in mead fermented with citrus

I’ve made mead using Natsumikan (summer mandarin) puree before.

The puree itself smelled wonderful, but the fermenting liquid developed a strong, chemical, band-aid-like aroma.

I’ve experienced this same scent when using other citrus fruits since then, to the point where I’ve personally formulated a rule: Citrus fermentation = chemical scent.

While researching blood oranges, I found research findings that hint at the cause of this phenomenon.

It turns out that storing the "Moro" variety for over 60 days after harvest can result in phenolic off-flavors. Specifically, hydroxycinnamic acid in the blood oranges is broken down by enzymes, dramatically increasing p-coumaric acid and ferulic acid. This, in turn, produces 4-VG and 4-VP, which are the culprits behind the medicinal smell. All of these are types of polyphenols.

These substances—4-VG (vinyl guaiacol) and 4-VP (vinyl phenol)—are also responsible for off-flavors in brewed alcohols like wine and beer. They are produced when coumaric acid and ferulic acid are converted by yeast.

In short, citrus fruits are rich in hydroxycinnamic acid, which serves as a precursor to coumaric and ferulic acid, likely leading to a higher-than-normal production of phenols like 4-VP by the yeast.

I feel like I finally understand the cause of that medicinal smell. Essentially, citrus fruits contain a high level of the substances that turn into that band-aid aroma. The brewer's job, then, is to figure out how to prevent this scent from developing.

The first strategy is to use yeast that does not possess the enzymes responsible for creating these phenols (known as "POF-"). Examples of wine yeast with this trait include Lalvin ICV Opale and Lalvin V1116.

As for the threshold at which humans detect this scent, it is reportedly 1.5 mg/L for 4-vinyl phenol and 0.38 mg/L for 4-vinyl guaiacol. In experiments using Koshu wine, where these are easily produced, most non-POF- yeasts exceeded these threshold values.

The second strategy is to avoid using the citrus peel or the white pith (albedo) as much as possible. Since the peel and pith are rich in polyphenols, eliminating them might reduce the precursors for these off-flavors.

I haven't personally tested either of these methods yet, but if you want to avoid that medicinal smell, using POF- yeast seems like the way to go.

Brewing with blood oranges

The hallmark of a blood orange is its rich anthocyanin content. Thinking about how to utilize this, why not try a process similar to making red wine?

Red wine production is characterized by crushing the skins and fermenting them along with the juice. Because of this, it’s said to have about ten times more polyphenols than white wine.

Given what I just discussed, you might think that more polyphenols would lead to a phenolic smell. However, 4-VP and 4-VG are rarely detected in red wine.

"Why is that?" you might ask.

There's a reason: the procyanidins and catechins found in red wine inhibit the enzymes that break down the source material, hydroxycinnamic acid. This means that in red wine, the substances that lead to 4-VP and 4-VG simply aren't produced.

Blood oranges also have a stronger antioxidant effect compared to other varieties like Navel oranges. This is thanks to the anthocyanins, and in wine-making, oxidation is essentially a reaction between phenols and enzymes.

In other words, the high antioxidant capacity of blood oranges might inhibit the enzymes that break down hydroxycinnamic acid, just like in red wine. Looking at it this way, it’s possible that brewing with blood oranges might not result in that phenolic smell at all.

That said, red wine can sometimes develop a phenolic smell if *Brettanomyces* yeast takes hold, which produces substances called 4-EP and 4-EG. While slightly different, these aromas are also basically medicinal in nature.

Therefore, when using blood oranges, you should still be very careful about contamination from unwanted bacteria.

It would be interesting to try fermenting them with the skins and pulp just like in red wine production, but I foresee it becoming incredibly bitter, so I'll save that experiment for when I have more free time.