The Coffee Protein Mystery

This post was moved from Coffee Hero.

In the post Black Coffee and Espresso – Not Calorie Free, I discovered there are trace amounts of protein in brewed coffee, but not in espresso. According to CalorieKing:

• 0.0 grams protein for Espresso (1 oz)
• 0.3 grams protein for Coffee Brewed (6 oz)

The first part of the mystery is figuring out if brewed coffee really has protein. I do not have a science background, but I’ll try my best to figure this out. If I make an error along the way, please let me know in the comments.

The January 1995 issue of Plant Physiology has an article titled A cDNA encoding a metallothionein I-like protein from coffee leaves (Coffea arabica). In this study, the authors state they found an 80 amino acid protein in a coffee plant.

We now report the isolation of a full-length cDNA from immature coffee (Coffea arabica) leaves encoding a metallothionein I-like protein (Table I). The deduced coding region encodes an 80-amino acid protein with 64% identity to a Ricinus communis metallothionein.

This Coffea arabica protein is also found on the WikiProteins.org site, where it also goes by the name coffee metallothionein I.

This study confirms the presence of protein in the Arabica plant. Unfortunately, I couldn’t find any information on the protein as it moves from the coffee plant itself to the brewed outcome. At least this study makes the presence of trace amounts of protein in coffee plausible.

Now to the next question in the coffee protein mystery: Where did the protein go in the espresso? Espresso is still derived from the coffee plant. I have two theories:

1. Number Rounding – The protein didn’t go anywhere. The number was rounded down to a single decimal. This gave it 0.0 grams of protein.
2. Protein Denaturation – The protein was destroyed due to a difference in brewing styles.

Brewed coffee has 0.3 grams of protein per 6 ounces. This works out to 0.05 grams of protein per ounce. If espresso extraction has no impact on the protein, then the 0.05 number should be rounded up to 0.1 grams. CalorieKing rounded a 1.2 number to 2.0 on calories, so they wouldn’t be shy about rounding a 0.05 to 0.1. So we are still missing some protein in the espresso.

What if the coffee brewing differences between brewed (saturation) and espresso (pressure) caused the protein to be denatured? TheFreeDictionary defines denatured as:

3a. To cause the tertiary structure of (a protein) to unfold, as with heat, alkali, or acid, so that some of its original properties, especially its biological activity, are diminished or eliminated.

The Wikipedia page on Denaturation also mentions temperature, but not pressure. Lets tackle temperature first. According the SCAA, the ideal brewed coffee temperature is 200 F (93 C). Although espresso temperatures can vary by blend, I’ll use the number EspressoQuest charted, which is 198 F (92 C). So our temperatures are basically the same. Is it the pressure?

A study titled On the physics of pressure denaturation of proteins from The Journal Of Physics: Condensed Matter authors Yuichi Harano and Masahiro Kinoshita write:

A protein, a long polypeptide chain, spontaneously folds into a unique native structure in aqueous solution under physiological conditions [1]. However, the native structure is unfolded by various perturbations such as the addition of chemical substances, the change in the temperature or pH, and the application of a high pressure [2 6].

I located other studies that link protein denaturation and pressure. So it appears the mystery is solved. A coffee plant has protein on the root. Even after the beans go through processing, roasting, grinding and brewing temperatures, there will be a trace amount of protein left. However, if exposed to the pressure used to extract espresso, that trace amount of protein becomes denatured.

Hope I got that right. If I made an error, please let me know in the comments.

Sources:

Espresso, unsweetened Calories – CalorieKing

Brewed Coffee, unsweetened Calories – CalorieKing

A cDNA encoding a metallothionein I-like protein from coffee leaves (Coffea arabica) – Plant Physiology study by S Moisyadi and J I Stiles (January 1995)

Metallothionein-like protein 1 (Coffea arabica) – WikiProteins.org (page gone)

Denatured Definition – TheFreeDictionary

Denaturation (biochemistry) – WikiPedia

The Espresso Quest – Blog post by Instaurator (Site down January 2015)

On the physics of pressure denaturation of proteins – Journal Of Physics: Condensed Matter study by Yuichi Harano and Masahiro Kinoshita (Issue 7: 22 February 2006)

Legacy Comments:

TigerAl – Forgot to mention that the resolution of the measuring (in this case, weighing) device also would play a part in the error. I dont think CalorieKing cares that much about decimal point precision in calorie reporting. Your research and case was impressive though, MAS

Paul – What a fantastic and fascinating blog. I love coffee and discovered this via the ineedcoffee website when looking at roasting my own coffee. It has been a few years since I studied biochemistry. I could be wrong and it wouldnt be the first time. Part of what makes a protein one kind and not another is the way it folds up and looks three dimensionally. Boiling and together with high pressures will denature a protein that is to say break the bonds between certain amino acids that cause it to have the particular 3-d shape. However, chemical tests for protein would still reveal its presence.