Sugars in roasted coffee – a conversation

I’m posting this (with consent of the other conversants) in an effort to kickstart some more open discussion about roasting. Right now there is very little out there when it comes to roasting speciality coffee. I am aware that most of the studies quoted below were probably done on C-grade coffees.

I hope we can get to a place where we can discuss profiling, roast development, densities and the like with a view to understanding what we do better and to reduce the time we put into trial and error profiling. Undoubtedly lots of great roasters aren’t just heating and hoping – they are applying a lot of what they’ve gained from previous experience and other knowledge.

Below is an email exchange between Deaton Pigot, Chris Kornman (both roaster with Intelligentsia) and myself. It had come about after a comment I’d made on someone else’s blog about there being no sugars in roasted coffee.

As becomes evident in the exchange – there really is a need to better define what we are talking about when we use the term sugars.

I wish I could say that all my emails are like this, that every day is an in depth discussion of theory combined with our experiences.

Unfortunately this conversation didn’t get any further – work has a terrible way of getting in the way of things like this. I can only speak for myself here, but feel free to pick apart things I’ve gotten wrong or misunderstood. If people would like to contribute anything – please post in the comments.


From: Chris Kornman [mailto:xxx@xxx.com]
Sent: Thursday, January 21, 2010 11:43 AM
To: Deaton Pigot
Cc: xxx@xxx.com
Subject: Re: This is interesting

So we got interested in this “no sugars” claim by JH below, and Josh &
I did some digging.

Sucrose (aka white table sugar, chemical formula C12H22O11) has a
melting point at around 370F (so sucrose, if present in coffee, would
caramelize at or before 1st Crack.) It breaks down into water vapor
(H20) and carbon dioxide (C02).

According to our colleagues Henry Schwartzberg (Professor Emeritus,
University of Massachusetts) and Joachim Eichner (Praxis International
Inc), during the roast, Polysaccharide (Starch chains containing sugar
polymers) Hydrolysis (water breakdown that splits polymers) in the
cell walls of roasting coffee results in the creation of sugars and
oligosaccharides (simple sugar chains). Also, sugars break down to
form Aliphatic (fatty) acids.

So roasting both creates and decays sugars.

Rapid decay begins later in the roast, (11min according to HS & JE),
so likely during or after first crack. The decay of sugars also
produces alcohols & furfural (“…an aromatic aldehyde… Its chemical
formula is C5H4O2. In its pure state, it is a colorless oily liquid
with the odor of almonds, but upon exposure to air it quickly becomes
yellow” – Wiki).

It seems unlikely that full degradation would take place before the
end of 2nd crack, however.

So, yes, there are sugars in roasted coffee.

-ck


On 21 Jan 2010, at 20:11, Deaton Pigot wrote:

Hey James,

I am emailing you because of your comment on the Double shot blog about
sugars left in coffee after roasting. I am sure all of us would love to hear
your thoughts on our mail chain. That is of course if you have the time to
answer.

Hope you are well!

Kind regards

Deaton

Chris and Josh that is a great read! Out here we (or at least I) have bought
into the Staub theory of developing sucrose. IE Polysaccharide split into
monosaccharide, creating a foundation of sucrose that would essentially be
dug into the later stages of the roast.

I went into it in some detail here

http://deaton.wordpress.com/2009/07/12/balancing-act-sweetness/

Plus some of it here as well.

http://deaton.wordpress.com/2009/05/07/how-sweet-is-your-cup/

I know that in James comment he stated;

“Nothing to do with sugars – of which there are none left in roasted coffee,
certainly not simple sugars which react away through maillard, caramlisation
and strecker degradation reactions.”

So it leaves me wondering what sugars he believes are left in the bean after
roasting, as we all know we do perceive varying levels of sweetness.


From: James Hoffmann [mailto:xxx@xxxx.com]
Sent: Thursday, January 21, 2010 2:13 PM
To: Deaton Pigot
Subject: Re: This is interesting

Hey Deaton,

I could well be wrong – it’s usually the case.

I will dig out the reading I’d done on this – by sugars I meant simple sugars, rather than long chain complex carbohydrates.
No doubt there is sweetness, but more things than simple sugars create the sensation of sweetness on our tongue. Sweetness reception, like bitterness reception, is complex and only reasonably understood.
I find the sweetness of coffee to be unlike the sweetness of simple sugars. I’ve had very sweet shots, for example, but not ones I would describe as sugary.

Give me a day and I will either get pwned or add something interesting.

Kind regards,
James Hoffmann


From: James Hoffmann [mailto:xxx@xxx.com]
Sent: Thursday, January 21, 2010 3:39 PM
To: Deaton Pigot
Subject: Re: This is interesting

Hi Deaton,

I think the tricky one here is going to be pinning down a useful definition of sugars.
Once polysaccharide chains get longer they are perceived as being less and less sweet. In the past i used an 18-chain multidextrose to make non-sweet syrups.

The simple sugars, as would be perceived as having a strong sweetness, are very reactive during roasting – through maillard, strecker and caramelisation. Certainly there are some longer chain sugars in the mix.
Most of the research has been done on lower quality coffees, which will always leave us a little in the dark I guess.

Having dug through a couple of textbooks I am up to the following to support my initial statement on the lack of sugar in roasted coffee. If you want the full paper titles referenced then let me know and I will dig them out.
When talking about sugars I am talking about low molecular weight carbohydrates (R.J. Clarke’s definition). On a side note – related to the original post – it seems Petracco et al (1999) isolated a polysaccharide that actually increased foam stability. I digress….

Trugo & Macrae (1985) did a study that showed 97% sucrose loss in light roasted coffee, creeping up to 99% in a dark roast. Hughes and Thorpe (1987) came up with 0.24% and 0.34% sucrose in roasted coffees. Sucrose can hydrolise into glucose and fructose but both degrade thermally more readily than sucrose.
Noyes and Chu (1993) found 0.1% sugars in 21 roasted Brazilian coffees – though most of this (0.08%) was sucrose.

My reading implies that most of the polysaccharides in coffee are extremely stable through the roasting process – according to Bradbury and Halliday (1990), and hydrolysis would have to happen at lower temperatures when there was still water available. I couldn’t find data on water activity and roasting temperatures – to understand when hydrolysis would stop. Will keep looking.

I guess what I would look to argue is the breakdown of the polysaccharides – which ones are breaking down, into what size chains? Do these chains have detectable sweetness? What molecular weight do we use to decide a cut of point for sugars?

Food for thought…..

Kind regards,
James Hoffmann

Related Reading

For reference this older post on the Maillard Reaction, Strecker Degradation and Caramelisation may be of interest.

18 Comments

  1. @Mike

    I presume the sugar application is calibrated to convert the refractive index of a simple sugar/water solution and output a strength. The meter won’t detect sugar.

  2. Perhaps Vince will clarify, but it’s my understanding that it does (I’ve never tried, obviously).

  3. The refractometer measure neither coffee strength nor sugar concentration, rather you could say it measures density – or refractive index which relates to density. In either the case of a sugar or coffee solution this can be converted using tables (done internally in the refractometer) – so although sugar will be contributing to the density of the coffee beverage the meter cannot discriminate…

    This is rather an aside to the main topic here, to which I might add to later…

  4. Most sugar refractometers read between 5 and 25% sugar. The coffee refractometer is calibrated to look for a total solute of under 2% in total, of which sugars would likely be a very small proportion.
    So, no, you can’t use an r2mini for sugar.

  5. James,

    I assume you’re reading Clarke and Vitzthum’s Coffee Recent Developments, from the World Agriculture Series. If so, here is something I’m wondering about, and might help start digging into how to eliminate the trial and error roasting process…

    In that same text, section 1.3.2 on high molecular weight carbohydrates states that in roasting, “much of the cell macrostructure survives”, the macrostructure, that is, being made of polysaccharides. They evidence that the polysaccharide linkages that were present in the green coffee were maintained after roasting, meaning that they did not break down into other monosaccharides and simple carbohydrates. However, this was the reporting for what they called ‘conventional roasting conditions’.

    So, my first question is, obviously, what are conventional roasting conditions? Later, in section 4.2.2 they characterize ‘conventional roasting’ as being in a horizontally rotating drum with high temperatures, long time (HTLT). Makes sense to me, until I realize that their high temperature is 450c (!!!) with roasting times from 8 to 12 minutes.

    Now, I can’t speak for any other specialty roasting company, but an incoming air process value of 450c is not in line with our roast parameters to produce a specialty product, and 8 minutes certainly doesn’t quite seem like a long time to me.

    Meanwhile, above, in the first e-mail in the correspondence, Chris states “According to our colleagues Henry Schwartzberg (Professor Emeritus, University of Massachusetts) and Joachim Eichner (Praxis International Inc), during the roast, Polysaccharide (Starch chains containing sugar polymers) Hydrolysis (water breakdown that splits polymers) in the cell walls of roasting coffee results in the creation of sugars and oligosaccharides (simple sugar chains). Also, sugars break down to form Aliphatic (fatty) acids.”

    If there is polysaccharide fractioning as described by Chris above, it must be very minimal, no? How then might ‘conventional specialty roasting conditions’ (wish I’m sure every specialty roaster would like to describe as adequate heat, adequate time [AHAT]) degrade the polysaccharides differently… perhaps a longer dehydrolysis/dehydration period prior to first crack?

  6. While on an origin trip to Cuba I visitted a small botanical garden which grew coffee, there was an old Haitian lady there who roasted the coffee in a pan over a fire and through in some actual sugar half way through the roast. She brewed it in a coffee sock and it actually tasted really good! I don’t drink coffee with sugar and we havent tried throwing sugar into a roast on our Whitmees or Uno! But it was interesting to see.

  7. James, your post highlights a real problem in the use of terminology by scientists and laypersons.

    Sugars, to a scientist, are a very broad category of chemicals. To a layperson, sugar is sucrose (a disaccharide comprised of a molecule each of glucose and fructose). Reports on coffee chemistry use sugar as a general term, not the sucrose-specific term. Hence the entire email conversation between James and the other folks. When laypersons read them, they take information out of context.

    As far as I know, compounds that humans might detect as “sweet” have not been looked at in coffee. We assume they are present and physiologically active because everyone talks about “sweetness” in a brew. Not having data to back up claims, we make educated guesses that polysaccharides break down into smaller pieces that we detect as sweet, though, as James wrote, not the same sweet we get from the low molecular weight (aka simple) sugars of glucose, fructose, and sucrose. BTW, all those polysaccharides initially are likely to be components of cell walls.

    Colleen, you make an astute observation by questioning how coffee science has been conducted in the past and how specialty coffee roasts, brews, and consumes coffee.

    Historically, coffee scientists have not been coffee geeks. Consequently, science hasn’t looked at specialty coffee the way coffee geeks do. Of course, coffee geekdom is relatively new and coffee roasting/brewing/consuming varies widely by location around the world. Hence, it is not a surprise that much coffee science work lacks a sense of authenticity to the new breed of coffee drinker.

    While this historical quirk doesn’t negate past research, it does suggest that past data should be considered in light of the procedure it was gathered in (this is a major reason why all journal articles come with a materials and methods section- to ensure everyone understands the process). Luckily, recent years have seen the arrival of coffee geek minded scientists who are stating to ask novel, coffee geek appropriate questions. This, along with the quality initiative Peter G is promoting (a brilliant idea, BTW), will begin to address questions that modern specialty coffee wants answered.

    Now, if we can just fund it all…

  8. Like the commentary above, I was under the impression that Cuban Style roasting included “cane sugar” wit the beans when roating. This deepens the flavours and gives the beans a glossy colour simular to French Roasting (but without the ‘burned’ flavour of the French). Nes pas?

  9. Colleen wrote: “How then might ‘conventional specialty roasting conditions’…degrade the polysaccharides differently… perhaps a longer dehydrolysis/dehydration period prior to first crack?”

    I don’t pretend to know all of the science involved, but I will say that through simple A/B testing and trial and error I’ve found that for softer beans that are considered to be “sweeter” or contain more sugars, introducing a “drying” phase of lower heat during the early part of the roast (not enough to stall the roast however) does result in a sweeter cup profile.

    It’s been my assumption–because I do believe that there are sugars in roasted coffee–that stretching out the dehydrolysis phase early on causes the more complex polysaccharides to break down into simple sugars just enough to sweeten the final result. But as I say, I haven’t found enough research to be sure that I’m guessing correctly, so I could be wrong.

  10. Hello Jim/James;

    Good conversation. Would you please tell me: what EXACTLY do you mean when you speak of “profiling”; I read various explanations of this, mostly from those who roast. Some refer to bean profile, others to temperature profile; it becomes confusing and maybe even dubious in terms of veracity. Anyway, thank you in advance. Cheers!

  11. Yes, the discussion begs the definition of “sugars” as Shawn very rightly writes.

    The presumption that polysaccharides from cell walls break down into sweet-tasting compounds is the dominant theory of sweetness in coffee. All around, however, we cite studies that disaccharides and monosaccharides are few in roasted coffee.

    It makes the ripe cherry phenomenon all the more interesting: As coffee cherry ripens, the mucilage tastes sweeter. And, consistent with that, roasted coffees from riper, sweeter cherries themselves taste riper and sweeter. Presumably, there is more simple-sugar content in ripe cherry mucilage. However, it is unlikely that those sugars persist into the final, roasted coffee. So why do our sweetness receptors likewise perceive riper ROASTED coffees to be sweeter? I have always assumed that there are more stable compounds in ripe fruit that trigger our sense of “sweetness”. Perhaps we, as fruit tasting machines, have gotten so accustomed to correlation of certain flavor compounds with sugar-sweetness that the simple existence of them triggers a sort of fantasia-sweetness flavor, which we also call “sweetness”.

    Peter G

  12. @Peter G – I think it has to be related to the quantity of simple sugars we start with. The reaction pathways seem so fuzzy that I guess it would be good to know exactly how the composition and ratio of simple sugars changes in coffee fruit, and in the seed itself, around the time of maturation.

    I had a quick chat to Harold McGee about this, and he said he’d seen a paper on sweet tasting compounds in coffee but couldn’t find it. (He did send me 4 other awesome ones though – so no complaining from me.) He mentioned that some phenolic compounds can taste sweet (I think! I wish I had taken notes!)

  13. @Ollie Profiling refers to the route taken by a coffee, in terms of the temperature of the bean mass, to get to a certain end temperature.

    A roaster is able to accelerate or slow down temperature change to alter the flavour of the coffee. It is easier to explain with a graph – most profiles are charted with time along the bottom and temperature up the side. This graph has two different roast profiles for two different coffees:

  14. I think it is a reasonable assumption to conclude that the cause of sweet tastes in coffee has to do with more than sugar (as others have pointed out). Aspartame for example is amino acid (protein building block) based instead of sugar based. Indeed I am sure there are compounds that enhance the perception of sweetness, without themselves directly tasting sweet (certain salts for example which would not be affected by roast).

    Nonetheless, interesting topic – and obviously carbohydrates are extremely important given they make up about 46% of green coffee by weight.

    Poly-, oligo- and mono- saccharides could all be considered sugars. However, typically the perception of sweetness decreases with sugar chain length – ie mono- > di- > oligo- > poly-

    Sucrose of course is a disaccharide, but it is less sweet than the monosaccharide fructose, which is a component (with glucose).

    Polysaccharides contribute in general to organoleptic properties of coffee brews – ie viscosity, mouthfeel, foam stability etc (Redgwell 2002). Hydrolysis during roasting decreases polysaccharide molecular weight (makes them smaller) and also decreases their branching. Basically breaking the chains off into smaller chunks – oligo- and mono-saccharides.

    Oosterveld et al (2003) did a reasonably thorough study on poly- oligo- and mono-saccharide composition of roasted (arabica) coffees.

    The first finding was that from a starting point of about 46% carbohydrate in green beans – you are left with between 29-37% in roasted coffee, with dark roasted coffee having the smallest amount. The rest are converted into degradation products.

    Importantly the proportion of (extractable in water at 90C) the various types of carbohydrates was noted.

    4.6g of polysaccharides per 100g coffee

    2.0g of oligosaccharides per 100g coffee

    28mg of monosaccharides per 100g coffee

    28mg = 0.028g

    The monosaccharide levels didn’t vary significantly between light and dark roasts – and even though the starting sucrose levels were quite large, and you might expect a decent amount of fructose and sucrose at light roasts – even at mild roasting conditions the sucrose had been converted into degradation products.

    While it would seem that the sugar – at least the sweet sugars in the final product are all but gone, I can’t help but be struck by the casual observation of fruit ripeness and cup quality (Peter’s point).

    Mazzafera (1999) found a correlation between sucrose levels in green and been quality (defective beans vs good quality beans) – and as such perhaps an empirical link between quality and sweetness. However, Farah et al (2006) did not replicate that finding.

    Murkovic (2006) found up to twice the sucrose concentration in green arabica vs green robusta (up to 90mg/g vs mean 45mg/g). Though I wonder the value in comparing the two varieties, at least for our interest.

    -David

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