The Fresh Loaf

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Gluten: W, P/L and the problem with durum

mwilson's picture
mwilson

Gluten: W, P/L and the problem with durum

Note: I wrote this, I've chosen to publish even though I feel it's not exactly complete. Should be accurate. Hopefully it will make sense.

 

Gluten: W, P/L and the problem with durum

Baking professionals in Italy commonly refer to the strength of flour by its W index, describing this as its "force".

Italian Millers provide much data in their flour specifications often including rheological properties recorded by the Chopin-Alveograph. Testing performed by the Alveograph involves forcing air into a piece of dough causing it to expand like a balloon until it bursts, at which point the test is complete. The data is recorded on a graph as a line measured in millimetres. The pressure required and therefore resistance of the dough is measured as P. The final size of the dough is measured as L, this being it's extensibility. The area under the line is represented by W which indicates overall strength.

The index of W.

Up to W170 (weak): for biscuits, waffles and tender baked sweets;  for béchamel and thickened sauces.

From W180 to W260 (average): French bread, bread rolls, pizza, pasta. 

From W280 to W350 (strong): classic bread, pizza, pasta, baba, brioche.

Above W350 (very strong): Made with particular types of wheat, that are used to reinforce weaker flours. Ideal for highly enriched doughs subjected to long leavening, Often referred to as "Manitoba".

Extensibility, elasticity and quality.

Gluten consisting of proteins gliadin and glutenin is the one, which supports the dough. The higher the content of the gluten, the stronger is the flour. However, the amount of gluten does not determine everything, its characteristics are also important. The strength of the flour is more dependent on the properties of gluten. Two flours may have the same amount of gluten, though, one may be stronger and the other weaker.

Gliadin in contact with the water forms a fluid sticky mass, while glutenin absorbing water, forms a compact mass, elastic and resistant. The wet gluten possesses all the mechanical properties of the two proteins. Obviously, for a flour to be strong it must have glutenins in the majority. If a meal has a high amount of gluten, but this consists mainly from gliadin, the flour can not be very strong, because its gluten is soft and slightly spongy.(1)

P/L and Durum

Instead of referring to them individually the balance between extensibility and resistance can be expressed with the P/L ratio, the optimal being between 0.5 - 0.6. With a P/L ratio higher than 0.7 the flour is very resistant, lower than 0.4 it is very weak and extensible. Soft wheat flour is naturally extensible while hard wheat flours are naturally more tenacious this is especially true with durum, the hardest wheat of all, where a typical P/L ratio can be above 1 and even exceeding 2.

Durum wheat doughs are distinguished by a high resistance to deformation and consequently limited extensibility. To be judged sustainable for bread making the semolina must have a protein content >12%, a good farinographic stability and alveograph P/L index value below 1. However it should be noted that the bread-making process preferred in the case of durum wheat requires the use of sourdough. A case in point is the pane di Altamura. The proteolytic activities of semolina and/or of the lactobacilli may produce considerable changes in the rheological properties of the gluten network, reducing the natural excessive elasticity of dough made from durum wheat.(2)

Numerous factors brought about by fermentation and inclusion of other ingredients will affect the properties of gluten, even water quality. Water too hard being rich in minerals will make gluten more tenacious reflecting an increase in the P/L ratio. Soft water will create the opposite effect making gluten softer and more extensible reflecting a reduction in the P/L ratio.

(1) Giorilli
(2) Handbook of Food Products Manufacturing

breadforfun's picture
breadforfun

Thanks very much for posting this, it helps me to understand some of the issues that I have had with using durum flours.  My dough often becomes overly sticky in a shorter than expected time frame, and probably this explains why.  A couple of questions:

Is there a list available that classifies the varieties of durum with respect to their W number?

Can you please provide additional details for your references?

Thanks,

-Brad

mwilson's picture
mwilson

Hi Brad,

Thanks, I was hoping someone would find this useful... I have more articles in the pipe.

I have seen some data regarding the different varieties which tells me they clearly blend them to achieve a flour suitable for making bread. Growing conditions will affect the properties of the grain. I'm not sure there is a list as such.

My citations included Piergiorgio Giorilli - La Tecnica Dell'Autolisi. He speaks of the properties gluten.

and

Front Cover

This has much information regarding durum wheat and the process used for making pane di Altamura.

Stay tuned for
- "Calculating inoculation"
-"Acidity, Sourness and LAB"

Cheers,
Michael

breadforfun's picture
breadforfun

Hi Michael,

Thanks for all the information.  The Giorelli article is interesting, but I haven't found the handbook yet.  I hope a library will be able to get it for me.  I am very interested in it because of the info on Pane di Altamura.  I have to admit that I fell in love with this bread when I was there a year and a half ago.  I even brought home 5 kg of flour, and I had some luck with it, but I haven't been able to consistently reproduce the shaping and oven spring that they get there.  And I don't have a wood fired oven, either, so of course that makes a difference, too.  Your post made me think that maybe I got the wrong variety of grano duro. Little did I suspect that they could be so different, but I'll be ready next time.

I'm looking forward to reading your next posts.

-Brad

mwilson's picture
mwilson

I'm not sure if you noticed but there is a link to the handbook, if you click the picture. You can preview a fair degree of the content from google books.

I have seen your posts regarding pane di Altamura, a great resource they are. I have also been trying to replicate this bread but after having worked my way through 25KGs of semola rimacinata I am still at a loss as to how they make it exactly. After continued researching and much thought I think I know how to proceed. But it means buying another sack, not cheap! The difficultly is understanding how they obtain a starter like the one they use and knowing the overall process. Clearly they are using a super active leaven with low acidity.

Rest assured, the semola you picked up from Italy was the right kind. Re-milled, Rimacinata is intended for bread baking or "per panificazione". The blending of durum wheat varieties I referred to is something that would happen at the mill.

 

Not long ago I discovered this website: http://www.panealtamuradop.it There is much to see and there's something about that golden yellow dough...

breadforfun's picture
breadforfun

Michael,

Thanks, I did look at the preview on Google, but could not find anything specific to durum, maybe I am missing it.

Your comment to DBM regarding the acidity of the starter for Pane di Altamura started me thinking again.  Is there a reference to the leaven in the regulations other than it should be enhanced 3 times before using in the final dough (I seem to have lost my link to the original regulations, so I can't check it).  I wonder if perhaps they traditionally use a biga from commercial yeast instead of a natural starter with LAB to keep it lower in acidity.

BTW, here are a few more sites that you may have already found, but if not, may provide more information:

http://murgiapride.com/2015/en/bread/

http://www.panificioduemila.it/

-Brad

mwilson's picture
mwilson

Hi Brad,

Thanks for the links. When you preview the book on google, use the search box on the left pane, for keywords like "durum" or "Altamura". You should find there is a diagram of the process used at one particular bakery on page 368.

There is evidence to suggest that the so-called "lievito naturale" may be obtained with three refreshments from a biga pre-ferment or old dough. Such a method would make sense as to how the process times are relatively short.

I will experiment. The key thing is to see if with refreshments the leaving power is sustained and if the pH drops to the desired 4.0.

-Michael

breadforfun's picture
breadforfun

Thanks Michael.  I've been away and unable to log in for the last week.

This thread has inspired me to give it another shot with some of the changes you outline.  My starter generally gets to a pH of around 4.2 (using pH paper, so I'm not sure how accurate it is).  I've never been able to get it below 4.0, though.  How are you measuring yours?

-Brad

mwilson's picture
mwilson

The precision of pH isn't actually that important and a difference of .2 doesn't mean anything in this context. What I deemed critical is where activity starts to slow. This occurs at a range of pH. Usually between 4.0-4.3.

I have a cheap pH pen which are abundantly found on eBay.

I look forward to your next bake..

Good luck. I hope I have been of some use.

regards,
Michael

breadforfun's picture
breadforfun

Michael,

I haven't used pH meters in a long time so I am not up on the latest technology.  In the old days, when the electrodes were filled with electrolytes, it would have seemed to me that you wouldn't be able to get an accurate reading of a viscous fluid like dough, even with 80% hydration.  I guess that has changed so I'll have to read up on the new ones.  Thanks.

-Brad

dabrownman's picture
dabrownman

Durum that you are bring to light.  Durum is the hardest wheat variety and it has the 2nd highest protein content , second to Red Spring Hard Wheat grown in the Pacific Northwest that is used for High gluten flour.  The only problem I know about durum wheat is that, even though it is very high in protein it is low in the two that count.  It lacks sufficient quantity of the 2 proteins that form gluten in the presence of water so the the gluten formed is not as strong as common wheat varieties and why most durum breads are made with some higher gluten wheat flour in the mix.  SD helps to strengthen the gluten strands in whole durum breads and why it is the preferred way to to make whole Durum breads.

Is this what you are saying or is there more that my feeble brain can't understand?

See you in June Michael

mwilson's picture
mwilson

There is more...

The problem as I call it, with durum isn't one that can't be overcome. However compromises must be made. What I think you have missed or I haven't emphasised clearly is the matter of gluten quality and how this relates to the P/L ratio. 

Unlike common wheat durum can be characterised as having very resistant gluten this is because it is unbalanced. It has too much glutenin in relation to gliadin and consequently a high P/L ratio. This problem is exaggerated with semola rimacinata. Working with durum hands on, it's clear to see and indeed feel that its gluten is different. A common mistake is to over hydrate this flour to compensate for its inherent toughness. Extra water will not change the nature of the gluten it will just flood the dough. Fermentation and a utilisation of native enzymes will transform the gluten and create a dough much more workable however.

It's too much of a broad statement to say that SD helps strengthen gluten, truth is while it does influence a temporary strengthening effect it mostly just weakens it but this weakening is highly desirable to make the most of durum wheat gluten.

To gain perspective take Spelt for example. It is on the other end of the spectrum as it can be characterised as having very weak, extensible gluten as it has too much gliadin in respect to glutenin. A typical P/L ratio for Spelt is 0.4 or less.

Hope that helps,

See you in June!

suave's picture
suave

Not all durums are the same - we have a couple of Italian brands in stores around where I live and they differ drastically in their behavior - one produces the most lovely loaves and is very easy to work with, the other - tried it a couple of times and chucked the outcome (won't call it bread) both times.

mwilson's picture
mwilson

Sure, as does all wheat vary. But in the majority I think durum being hard wheat as with all hard wheat leans toward a more tenacious gluten. The evidence is there. Glutenin being resistant is at the heart of what make flour strong after all. This problem of very high tenacity may only be with rimacinata and perhaps even millers and bakers want this. All the specs I've seen look very similar with a P/L ratio around 2 and ~W200. Thanks for your input Suave.

mwilson's picture
mwilson

http://www.globalsciencebooks.info/Online/GSBOnline/images/0812/FOOD_2(2)/FOOD_2(2)75-90o.pdf

Typically many durum wheats have inferior gluten strength needed in the fermentation process to produce bread (Boyacioglu and D'Appolonia 1994). The dough tends to be inextensible and this can reduce oven spring in bread and therefore reduce loaf volume. In farinograph tests durum flours have higher water absorption than bread wheat flours due to the higher starch damage during milling, especially when semolina is re-ground into flour (Saperstein et at 2007). Also, farinograph development times are often shorter than bread wheat flours and durum flours have unsuitable doughs for bread making when measured using the
extensograph and alveograph (Boyacioglu and D'Appolonia 1994).

The inextensibility typically associated with durum wheat is one reason thought to explain why durum bread has lower loaf volume than when made from hexaploid wheat (Ammar et at 2000). Typical dough characteristics of durum wheats varying in dough strength used for baking are shown in Table 3. Mixograph mixing curves showed wide variation in dough mixing among the cultivars. The extra strong varieties have longer mixing times and this trend agrees with the mixing energy and mixing time data obtained during mixing using either baking process.

The baking strength index (an indicator of loaf volume potential at a given protein) values obtained are lower than for common wheat, typically of 100. The strong durum exhibited higher P/L ratios than common wheat, indicative of a less extensible dough. In addition, a less extensible dough is not desirable because of dough handling (sheeting) properties are inferior. A dough can be firm but also needs elasticity to avoid a doughy product. Durum with weak gluten exhibits more viscous and less elastic dough than bread wheat flours. This can be shown by a low extensibility using the alveograph (L) or extensograph. But the alveograph pressure and overall strength also tends to be too low in durum. To achieve an improvement in loaf volume a better balance of P/L and W is needed. By increasing the gluten strength, better loaf volumes and fermentation tolerance were obtained but they still fall short of loaves made from bread wheat (Marchylo et at 2001).

mwilson's picture
mwilson

Relevant information:

La forza della farina

And better explained than myself.

mwilson's picture
mwilson