Plant cell wall engineering

The amazing structural properties of plants” – “Via “ScienceWise”  at the Australian National University.

I came across this when I was searching for the strategies used by other people and institutions regarding efforts to expand the public awareness of science. It’s a little old, from February 2008, but I thought it of interest.

Plant cell walls are incredibly important in all sorts of places in foods: from the texture of fruits and vegetables, and how texture softens during ripening, often due to concerted action by enzymes like pectinases, to the efficacy of extraction techniques where plant cell walls that need to be degraded for access to the internal contents e.g. wine grape crushing. Plant cell walls also soften under the impact of enzymes produced by post-harvest microbial growth. Any one who has experienced the effects of Erwinia carotovora soft rot on potatoes or carrots has seen first hand what the concerted effects of pectinases, cellulases, and xylanases can do to the integrity of the plant tissue. Cell walls  are important in cereal processing as well. Depending on their solubility arabinoxylans (AX) in wheat can be beneficial or detrimental to baking properties of flour, and AX create a second elastic polymer network in cookies that can limit their spread. Soluble beta-glucans [closely related to cellulose] are a benefit as soluble fiber in oats and barley, but can be a nightmare  for brewers trying to drain a mash tank.

Daniel J. Cosgrove, of the Department of Biology at Penn State University, got it right when he wrote;

Without cell walls, plants would be pliant piles of PROTOPLASM, more like slime moulds than the stately trees and other greenery that grace our planet“.

(Cosgrove DJ. 2005. Growth of the plant cell wall. Nature Reviews Molecular Cell Biology 6, 850-861 | doi:10.1038/nrm1746)

Anyone who has mistakenly grabbed a Erwinia rotted potato has experienced what the whole plant kingdom would feel like, and what its TEXTURE would be, without the cell walls – YEECH!

One problem about plant cell walls is their complexity. It has been hard to model their fine structure, and even harder to define the sequence of events in their synthesis.

In the article “Mixing cell biology with mechanical engineering” Shankar Kalyanasundaram, Hung Kha and Richard Williamson, biologist and engineers team up to model primary cell wall structure.

Williamson is quoted…

The mechanical properties of any material always reflect its underlying structure” of course for food scientists the mechanical properties are also the properties we perceive as texture when we eat the material.

Dr Kalyanasundaram reported; “… biologists might be able to test the individual components that make up the structure of the cell wall, but they don’t have the expertise to model the various components as a system.. How the structure of a cell wall gives rise to its mechanical properties is an important research area, and we need this understanding if we are to better understand cell expansion and the role it plays in plant growth“.

This is entirely aligned in its strategy with  the systems approach to understanding plant cell walls published by  Chris Somerville and colleagues from Stanford in 2004

(Somerville et al. 2004. Toward a Systems Approach to Understanding Plant Cell Walls.  Science 24: Vol. 306. no. 5705, pp. 2206 – 2211. DOI: 10.1126/science.1102765)

The abstract of Kha et al can be found here –  —  —  —  Kha H, Tuble S, Kalyanasundaram S, Williamson RE. (2008) Finite element analysis of plant cell wall materials. Advanced Materials Research 32: 197-201.

Open you fermentation horizons & mobile microscopy

A new post “Forays in Fermentation” from Jeremy at the Agricultural Biodiversity Weblog, via Research Blogging highlights two recent papers on fermentation that go beyond the usual beer/wine paradigm that I see in some students that choose our fermentation option.

The papers are

  1. Nout, M. (2009). Rich nutrition from the poorest – cereal fermentations in Africa and Asia Food Microbiology DOI: 10.1016/j.fm.2009.07.002 []
  2. Poutanen, K., Flander, L., & Katina, K. (2009). Sourdough and cereal fermentation in a nutritional perspective Food Microbiology DOI: 10.1016/j.fm.2009.07.011

We have used idli (rice & mung beans & a small amount of fenugreek) and injera (teff –  Eragrostis teff) as demonstration fermentations in the Topics in Fermentation – Science of Baking class. They are quite interesting. The idli ferment smells for all the world like yoghurt, apparently from a colonization of lactic producing bacteria. We kicked off our injera by chewing some of the grain and returning it to the mix, giving an inocculum of acid forming bacteria [better not done immediately after cleaning your teeth] and amylase from saliva to provide the two essentials – fermentable sugars and fermentation organisms.

The paper by Nout looks like a good read.

Microscopy comes to Web 2.0

I have been looking for ways to streamline our experience of viewing the diversity and behavior of starch granules outside the traditional transmission microscope exercise we have done in Food Chem labs – most students, and I, who don’t use microscopes everyday, often have trouble setting them up, and as an instructor, with multiple microscopes in a lab, I don’t know if students are seeing what te ought to be.

A new development in clinical microscopy…

Breslauer, D., Maamari, R., Switz, N., Lam, W., & Fletcher, D. (2009) Mobile Phone Based Clinical Microscopy for Global Health Applications. PLoS ONE, 4(7). DOI: 10.1371/journal.pone.0006320

for adaptation to a mobile phone (or I guess, my FlipCam) would let us all see a share our visions of starch granules, and share in real time the excitement [well, I am a food chemist] of seeing starch granules literally explode when we douse them with 1 normal hydroxide.

For more see Dan Gorelick’s post at Science Planet , which I also found via my RSS feed from Research Blogging.

Why does my pita puff ?

Pita is made from one layer of dough, not as some think 2 layers that are joined at the edges.

So how does if puff?

The dough is often given a final proof that is drier than for risen breads. When the bread hits the hot oven the slightly dry skin seals. Really thin flat breads like pita can be baked at extreme temperatures. My lab in Sydney when I worked there used a pottery kiln for our routine test-baking of pita, and we baked them for 30 seconds at 550 degrees CELCIUS (about 1020 degrees F). Not unlike the conditions in a tandoori oven.

The sealed skin first constrains the existing gases in the small amount of dough that will turn to crumb. These expand and exert more pressure in line with the gas laws. There MAY be a VERY brief moment of additional carbon dioxide production from yeast but this will be really limited in the thinner types. But the greatest gas production and pressure comes from the water in the dough that turns to steam, lots of gas an pressure now. If you see the video in my last post you’ll see the outcome. The interior splits at the weakest point creating the taste sensation of fresh, high temperature baked pita bread. Good enough to eat on its own.

You can access a schematic of this on the google books preview of Jalal Qarooni’s Flatbread Technology book (page 71).

And why does my poolish lose weight ?

I commonly bake using a yeast poolish, a mixture of equal weights of flour and water, and a vanishingly small amount of instant yeast that is allowed to ferment overnight or longer. This long pre-ferment creates bread of outstanding flavor.

Anyway, I am, as many bakers are, in the habit of scaling out the exact amount of poolish I will need for a dough (I always bake using formulations that list ingredients by weight).In the morning i use the poolish assuming the same weight. Duh !! Some chemist I am.

Out of error the other day I had more poolish than I needed. When I weighed what I had I had 952 g. The evening before we had scaled out 500 g flour, 500 g water, and 1.5 g yeast (1001.5 g). It is clear that we’d lost in the order of 5% of the poolish weight. As I had covered the poolish I assumed this was not water loss, but loss of carbon dioxide (and maybe volatilized ethanol). Poolishes rise but at 100% hydration they are pretty weak and sloppy and gas loss would be expected.

If this was the case the weight loss would come from the conversion of starch to maltose , and the conversion of the maltose, via the fermentation pathway to CO2 and ethanol with the subsequent loss of volatiles. Anyway the result is a poolish that is more than 100% hydration (1.11 %) as the substrate comes mostly from the dry solids part of the poolish (although maltose requires the addition of 1 water molecule when hydrolysed to 2 glucose). This probably isn’t enough difference to cause dough handling problems and I can just go back to my weigh out the poolish the night before assumptions and not worry about it (until I do any poolish research, with the aim of eating the experimental outcomes of course.)

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