Microflora analysis of my starter. I never intended for this analysis but I was quite thrilled when it was provided. Originally I contacted Professor Gaenzle because I was having trouble with some flour and he very kindly offered some help. I asked him if he would analyze the flour because it wasn't working and he suggested that I send a little of my culture along with the flour. Off they both went in the mail and a couple of weeks later he emailed me this Letter. So I post it now.
Department of Agricultural, Food and Nutritional Science
Faculty of Agriculture, Forestry, and Home Economics
Dr. Michael Gänzle, Assistant Professor
Canada Research Chair in Food Microbiology and Probiotics
410 Agriculture/Forestry Centre www.afns.ualberta.ca Tel: + 1 780.492.0774
Edmonton, Alberta, Canada T6G 2P5 [email protected] Fax: + 1 780.492.4265
Salt Spring Island
Edmonton, Dec. 12th, 2007
Characterisation of microflora of Salt Spring Island sourdough.
Dear Tony DePasquale,
As discussed, we have analysed the microflora of the sourdough of your bakery on Salt Spring Island in the last weeks. The analysis was done by Dr. Clarissa Schwab, who is working as a post doctoral fellow my lab. The analysis was done within two days after the shipment reached us. I expect that the sourdough was kept at ambient temperature for much of the time during shipment and probably was fully fermented or over fermented.
Bacteria and yeasts were counted by surface plating on mMRS medium, which is known to support growth of all sourdough microorganisms. Single colonies of yeasts and bacteria were sub-cultured to obtain pure strains for further identification. From pure cultures, DNA was isolated to identify the organisms on species level by sequencing of the bacterial 16S rRNA genes, the current standard method for identification of bacteria. We did not characterize the yeasts on species level (yet).
The sourdough contained 1 x 1010 bacteria / g and 1 x 108 yeasts / g. The bacteria belonged to three different species, Lactobacillus rossiae (about 50% of cell counts), Lactobacillus spicheri (about 25% of the total cell counts) and Lactobacillus paralimentarius (about 25% of total cell counts). L. rossiae and L. spicheri are heterofermentative lactobacilli producing lactic acid and acetic acid or ethanol from glucose, L. paralimentarius is a homofermentative organsism which produces only lactate.
Heterofermentative organisms also produce carbon dioxide carbohydrates and thus contribute to dough leavening. Maltose (which is the most abundant carbohydrate in dough) is the preferred carbohydrate source. Furthermore, virtually all strains of L. rossiae and L. spicheri will use the sucrose that is present in wheat flour (about 10 g / kg) or added as part of the dough formula to produce acetic acid instead of ethanol. L. rossiae is known to convert the amino acid arginine, which is present in flour, to ornithine, an amino acid which is not naturally present in flours. Ornithine, in turn, reacts during baking to form the pleasant, roasty smell of the bread crust.
We keep all strains in our the strain collection at -80°C for future reference.
The species composition (combination of homo- and heterofermentative lactobacilli) of the sourdough is typical for wheat sourdoughs that are used as sole leavening agent. The combination of L. rossiae or L. spicheri and L. paralimentarius is much more frequent in Italian and French (wheat) sourdoughs compared to other areas of Europe or North America.
In the sourdough, bacteria outnumber yeasts by 100:1. Typically, the ratio of bacteria to yeasts is in the range of 10:1 to 100:1. Because yeast cells are much larger than bacterial cells (10 μm diameter for a spherical yeast cell compared to 0.5 μm diameter and 2 μm length for a rodshaped lactobacillus), a single yeast cell has a higher metabolic rate (glucose consumption) compared to a single bacterial cell. At a ratio of cell counts of 100:1 as found in your sourdough, the yeast metabolism in dough is about equal to the bacterial metabolism.
We were not yet able to determine the growth of the organisms in the flour samples that you shipped as well. We kept the flour and I hope that I can give you an update on these analyses in the next weeks.
with kind regards
Lactobacillus rossiae (about 50% of cell counts), Lactobacillus spicheri (about 25% of the total cell counts) and Lactobacillus paralimentarius (about 25% of total cell counts). L. rossiae and L. spicheri are heterofermentative lactobacilli producing lactic acid and acetic acid or ethanol from glucose, L. paralimentarius is a homofermentative organsism which produces only lactate.
Heterofermentative organisms also produce carbon dioxide carbohydrates and thus contribute to dough leavening.
What direct role does bacteria play in raising bread, in comparison to the role of yeast? A popular sourdough perception is that yeast is responsible for leavening, and bacteria is responsible for flavour.
How accurate is this perception?
Originally I contacted Professor Gaenzle because I was having trouble with some flour and he very kindly offered some help. I asked him if he would analyze the flour because it wasn't working .......
I've been curious what trouble you're having in regard to your flour not working!
What has been happening that makes you suspect your flour quality?
After I received the analysis, I was intrigued because when I started baking for the public, I always intended to make "Italian" style naturally leavened bread so when that bacteria showed up, I was surprised because I really did not know what I was doing and I certainly didn't spit into the starter jar. That's where I'm at.
edit - it was the sluggish flour thread which then led into a d malt discussion
I'd be interested in knowing which yeast species were isolated from your starter also, if that's also in the wings.
Re yeast species, I sent off a refresher email last week but like all scientists, he's probably really busy. I hope it comes through though and will post it if it does. Very curious.
PS: thanks for coverting the PDF. Everytime I tried I got the Mac Hand.