|Plants & Human Affairs (BIOL106) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; email@example.com; http://www.employees.csbsju.edu/ssaupe|
The term fermentation is used in two ways: (1) as a general term to describe any activity where microorganisms digest organic materials. For example, during the processing of chocolate, the seeds are left in piles surrounded by pulp that is fermented by microorganisms; and (2) to refer to a particular type of chemical process in which sugar is metabolized without oxygen.
II. Glycolysis the first step of glucose breakdown
Refers to the breakdown of sugar (glucose C6) to 2 molecules of pyruvic acid (C3).
Occurs in the cytoplasm of the cell
Occurs in virtually all organisms suggests it is an ancient biochemical pathway.
Initial stage of sugar breakdown
Function is to supply the cell with: (1) energy (2 ATP's) and (b) building blocks, other molecules, that the cell can use for its activities (i.e., pyruvic acid can be used for making certain amino acids).
Glycolysis doesn't require oxygen occurs in the presence or absence
Results in the production of only a small amount of ATP.
If oxygen (aerobic conditions) is present, the pyruvic acid is further metabolized in the mitochondria. This is the preferred route since much more ATP is made.
If oxygen is absent (anaerobic conditions), the pyruvic acid undergoes fermentation.
in cytoplasm of cell
Only occurs under anaerobic, no oxygen, conditions
The purpose of fermentation is to keep glycolysis going by recycling materials needed for glycolysis.
Types of fermentations - based upon end product
Alcohol: pyruvic acid → ethyl alcohol + CO2. This type occurs in plants and yeast (fungi)
Lactic acid: pyruvic acid → lactic acid (3 carbons). Occurs in animals, bacteria
IV. Alcohol Fermentations
Many microbes can ferment sugars to alcohol. Yeasts are the most important group because they are: (a) common, found of the surface of plant structures; and (b) produce palatable products; many others have offensive byproducts. Yeasts are single-celled fungi. The most important yeast is Saccharomyces. It tolerates more alcohol than other fungi.
B. Products of alcohol fermentations
Two commercially important products are alcohol and carbon dioxide. Alcohol is obviously the product of choice in the beer, wine and distilled spirits industries, whereas the carbon dioxide is used by the bread makers.
one of the oldest industries, dates back at least 6000 years
important beverage that serves as:
to improve flavor of water;
to preserve materials - i.e., water is safer to drink (acidity, anti-microbial activity of hops, alcohol)
largest of the microbial-based industries; about 20 billion gal/year are consumed
beer production occurs in 4 major steps
B. Step 1. Malting
The function of this step is to induce hydrolytic (digestive) enzymes that breakdown the starches in the grains to fermentable (simple) sugars like glucose.
Steps in the malting process are: barley → cleaned → soak → germinate (rotating drums) → dry in kiln (raise temp to about 180 F) → grind → screen → malt
Malt - gets its name from the maltose (one of products of enzyme breakdown of starch)
Preparation of malt will determine flavor of beer. For example, stouts use malt that is caramelized.
C. Step 2. Mashing.
The purpose of this step is to convert starch to simple (fermentable) sugar
Malt + water + adjuncts (other materials, i.e., rice & corn added to US beers) → enzymes continue to convert starch into fermentable sugar → spent grain removed feed to animals; liquid → wort
D. Step 3. Brewing.
The function of this step is to convert sugars into alcohol (and other flavor components)
wort + hops (related to marijuana; use the female flowers; provides flavor, disguises sweetness from sugars in the beer; stops enzyme action, precipitates proteins, antimicrobial action) → boil (stops enzyme action, extracts flavor, sterilizes) → spent hops removed (used for fertilizer) → add yeast to liquid → brew 5-12 days
Saccharomyces cerevisae - ales top fermented (yeast floats), higher alcohol content, higher hops, paler color; lagers bottom fermented (yeast sinks).
The purpose of the yeast is to convert sugars to carbon dioxide and alcohol and to impart flavor from other metabolic products.
E. Step 4. Lagering
This is the aging and maturing step.
E. Step 4. Lagering
Beer is aged from 14 days to 3+ months.
The yeast is removed → transferred to lagering tanks where the proteins settle out and flavor improves → carbonated → pasteurized → bottled
Carbonation can be done by: (a) adding beechwood chips and green beer to stimulate a final yeast growth (krausening); (b) introduce CO2 under pressure.
F. Some notes
The alcohol content ranges from 3-12%, usually around 4-6%.
Light beer - allow yeast to ferment more of sugars.
Non-alcoholic beers - remove alcohol by evaporation/distillation/dilution
VI. Wine Making enology
Enology - science of wine making
Viticulture - grape growing
Wine is fermented from grapes, though any berry can be used.
Wine vs. beer.
In wines, the sugars are already in a fermentable stage and they are fermented from grapes or sweet fruits; in beers the sugar must be converted to a fermentable form and beers typically fermented from grains
Viticulture - the grapevines are pruned and trained to a trellis
Commercially important wine grapes are a comprised of shoots of Old World plants (Vitis vinifera, native to area around Caspian Sea) grafted onto the roots of a N. American species (Vitis riparia). North American grapes are not suitable for wine making, although Vitis labrusca can be used. Red or green grape varieties can be used for making wine
C. Grape Harvest
Grapes must have proper sugar content; tested at regular intervals to pick at proper moment. The enologist must monitor the crop carefully.
D. Preparation of the must
The grapes are crushed and de-stemmed to release the juice from the grape. The crushed grape/stem/skin mix is called the must. It can be (a) pressed immediately to express the juice (as is done for white wines) or the mixture can be used intact (in red wine the mixture is pressed after fermentation)
Naturally occurring microbes are killed with sulfur dioxide (also remove excess oxygen from the juice to keep it anaerobic). Yeast is added. Specialized wine strains are used and it is allowed to ferment - white wine (10-15 C; 1.5-6 weeks); for reds (25-30; 3-14 days). Conversion of the sugar to alcohol occurs during the fermentation. The yeast also produces other flavorings. The fermentation is complete when the sugar runs out (dry wine, no residual sweetness) or is stopped early (filtering, more sulfur dioxide, lowering temperature) which will result in a sweeter wine.
Remove suspended sediments by: (a) racking (allowing them to settle out); (b) adding clarifiers (gelatin, clay, egg whites); (c) centrifuging.
Kill yeast and any other microbes by (a) cold sterilizing; (b) pasteurization, (c) adding preservatives
Flavor development, especially important in red wines. Age in oak barrels. In some wine, a secondary fermentation removes excess acids using Lactobacillus bacteria). Then, wine is bottled and corked.
Phylloxera is a root aphid that does serious damage to V. vinifera grapes. When American grapes were transplanted to France, they also brought the root parasite with them. It nearly wiped out the French wine industry. To solve problem, they grafted V. vinifera stocks on rootstocks of less susceptible V. riparia.
Another problem is powdery mildew, a fungus also introduced from N. America. These fungi grow on leaves and form a whitish powder on the surface. You've probably seen them in the fall and didn't realize it - for example, lilac leaves often have lots of powdery mildew by the end of the summer. Again, this caused great problems for the French until a French botanist noticed that plants growing along a road were doing well. These had been sprayed with a copper mixture by a farmer to prevent people from eating the grapes. Thus, copper sulfate became a big component of the standard treatment, called the Bordeaux mix.
J. Alcohol Content
Naturally fermented wine has maximum alcohol content of ca 14%. Why no higher? cuz the yeast pickle themselves. Why is the alcohol content of wine higher than beer? (a) more tolerant yeast; and (b) beer yeast has less sugar to start with therefore makes less alcohol.
What about fortified wines like sherry and port that have higher alcohol contents (15 - 21%)? These have brandy added to them. This practice presumably arose from the need to insure wine preservation by increasing the alcohol content.
VII. Distilled Spirits.
VIII. Interesting alcoholic beverages.
X. Lactic Acid
Commercially important in products like: (a) cultured dairy products (cheese, yogurt, buttermilk - i.e., preserving milk); (b) silage (fermentation lowers pH so no other organisms can grow) and (c) preservation of olives, pickles, sauerkraut, cucumbers.
The type of cheese is determined by: (1) bacterial flora that is present; (b) temperature of manufacture; and (c) presence/absence of secondary microbes on the cheese.
B. Basic process.
Milk → inoculate with bacteria → incubate, souring begins → add proteolytic enzymes (rennet) → coagulates proteins → liquid (whey) removed and discarded or used to make wine or feed pigs; curd → dried → processing further.
and water content.
Cheese is drained, heat, pressed. Soft cheese has a hi water content (50-80%; Camembert, Brie); semi-hard cheese - cooked briefly, reduce water content to ca 45% (blue, Muenster, Monterrey); hard 40% or less water (Colby, Cheddar, Gouda, Edam, Swiss).
Unripened cheese - finished product undergoes additional processing. ripened cheese -allow for secondary growth of bacteria or fungi for flavor (blue, camembert).
E. Some assorted products
Last updated: 10/24/2005 � Copyright by SG Saupe