Plants & Human Affairs - Introduction
Cherries.wmf (7140 bytes) Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Fermentation Products

 I.  Definitions
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      

III.  Fermentation

  1. Alcohol:    pyruvic acid ethyl alcohol + CO2.   This type occurs in plants and yeast (fungi)

  2. Lactic acid:   pyruvic acid  lactic acid (3 carbons).   Occurs in animals, bacteria

IV.  Alcohol Fermentations

 A.  Organisms
   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.

V.  Beer

A.  General

  1. an inebrient;

  2. to improve flavor of water;

  3. to preserve materials - i.e., water is safer to drink (acidity, anti-microbial activity of hops, alcohol)

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.

C.  Step 2.  Mashing.
The purpose of this step is to convert starch to simple (fermentable) sugar

D.  Step 3.  Brewing.
The function of this step is to convert sugars into alcohol (and other flavor components)

E.  Step 4.  Lagering
    This is the aging and maturing step.  

F.  Some notes

VI.  Wine Making – enology

A.  General

B.  Grapes

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)

E.  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.

F.  Clarification
    Remove suspended sediments by: (a) racking (allowing them to settle out); (b) adding clarifiers (gelatin, clay, egg whites); (c) centrifuging.

G.  Sterilization
   Kill yeast and any other microbes by (a) cold sterilizing; (b) pasteurization, (c) adding preservatives

H.  Aging/bottling
   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. 

I.  Diseases
   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.

  1. Champagne - Dom Perignon, 17th Century Benedictine monk, one of first to produce.  A wine undergoes a secondary fermentation in the bottle (although cheaper products may artificially introduce carbon dioxide).  The final pressure of carbon dioxide in the bottle is 75-90 lbs/in2.  Riddling is the process of removing the sediments. 
  2. Mead - honey
  3. Pulque - fermented agave
  4. Mescal - distilled pulque.  Tequila is mescal from a particular region in Mexico.
  5. Sake - fermented steamed rice.  Starch is hydrolyzed by black bread mold (Rhizopus) prior to fermentation
  6. Chicha - salivary amylase is to hydrolyze the starch in the grains.  Can be made from corn, peanuts, potatoes.
  7. Gin - grains or potatoes; flavored with juniper berries
  8. Vodka - potatoes
  9. Rum - molasses
  10. Whiskeys:  scotch - barley malt, heated over a peat fire; bourbon - corn, aged in charred barrels; rye - rye
  11. Brandy - distilled grape wine; aged in oak.  VSOP - long aging, sign of quality

 IX.  Bread

X.  Lactic Acid Fermentations.
   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.

A.  Cheese Production.  
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.
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.

C.  Cheese 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).

D.  Processing. 
    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

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Last updated:  10/24/2005     � Copyright  by SG Saupe