What are the biochemical processes occurring when food spoils?

What are the biochemical processes occurring when food spoils?

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Let's assume for a minute that microbes themselves and their direct toxic products (i.e. endotoxins) aren't toxic to humans. Let's also discount any innate immune responses the body mounts against the invading microbe (i.e. inflammation and production of cytokines).

What happens to food molecules (mechanistically) as it spoils and what deleterious effects do these "spoil products" have on the body if ingested? I'm looking for compounds that can result from the spontaneous breakdown of food or the byproducts of microbial metabolism (that is NOT a "direct" toxin) that is harmful to the body.

For example, do the proteins in food break down into some toxic nitrogenous substance?

During putrefaction of animal tissue, lysine is decarboxylated into cadaverine and arginine is decarboxylated into putrescine. These compounds are deemed to be toxic.

A serving of meat contains 8 g of protein, corresponding to 640 mg lysine and a little bit less of arginine. Let's go straight and say that a spoiled meat serving contains 640 mg cadaverine and a little bit less of putrescine.

In rats, the acute oral toxicity for both polyamines is around 2000 mg/kg, let'assume that this is valid for humans also. According to these rough calculations, to have an acute toxic effect, a 70kg man that is resistant to the direct toxic effects of microbes, should eat 140 grams of cadaverine, corresponding to 218 smelly rotten meat servings.

[composition and toxicity data taken from wikipedia]

How Food Spoils

Food spoilage and deterioration is no accident. It is a naturally occurring process. To understand how to maintain the quality of food and prevent spoilage, we need to know what can cause it. Factors that affect food spoilage include:

  • Microorganisms
  • Enzymes
  • Air
  • Light
  • Insects, Rodents, Parasites and Other Creatures
  • Physical Damage
  • Temperature
  • Time


Many types of microorganisms can cause food problems. The microorganisms that can cause food-borne illness are called pathogenic microorganisms. These microorganisms grow best at room temperatures (60-90°F), but most do not grow well at refrigerator or freezer temperatures. Pathogenic microorganisms may grow in foods without any noticeable change in odor, appearance or taste. Spoilage microorganisms, including some kinds of bacteria, yeasts and molds, can grow well at temperatures as low as 40°F. When spoilage microorganisms are present, the food usually looks and/or smells awful. Read more about pathogenic microorganisms under Food Poisoning/Foodborne Illness?


Enzymes, substances naturally present in food, are responsible for the ripening process in fruits and vegetables. Enzymes are responsible for texture, color and flavor changes. For example, as a banana turns from green to yellow to brown, not only does the color change, but there is also a change in the fruit'ss texture. Unblanched, frozen corn-on-the-cob may taste like the cob over time. This is the result of enzyme action.

Oxidation, a chemical process that produces undesirable changes in color, flavor and nutrient content, results when air reacts with food components. When fats in foods become rancid, oxidation is responsible. Discoloration of light-colored fruits can be reduced by using an antioxidant, such as ascorbic acid or citric acid, before freezing. Vapor-proof packaging that keeps air out helps reduce oxidation problems.


Light exposure could result in color and vitamin loss. Light also may be responsible for the oxidation of fats.

Glycogenesis Process

To start the process, the cell must have an excess of glucose. Glucose is the starting molecule, and is modified through the process of glycogenesis. Through the modifications, it gains the ability to be stored in long chains. The process starts when the cell receives a signal from the body to enter glycogenesis. These signals could come from a number of different routes, and are discussed in a later section. When glucose enters the glycogenesis process, it must be acted on by a number of enzymes as seen in the image below.

First, the glucose molecule interacts with the enzyme glucokinase, which adds a phosphate group to the glucose. In the next step of glycogenesis, the phosphate group is transferred to the other side of the molecule, using the enzyme phosphoglucomutase. A third enzyme, UDP-glucose pyrophosphorylase, takes this molecule and creates uracil-diphosphate glucose. This form of glucose has two phosphate groups, as well as the nucleic acid uracil. These additions aid in the next step, creating a chain of molecules.

A special enzyme, glycogenin, takes the lead in this part of glycogenesis. The UDP-diphosphate glucose can form short chains by binding to this molecule. After around 8 of these molecules chain together, more enzymes come in to finish the process. Glycogen synthase adds to the chain, while glycogen branching enzyme helps create branches in the chains. This leads to a more compact macromolecule, and thus more efficient storage of energy.

Processing Technologies


The biochemical process is based on breaking down the cellulosic part of the organic fraction of the waste stream. This would include certain foods (e.g., vegetables, fruits), paper products, and yard vegetation. Biosolids can also be added as a waste material. All other materials in the waste stream should be removed prior to the process.

In the process, following drying and shredding of the waste, the prepared waste stream is mixed with water and sulfuric acid in a closed reactor vessel. This causes a reaction that in conjunction with common bacteria already in the waste breaks down the material into sugar compounds and a by-product known as lignin. There are some companies that are testing natural enzymes, instead of the strong acid chemical, to initiate this reaction.

The resulting sugar compounds and water are sent to a fermentation unit where yeast is added. The yeast reacts with the sugars to convert them to alcohol. The alcohol mixture is then heated and distilled to remove the solids. The resulting distilled alcohol (grain alcohol or ethanol) can be used as fuel. The lignin by-product is sent to a gasifier where it is used to produce heat for the drying process or can potentially be further processed for use as a fuel substitute in power plants [17] . A basic biochemical process is shown in Figure 4.39 .

Figure 4.39 . Basic biochemical process.

30.3.2. Metabolic Derangements in Diabetes Result from Relative Insulin Insufficiency and Glucagon Excess

We now consider diabetes mellitus, a complex disease characterized by grossly abnormal fuel usage: glucose is overproduced by the liver and underutilized by other organs. The incidence of diabetes mellitus (usually referred to simply as diabetes) is about 5% of the population. Indeed, diabetes is the most common serious metabolic disease in the world it affects hundreds of millions. Type I diabetes, or insulin-dependent diabetes mellitus (IDDM), is caused by autoimmune destruction of the insulinsecreting β cells in the pancreas and usually begins before age 20. The term insulin-dependent means that the individual requires insulin to live. Most diabetics, in contrast, have a normal or even higher level of insulin in their blood, but they are quite unresponsive to the hormone. This form of the disease—known as type II, or non-insulin-dependent, diabetes mellitus (NIDDM)—typically arises later in life than does the insulin-dependent form.


Named for the excessive urination in the disease. Aretaeus, a Cappadocian physician of the second century a.d ., wrote: “The epithet diabetes has been assigned to the disorder, being something like passing of water by a siphon.” He perceptively characterized diabetes as �ing a melting-down of the flesh and limbs into urine.”

From Latin, meaning “sweetened with honey.” Refers to the presence of sugar in the urine of patients having the disease.

Mellitus distinguishes this disease from diabetes insipidus, which is caused by impaired renal reabsorption of water.

In type I diabetes, insulin is absent and consequently glucagon is present at higher-than-normal levels. In essence, the diabetic person is in biochemical starvation mode despite a high concentration of blood glucose. Because insulin is deficient, the entry of glucose into cells is impaired. The liver becomes stuck in a gluconeogenic and ketogenic state. The excessive level of glucagon relative to insulin leads to a decrease in the amount of F-2,6-BP in the liver. Hence, glycolysis is inhibited and gluconeogenesis is stimulated because of the opposite effects of F-2,6-BP on phosphofructokinase and fructose-1,6-bisphosphatase (Section 16.4 see also Figures 30.4 and 30.6). The high glucagon/insulin ratio in diabetes also promotes glycogen breakdown. Hence, an excessive amount of glucose is produced by the liver and released into the blood. Glucose is excreted in the urine (hence the name mellitus) when its concentration in the blood exceeds the reabsorptive capacity of the renal tubules. Water accompanies the excreted glucose, and so an untreated diabetic in the acute phase of the disease is hungry and thirsty.

Because carbohydrate utilization is impaired, a lack of insulin leads to the uncontrolled breakdown of lipids and proteins. Large amounts of acetyl CoA are then produced by β-oxidation. However, much of the acetyl CoA cannot enter the citric acid cycle, because there is insufficient oxaloacetate for the condensation step. Recall that mammals can synthesize oxaloacetate from pyruvate, a product of glycolysis, but not from acetyl CoA instead, they generate ketone bodies. A striking feature of diabetes is the shift in fuel usage from carbohydrates to fats glucose, more abundant than ever, is spurned. In high concentrations, ketone bodies overwhelm the kidney's capacity to maintain acid-base balance. The untreated diabetic can go into a coma because of a lowered blood pH level and dehydration.

Type II, or non-insulin-dependent, diabetes accounts for more than 90% of the cases and usually develops in middle-aged, obese people. The exact cause of type II diabetes remains to be elucidated, although a genetic basis seems likely.

Methods to Investigate Cardiac Metabolism

Moritz Osterholt , . Torsten Doenst , in The Scientist's Guide to Cardiac Metabolism , 2016


Cardiac metabolism encompasses all biochemical processes that result in the conversion of substrates or intermediates of metabolic pathways and cycles for the purpose of cell function, growth, and contraction. Methods investigating metabolism must therefore address the moieties of metabolic pathways and cycles, flux through them and the activity and regulation of their enzymes. Such investigations can be performed in vivo, in vitro, or ex vivo in humans, in animals, or in cell culture. In this chapter, we will describe the principles of the main methods used to examine cardiac metabolism in health and disease. As mitochondria have moved into the focus of attention in recent years, we will put a focus on the assessment of mitochondrial function. We will start with the methods of assessing moieties of pathways and cycles, that is, the amounts of RNA, proteins, and metabolites, then continue with the description of methods to assess enzyme activities in vitro, and move on to methods of addressing fluxes and imaging of metabolic activity in intact organs or in vivo. This chapter is not meant to describe individual techniques in detail and is not meant to be complete but to provide an overview and to illustrate principles of the currently available methodology.


You can preserve high-acid foods using a traditional process called canning. Apples, berries, peaches and tomatoes are just a few foods that may be canned safely. Boiling water kills spoilage bacteria and creates a vacuum seal around the jar lid. Canned food items must be cooked for a minimum amount of time to ensure that all bacteria are killed. Botulism, a deadly bacterial toxin, grows quickly in canned goods that have been improperly processed. The Virginia Cooperative Extension Office recommends closed-kettle boiling with heat-tempered jars and lids.

The Phosphorus Cycle

Phosphorus is an essential nutrient for living processes. It is a major component of nucleic acids and phospholipids, and, as calcium phosphate, it makes up the supportive components of our bones. Phosphorus is often the limiting nutrient (necessary for growth) in aquatic, particularly freshwater, ecosystems.

Phosphorus occurs in nature as the phosphate ion (PO4 3- ). In addition to phosphate runoff as a result of human activity, natural surface runoff occurs when it is leached from phosphate-containing rock by weathering, thus sending phosphates into rivers, lakes, and the ocean. This rock has its origins in the ocean. Phosphate-containing ocean sediments form primarily from the bodies of ocean organisms and from their excretions. However, volcanic ash, aerosols, and mineral dust may also be significant phosphate sources. This sediment then is moved to land over geologic time by the uplifting of Earth’s surface. (Figure below)

Phosphorus is also reciprocally exchanged between phosphate dissolved in the ocean and marine organisms. The movement of phosphate from the ocean to the land and through the soil is extremely slow, with the average phosphate ion having an oceanic residence time between 20,000 and 100,000 years.

Excess phosphorus and nitrogen that enter these ecosystems from fertilizer runoff and from sewage cause excessive growth of algae. The subsequent death and decay of these organisms depletes dissolved oxygen, which leads to the death of aquatic organisms such as shellfish and fish. This process is responsible for dead zones in lakes and at the mouths of many major rivers and for massive fish kills, which often occur during the summer months (see Figure 6 below).

Figure 6. Dead zones occur when phosphorus and nitrogen from fertilizers cause excessive growth of microorganisms, which depletes oxygen and kills fauna. Worldwide, large dead zones are found in coastal areas of high population density. (credit: NASA Earth Observatory)

A dead zone is an area in lakes and oceans near the mouths of rivers where large areas are periodically depleted of their normal flora and fauna. These zones are caused by eutrophication coupled with other factors including oil spills, dumping toxic chemicals, and other human activities. The number of dead zones has increased for several years, and more than 400 of these zones were present as of 2008. One of the worst dead zones is off the coast of the United States in the Gulf of Mexico: fertilizer runoff from the Mississippi River basin created a dead zone of over 8,463 square miles. Phosphate and nitrate runoff from fertilizers also negatively affect several lake and bay ecosystems including the Chesapeake Bay in the eastern United States.


Digestion of food is a form of catabolism, in which the food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs when food is moved through the digestive system. It begins in the mouth and ends in the small intestine. The final products of digestion are absorbed from the digestive tract, primarily in the small intestine. There are two different types of digestion that occur in the digestive system: mechanical digestion and chemical digestion. Figure (PageIndex<2>) summarizes the roles played by different digestive organs in mechanical and chemical digestion, both of which are described in detail in the text.

Figure (PageIndex<2>): Both, mechanical and chemical digestion take place throughout the gastrointestinal tract as indicated in this diagram, but absorption takes place only in the stomach and small and large intestines.

Mechanical Digestion

Mechanical digestion is a physical process in which food is broken into smaller pieces without becoming changed chemically. It begins with your first bite of food and continues as you chew food with your teeth into smaller pieces. The process of mechanical digestion continues in the stomach. This muscular organ churns and mixes the food it contains, an action that breaks any solid food into still smaller pieces.

Although some mechanical digestion also occurs in the intestines, it is mostly completed by the time food leaves the stomach. At that stage, food in the GI tract has been changed to the thick semi-fluid called chyme. Mechanical digestion is necessary so that chemical digestion can be effective. Mechanical digestion tremendously increases the surface area of food particles so they can be acted upon more effectively by digestive enzymes.

Chemical Digestion

Chemical digestion is the biochemical process in which macromolecules in food are changed into smaller molecules that can be absorbed into body fluids and transported to cells throughout the body. Substances in food that must be chemically digested include carbohydrates, proteins, lipids, and nucleic acids. Carbohydrates must be broken down into simple sugars, proteins into amino acids, lipids into fatty acids and glycerol, and nucleic acids into nitrogen bases and sugars. Some chemical digestion takes place in the mouth and stomach, but most of it occurs in the first part of the small intestine (duodenum).

Digestive Enzymes

Chemical digestion could not occur without the help of many different digestive enzymes. Enzymes are proteins that catalyze or speed up biochemical reactions. Digestive enzymes are secreted by exocrine glands or by the mucosal layer of the epithelium lining the gastrointestinal tract. In the mouth, digestive enzymes are secreted by salivary glands. The lining of the stomach secretes enzymes, as does the lining of the small intestine. Many more digestive enzymes are secreted by exocrine cells in the pancreas and carried by ducts to the small intestine. Table (PageIndex<1>) lists several important digestive enzymes, the organs and/or glands that secrete them, and the compounds they digest. You can read more about them in the text.

Digestive Enzyme

Organ, Glands That Secretes It

Compound It Digests

Chemical Digestion of Carbohydrates

About 80 percent of digestible carbohydrates in a typical Western diet are in the form of the plant polysaccharide amylose, which consists mainly of long chains of glucose and is one of two major components of starch. Additional dietary carbohydrates include the animal polysaccharide glycogen, along with some sugars, which are mainly disaccharides.

To chemically digest amylose and glycogen, the enzyme amylase is required. The chemical digestion of these polysaccharides begins in the mouth, aided by amylase in saliva. Saliva also contains mucus, which lubricates the food, and hydrogen carbonate, which provides the ideal alkaline conditions for amylase to work. Carbohydrate digestion is completed in the small intestine, with the help of amylase secreted by the pancreas. In the digestive process, polysaccharides are reduced in length by the breaking of bonds between glucose monomers. The macromolecules are broken down to shorter polysaccharides and disaccharides, resulting in progressively shorter chains of glucose. The end result is molecules of the simple sugars glucose and maltose (which consists of two glucose molecules), both of which can be absorbed by the small intestine.

Other sugars are digested with the help of different enzymes produced by the small intestine. For example, sucrose, or table sugar, is a disaccharide that is broken down by the enzyme sucrase to form glucose and fructose, which are readily absorbed by the small intestine. Digestion of the sugar lactose, which is found in milk, requires the enzyme lactase, which breaks down lactose into glucose and galactose, which are then absorbed by the small intestine. Fewer than half of all adults produce sufficient lactase to be able to digest lactose. Those who cannot are said to be lactose intolerant.

Chemical Digestion of Proteins

Proteins consist of polypeptides, which must be broken down into their constituent amino acids before they can be absorbed. Protein digestion occurs in the stomach and small intestine through the action of three primary enzymes: pepsin, secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas. The stomach also secretes hydrochloric acid, making the contents highly acidic, which is required for pepsin to work. Trypsin and chymotrypsin in the small intestine require an alkaline environment to work. Bile from the liver and bicarbonate from the pancreas neutralize the acidic chyme as it empties into the small intestine. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, these are further broken down into amino acids by other enzymes called peptidases, also secreted by the pancreas.

Chemical Digestion of Lipids

The chemical digestion of lipids begins in the mouth. The salivary glands secrete the digestive enzyme lipase, which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.

Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas as well as bile secreted by the liver. Bile is required for the digestion of lipids because lipids are oily and do not dissolve in the watery chyme. Bile emulsifies, or breaks up, large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease. The micelles provide a great deal more surface area to be acted upon by lipase and also point the hydrophilic (&ldquowater-loving&rdquo) heads of the fatty acids outward into the watery chyme. Lipase can then access and break down the micelles into individual fatty acid molecules.

Chemical Digestion of Nucleic Acids

Nucleic acids (DNA and RNA) in foods are digested in the small intestine with the help of both pancreatic enzymes and enzymes produced by the small intestine itself. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.

Chemical Digestion by Gut Flora

The human gastrointestinal tract is normally inhabited by trillions of bacteria, some of which contribute to digestion. Here are just two of dozens of examples:

  1. The most common carbohydrate in plants, which is cellulose, cannot be digested by the human digestive system. However, tiny amounts of cellulose are digested by bacteria in the large intestine.
  2. Certain bacteria in the small intestine help digest lactose, which many adults cannot otherwise digest. As a byproduct of this process, the bacteria produce lactic acid, which increases the release of digestive enzymes and the absorption of minerals such as calcium and iron.


Food spoilage has been an important problem throughout human history. Finding ways to overcome this problem was crucial as communities became larger and individuals no longer grew their own food. Some kind of system was needed to maintain the nutrient content of various food stuffs for long periods of time and prevent them from rotting and becoming inedible.

Early solutions to food spoilage

Food spoilage is caused by the growth of microorganisms, primarily bacteria and fungi, that convert nutrients into energy which they use for their own growth. Depletion of the nutrient content of food as well as the secretion of byproducts from this biochemical process are two things which contribute to the spoilage of food rendering it inedible. Since ancient times, humans have used many methods to extend the shelf life of food although not always understanding how these processes worked. Salting and drying are two very simple techniques that prevent rotting both make the food an inhospitable environment for microorganisms. Canning is another technique first developed in the late 18th century by Nicholas Appert, a French confectioner, who, after 15 years of research, realized that if food is sufficiently heated and then sealed in an air tight container it will not spoil. Here the heating of food, kills all residual microorganisms present in the food and immediate sealing prevents the reentry of other contaminanting organisms. Napoleon immediately put this discovery to work in his armed forces and awarded Appert a prize of 12,000 francs for his discovery. Later, an Englishman, Peter Durand, took the process one step further and developed a method of sealing food into unbreakable tin containers. This was perfected by Bryan Dorkin and John Hall, who set up the first commercial canning factory in England in 1813. In 1859, Louis Pasteur definitively showed that microorganisms were responsible for food spoilage for the first time. This discovery led to the coining of the term “pasteurization” to describe the process where liquids with the potential to spoil (milk in particular) are heated for preservation.

In some cases, the growth of microorganisms in food can be put to good use for the production and preservation of various types of food. Fermentation is arguably the earliest example of biotechnology and refers to the metabolic process by which microbes produce energy in the absence of oxygen and other terminal electron acceptors in the electron transport chain such as fumarate or nitrate. In ancient times, it was considered as a way to both preserve food and to retain nutritional value. It was probably accidentally discovered in ancient Egypt when dough, made from ground up wheat and rye, was left for a period of time before cooking. In contrast to dough that was immediately cooked, it was observed that the aged dough expanded in size and when cooked produced tastier, lighter bread. The process was not completely reproducible: sometimes the uncooked dough yielded good bread and other times it did not. However if small amounts of good dough was added to the next batch, the bread was again tasty. The Romans went onto improve and perfect this process and popularized this sort of bread throughout the European continent. The discovery of fermentation in Egypt also led to the first production of wine and alcohol. All these discoveries were largely phenomenological and it would be another 3000 years before the exact cause of fermentation was uncovered. It was Louis Pasteur, again, in 1857 who was able to demonstrate that alcohol can be produced by yeast when grown in particular conditions. This discovery revolutionized the modern food industry: for the first time the agent of fermentation was identified and could be used commercially.

Industrial processes using fermentation

Fermentation by bacteria, yeast and mold is key to the production of fermented foods. Fermenting yeast produces the alcohol in beer and wine. In fact, the smell of fresh baked bread and rising dough can be attributed to alcohol produced from yeast. Fermentation is used to make many ethnic foods such as sauerkraut and miso. Soy sauce is produced by fermenting Aspergillus ortzae, a fungus, growing on soy beans. Erwinia dissolvens, another type of bacteria, is essential for coffee bean production it is used to soften and remove the outer husk of beans. Finally, fermentation of milk produces most dairy products. Without microbes, we would not be able to eat many types of different food that we enjoy today. Table 1 shows example of several foods that are produced through fermentation with specific organisms.

Table 1. Some examples of foods which uses fermentation in their production. Dairy products are described in more detail below.

The biochemical process

All organisms need energy to grow. This energy comes from the reduction of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and results in the release of energy and a phosphate group. In this way ATP serves as a storage molecule of energy which can be used by the cell. But where does the ATP come from? Cells get their ATP from the controlled chemical breakdown of glucose to form two molecules of pyruvate. This process requires two molecules of ATP but results in the release of four molecules or a net gain of two molecules of ATP. This process is referred to as glycolysis and is illustrated in Figure 1. Once pyruvate is formed, it can be processed in several different ways. Mammalian cells usually process pyruvate by putting it into the tricarboxylic or Kreb’s cycle. In the presence of oxygen, oxidative phosphorylation produces more ATP from the byproducts of the Kreb’s cycle reactions. This is referred to as aerobic respiration. However when oxygen is limiting, other processes must be used in order to deal with pyruvate. This is done through anaerobic respiration or fermentation and involves the breakdown of pyruvate into simpler compounds. Two of the most important fermentation processes which are used on an industrial scale are ethanol or lactic acid fermentation. This is illustrated in Figure 1.

Figure 1. Glycolysis and fermentation.

Milk is an excellent food source for humans and bacteria alike. It is full of vitamins, fats, minerals, nutrients and carbohydrates. It is rich in the protein casein which gives milk its characteristic white color. The most abundant carbohydrate is the disaccharide lactose, “milk sugar.” At room temperature, milk undergoes natural souring caused by lactic acid produced from fermentation of lactose by fermentative lactic acid bacteria. This accumulation of acid (H+ ions) decreases the pH of the milk and cause the casein to coagulate and curdle into curds and whey. Curds are large, white clumps of casein and other proteins. Whey is the yellow liquid that is left behind after the casein has formed curds. Thus, bacteria obtain nutrients from the milk, inadvertently curdle it and humans use it as the first step in making many dairy products.

The microbes important for dairy product manufacturing can be divided into two groups, primary and secondary microflora. Products undergoing fermentation by only primary microflora are called unripened and those processed by both primary and secondary microflora are called ripened. Primary microflora are fermentative lactic acid bacteria which cause the milk to curdle. During dairy product production, milk is first pasteurized to kill bacteria that cause unwanted spoilage of the milk and of the downstream milk products. Primary microflora consists of certain kinds of Lactococcus, Lactobacillus and Streptococcus that are intentionally added to pasteurized milk and grown at 30°C or 37°C (temperature depends on the bacteria added). Secondary microflora include several different types of bacteria (Leuconstoc, Lactobacillus, and Propionibacterium), yeasts and molds they are only used for some types of surface ripened and mold ripened cheeses. The various combinations of microflora determine what milk product you will end up with.

Different unripened milk products are created by using various starting products and bacteria. For buttermilk production, Lactobacillus bulgaris (named for its country of discovery, Bulgaria) is added to skim milk to curdle it. Leuconostoc is then added to thicken it. Sour cream is made the same way except cream is used instead of skim milk. During yogurt production, dry milk protein is added to milk to concentrate the milk before addition of actively growing Streptococci and Lactobacilli. Butter is produced by curdling and slight souring from Streptococci growing in sweet cream. Leuconostoc is then added so it can synthesize diacetyl, a compound that gives butter its characteristic aroma and taste. The milk is then churned to aggregate the fat globules into solid butter.
Thus milk type and bacteria will determine the dairy product produced.

Cheese is an important product of fermentative lactic acid bacteria. Particularly in the past, cheese was valued for its long shelf life. Due to its reduced water content, and acidic pH, bacterial growth is severely inhibited. This causes cheese to spoil much more slowly than other milk products. Consequently, the art of cheese production has spread throughout Europe, each country manufacturing many different types of cheeses. Cheese production has three steps: curd formation, curd treatment and curd ripening.

1. Curd formation can use mare, ewe, cow or goat milk to produce “sour” or “sweet” curd. Sour curd is produced by fermentative lactic acid bacteria as mentioned above. Sweet curd is produced by adding an enzyme called renin instead of bacteria to curdle the milk. The curd is separated from the whey by draining. The curd can be used directly to make unripened cheeses such as ricotta or cottage cheese or can undergo further processing to make a ripened cheese.

2. Curd treatment consists of condensing and squeezing to form dense, hard curd. It is then molded into the desired shape, salted and mixed with different types of secondary microflora.

3. Secondary microflora ripen the cheese and will determine the final texture and aroma of each type of cheese. For hard ripened cheeses such as Cheddar, curds are further compressed and the bacteria particular for the cheese is added. The Cheddar is wrapped in wax or plastic to prevent contamination and then incubated to allow the bacteria to do its work. For soft ripened cheeses such as Camembert and Limburger, a microbe, usually mold, is added to the surface of the cheese that produces a protein-digesting enzyme. This enzyme breaks apart the curds and causes the cheese to become creamy and spreadable.

Many cities have long held traditions and nuances for producing a particular cheese i.e. the limestone caves in Roquefort, France which have constant heat and humidity that create unique and delightful cheeses. Figure 2 shows a schematic diagram of the cheese manufacturing process.

Figure 2. The cheese manufacturing process.

Thus, microbes can not only be harmful to society but also can be manipulated in a variety of ways for the benefit of society. Particularly in the preservation and production of food, microbes have proven to be useful and essential.

1. Alcamo, I. (2003). Microbes and Society. Missassauga, Ontario, Jones and Bartlett.

2. Doyle, M., L. Beuchat, et al., Eds. (1997). Food Microbiology: Fundamentals and Frontiers. Washington, DC, ASM Press.

3. Foster, E., F. Nelson, et al. (1957). Dairy Microbiology. Englewood Cliffs, Prentice Hall.

Sauerkraut Fermentation: Process, Microbiology, Defects and Spoilage | Industrial Microbiology

In this article we will discuss about the sauerkraut fermentation:- 1. Introduction to Sauerkraut 2. Process for Sauerkraut Fermentation 3. Microbiology of the Sauerkraut Fermentation 4. Defects and Spoilage of Sauerkraut.

Introduction to Sauerkraut:

The use of cabbage (Brassica oleracea) as a food antedates known re­corded history. Sauerkraut, a product resulting from the lactic acid fermen­tation of shredded cabbage, is literally acid (sour) cabbage. The antecedents of sauerkraut differed considerably from that prepared at present. At first the cabbage leaves were dressed with sour wine or vinegar.

Later the cabbage was broken or cut into pieces, packed into containers, and covered with verjuice (the juice expressed from immature apples or grapes), sour wine, or vinegar. Gradually the acid liquids were replaced by salt and a spontaneous fermentation resulted.

One may speculate that sauerkraut manufacture comparable to the method used today developed during the period of 1550 to 1750 A.D. although cabbage has been known and used commonly for about 4000 years. Those readers particularly interested in the historical evolution of the sauerkraut fermentation should consult Pederson (1960, 1979) and Pederson and Albury (1969).

Originally sauerkraut was made only in the home because it provided a means for utilizing fresh cabbage which otherwise would spoil before it could be used Now the commercial production of sauerkraut has become an important food industry. Even so, a significant quantity is still produced in the home, particularly in rural and suburban areas where home vegetable gardens still exist.

Cabbage varieties best suited for growth in the major production areas are used early, midseason, and late types are grown. Varieties formerly used such as Early Flat Dutch, Late Flat Dutch, Early Jersey Wakefield, and others have been replaced in part by new cultivars which have been bred to be well-adapted to mechanical harvesting and at the same time inherently contain less water, thus reducing the generation of in-plant liquid wastes. Mild-flavored, sweet, solid, white-headed cabbage is the choice as it makes a superior kraut.

Process for Sauerkraut Fermentation :

Properly matured sound heads of cabbage are first trimmed to remove the outer green broken or dirty leaves. The cores are cut mechanically by a reversing corer that leaves the core in the head. Then the cabbage is sliced by power-driven, rotary, adjustable knives into long shreds as fine as 0.16 to 0.08 cm (1/16 to 1/32 inches) in thickness.

In general, long, finely cut shreds are preferred, but the thickness is determined by the judgment of the manufacturer. The shredded cabbage (known also as slaw) is then conveyed by belts or by carts to the vats or tanks for salting and fermentation.

Salt plays a primary role in the making of sauerkraut and the concen­trations used are carefully controlled. According to the legal standard of identity the concentration of salt must not be less than 2%, nor more than 3%. As a result most producers use a concentration in the range of 2.25 to 2.5% of salt. Salt is required for several reasons.

It extracts water from the shredded cabbage by osmosis, thus forming the fermentation brine It suppresses the growth of some undesirable bacteria which might cause deterioration of the product and, at the same time, makes conditions favorable for the desirable lactic acid bacteria. Salt also contributes to the flavor of the finished sauerkraut by yielding a proper salt-acid ratio (bal­ance) if the cabbage is properly salted.

The use of too little salt causes softening of the tissue and produces a product lacking m flavor. Too much salt interferes with the natural sequence of lactic acid bacteria, delays fermentation and, depending on the amount of over-salting, may produce a product with a sharp, bitter taste, cause darkening of color, or favor growth of pink yeasts.

Uniform distribution of salt throughout the mass of shredded cabbage cannot be overemphasized. In some factories the slaw is weighed on con­veyor belt lines and the desired amount of salt is sprinkled on the shreds by means of a suitable proportioner as it moves along the conveyor to the vat.

In other plants hand-carts are used to carry the shredded cabbage to the vat. Some prefer to salt the weighed cabbage in each cart. Others transport the slaw in carts which are weighed occasionally to check the capacity. The shreds are then dumped into the vat, distributed by forks, and then salted with a specific weight of salt.

The variations of salt concentrations in the brines covering kraut have been thoroughly investigated by Pederson and Albury (1969) and discussed by Pederson (1975, 1979). No mention of recirculation of the brines to gain uniformity in concentration of salt was noted.

It would seem that this method of ensuring uniform salt distribution in sauerkraut brines would be as effective as it is in the olive industry. Only small alterations in tank or vat design would be required to make it possible to completely recirculate the brine, pumping from the bottom and discharging at the surface.

Brine begins to form once the shreds are salted, and the tank is closed once it has been filled to the proper level. Formerly, the slaw was covered with a thick layer of outer leaves and then fitted with a wood cover (head) which was heavily weighted. Within a few hours the brine had formed and the fermentation had started. The head then was fixed in position in much the same manner as with pickle or olive tanks.

Now, however, a sheet plastic cover is used. This cover is much larger in area than the top of the vat or tank itself. The plastic sheeting is placed firmly against the top of the shredded cabbage with the edges draped over the sides of the container to form an open bag. Then enough water or preferably salt brine is placed in this bag so that the weight of the liquid added forces the cabbage shreds down into the brine until the brine covers the surface of the uppermost shreds. Unless the shreds are completely covered with brine, undesirable discoloration together with undesirable flavor changes will occur. This newer method of covering and weighting provides nearly anaerobic condi­tions, particularly after fermentation becomes acid and quantities of carbon dioxide are produced. Precautions to avoid pin holes or tears in the plastic are mandatory if aerobic yeast growth is to be avoided.

With the old method of closure film forming yeasts always were a problem and if the scum was not removed at intervals a yeasty flavor was imparted to the kraut. Pichia membranaefaciens yeast strains, in particular, voraciously oxidize lactic acid contained in salt brines. Other genera also may be involved and besides destroying acid also contribute to yeasty flavor.

By the time the tank or vat is filled with the salted shreds and weighted, brine has formed and fermentation has started in a sequence of bacterial species responsible for the lactic acid fermentation.

Microbiology of the Sauerkraut Fermentation :

Although the lactic acid fermentation was described by Pasteur in 1858 and much work had been done in the intervening years with various lactic bacteria from cabbage and cucumber fermentations, it was not established that a definite sequence of bacterial species of lactic acid bacteria were responsible for the fermentation of either vegetable until 1930 when Peder­son first described the lactic acid bacteria he observed in fermenting sauer­kraut.

Pederson found that the fermentation was initiated by the species Leuconostoc mesenteroides. This species was followed by gas-forming rods and finally by non-gas-forming rods and cocci. Since 1930 additional studies by Pederson and Albury (1954, 1969) have firmly established the impor­tance of Leuconostoc mesenteroides in initiating the lactic fermentation of sauerkraut.

Also they more closely identified the species and sequence of the other lactic acid bacteria involved. Now it is accepted that the kraut, fermentation is initiated by Leuconostoc mesenteroides, a heterofermentative species, whose early growth is more rapid than other lactic acid bacte­ria and is active over a wide range of temperatures and salt concentrations.

It produces acids and carbon dioxide that rapidly lower the pH, thus inhibit­ing the activity of undesirable microorganisms and enzymes that may soften the shredded cabbage. The carbon dioxide replaces air and creates an anaerobic condition favorable to prevention of oxidation of ascorbic acid and the natural color of the cabbage. Also carbon dioxide stimulates the growth of many lactic acid bacteria. It also may be that this species provides growth factors needed by the more fastidious types found in the fermentation.

While this initial fermentation is developing, the heterofermentative species Lactobacillus brevis and the homofermentative species Lactobacil­lus plantarum and sometimes Pediococcus cerevisiae begin to grow rapidly and contribute to the major end products including lactic acid, carbon dioxide, ethanol, and acetic acid. Minor end products also appear.

These are a variety of additional volatile compounds produced by the various bacteria responsible for the fermentation, by auto-chemical reactions, or the intrin­sic enzymes of the fermenting cabbage itself. Hrdlicka et al (1967) reported the formation of diacetyl and acetaldehyde, the primary carbonyls formed during cabbage fermentation.

Volatile sulfur compounds are major flavor components of fresh cabbage according to Bailey et al. (1961) and Clapp et al. (1959) and also of sauerkraut. However, according to Lee et al. (1976), the major portion of the volatiles of sauerkraut is accounted for by acetal, isoamyl alcohol, n-hexanol, ethyl lactate, cis-hex-3-ene-l-ol, and allyl isothiocyanate. Of these, only the latter two have been identified as major constituents of fresh cabbage.

These latter authors concluded that although these two compounds define the character of cabbage products (kraut) they do not contribute significantly to the determination of its quality. They further believe that the fresh and fruity odor of such compounds as ethyl butyrate, isoamyl acetate, n-hexyl acetate, and mesityl oxide are probably more important in determining the acceptability of sauerkraut.

Temperature is a controlling factor in the sequence of desirable bacteria in the sauerkraut fermentation at a salt concentration of 2.25%. At the optimum of 18.3°C (65°F) or lower the quality of the sauerkraut is generally superior in flavor, color and ascorbic acid content because the hetero­fermentative lactic acid bacteria exert a greater effect.

According to Pederson and Albury (1969) an average temperature of about 18°C (65°F) with a salt concentration of 2.25% may be considered normal in the kraut-producing areas of the United States. At (or near) this temperature, fermentation is initiated by Leuconostoc mesenteroides and continued by Lactobacillus brevis and Lactobacillus plantarum, the latter species being most active in the final stages of fermentation.

Under these conditions a final total acidity of 1.7 to 2.3% acid (calculated as lactic acid) is formed, and the ratio of volatile to nonvolatile acid (acetic/lactic) is about 1 to 4. The fermentation is completed in 1 to 2 months, more or less, depending upon the quantity of fermentable materials, concentration of salt, and fluctuations in temperature.

At higher temperatures, as would be expected, they found that the rate of acid production was faster. For example, at 23°C (73.4°F) a brine acidity of 1.0 to 1.5% (calculated as lactic acid) may be observed in 8 to 10 days and the sauerkraut may be completely fermented in about 1 month.

At a still higher temperature of 32°C (89.6°F), the produc­tion of acid generally is very rapid with acid production of 1.8 to 2.0% being obtained in 8 to 10 days. As the temperature increased, they observed a change in the sequence of lactic acid bacteria. First, the growth of Leuco­nostoc mesenteroides was retarded and Lactobacillus brevis and Lactobacil­lus plantarum dominated the fermentation. At higher temperatures the kraut fermentation became essentially a homofermentation dominated by Lactobacillus plantarum and Pediococcus cerevisiae.

As a result, the quality attributes of flavor and aroma deteriorated and the kraut was reminiscent of acidified cabbage because of the large quantity of lactic acid and little acetic acid produced by the homo-fermentative species. They also observed that sauerkraut fermented at higher temperatures would darken readily and, therefore, should be canned as quickly as possible after the fermenta­tion was completed.

An extremely important observation they made was that kraut could be successfully fermented even when started at the low temperature of 7.5°C (45.5°F). Leuconostoc mesenteroides can grow at lower temperatures than the other lactic acid bacteria involved in the fermentation. At this low temperature (7.5°C or 45.5°F) an acidity of 0.4% (as lactic acid) is produced in about 10 days and 0.8 to 0.9% in less than a month.

This amount of acidity coupled with saturation of the mass of kraut and brine with carbon dioxide is sufficient to provide the conditions necessary for preservation and later completion of the fermentation provided that anaerobiosis is maintained throughout the period of latency. When the kraut mass warms enough, the fermentation then is completed by the lactic acid bacteria of the genera Lactobacillus and Pediococcus, known to grow poorly if at all at 7.5°C (45.5°F).

Thus, it may require 6 months or more before the fermentation is completed. Such kraut is generally of superior quality because it remains cool and is not subjected to high temperature during-fermentation. In good commercial practice this variation in temperature permits the processor to maintain a supply of new, completely fermented sauerkraut throughout most of the year.

Precedent for the recommendation by Pederson and Albury that sauer­kraut be fermented at not over 18.3°C (65°F) had already been recorded by Parmele et al. (1927), Marten et al. (1929), and others.

Defects and Spoilage of Sauerkraut :

Abnormalities of sauerkraut, although varied, with few exceptions can be and generally have been avoided by application of scientific knowledge already available to the industry. For example, the simple expedient of providing anaerobiosis has eliminated most of the problems involving dis­coloration (auto-chemical oxidation), loss of acidity caused by growth of, molds and yeasts, off-flavors and odors (yeasty and rancid) caused by exces­sive aerobic growth of molds and yeasts, slimy, softened kraut caused by pectolytic activity of these same molds and yeasts, and pink kraut caused by aerobic growth of asporogenous yeasts, presumably members of the genus Rhodotorula.

Stamer et al. (1973) described the induction of red color in white cabbage juice by L. brevis while studying the effects of pH on the growth rates of the 5 species of lactic acid bacteria commonly associated with the kraut fermen­tation. L. brevis was the only species which produced such color formation in white cabbage juice and did so only when the juice was buffered with either calcium carbonate or sodium hydroxide.

No color development occur­red when the pH of the juice (3.9) was not adjusted or when the pH of the juice was raised to 5.5 and the juice sterilized by filtration before it was re-incubated. Therefore, red color formation was caused by L. brevis and did not arise as the result of chemical or inherent enzymatic reactions of the juice.

It remains to be seen whether this interesting phenomenon will be ob­served in industrial kraut fermentations. Since color induction by L. brevis was found to be pH dependent it seems unlikely to be found in normal kraut fermentations but could easily result from accidental addition of alkali to the shredded cabbage during salting.

Slimy or ropy kraut has been observed for many years. It is generally caused by dextran formation induced by Leuconostoc mesenteroides and is transitory in nature. This species prefers to ferment fructose rather than glucose. Therefore, in the fermentation of sucrose, the fructose is fermented leaving the glucose which interacts to form the slimy, ropy, water-insoluble dextrans.

These vary from an almost solid, gelatinous mass to a ropy slime surrounding the bacterial cells. These variations are easily demonstrated by growing L. mesenteroides in a 10% sucrose solution containing adequate accessory nutrients. The fermenting kraut may become very slimy during the intermediate stage of fermentation but with additional time the dex­trans are utilized by other lactic acid bacteria. Thus, it is imperative to distinguish between dextran induced slimy kraut and permanently slimy kraut caused by pectolytic activity. The former condition certainly is not a defect but should be considered a normal step in a natural progression.


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