182 ◾ Fundamental Food Microbiology
During curing, breakdown of remaining lactose in the curd continues. However, a large
change occurs in proteins and other nitrogenous compounds. By the actions of rennin (retained
in curd) and cellular exoproteinases and endoproteinases and peptidases, peptides of different
sizes and amino acids are released. Further breakdown of amino acids produces hydrogen sulfide,
methanethiol and related sulfur compounds, amines, and other products. Lipids also undergo
lipolysis, releasing fatty acids, including the C4–C8 fatty acids (which are present in milk fat).
Other reactions produce lactones, ketones, and thioesters. Some of the reactions are nonenzymatic. The typical Cheddar cheese flavor is the result of a delicate balance among the products
produced from carbohydrate, protein, and lipid breakdown during processing and curing. The
concentrations of these components change with curing time.
Some secondary microflora that survive heating or gain entrance later in the milk and curd
during processing have definite roles in the flavor of Cheddar cheese. These include some enterococci, lactobacilli, pediococci, micrococci, and some Gram-negative rods. They probably contribute to the typical intense flavor that could be missing in cheese made with defined starter strains
only. Some of these flora are known to produce several flavor compounds rather rapidly and at
higher concentrations (e.g., volatile fatty acids and H 2 S).
Genetics
Phenotypic characteristics, as described before for these species, should be considered. Lac + strains
capable of producing lactic acid rapidly at the initial stage are preferred. Also, strains with weak
proteinase (Pro + ) activity are desirable because they do not cause rapid proteolysis with the accumulation of some peptides and the appearance of bitter flavor in the products. In mixed starters,
they should not have an antagonistic effect. Also, the strains should preferably be resistant to
multiple phages.
Microbial Problems
Bitter flavor in Cheddar cheese, especially in the aged product, results from the accumulation of
bitter peptides that are approximately 1000–12,000 Da and rich in hydrophobic amino acids.
Starters capable of hydrolyzing proteins rapidly (fast starters) tend to produce bitter peptides more
than slow starters do. Their enzymes hydrolyze proteins quickly, releasing large amounts of peptides that are subsequently hydrolyzed slowly to smaller peptides and amino acid by peptidases,
resulting in the accumulation of peptides. Because they are hydrophobic, bitter peptides are generally hydrolyzed slowly, causing them to accumulate. Use of slow starters for protein breakdown at
a slow rate or treatment of cheese with peptidase, or both, are effective in reducing bitterness (also
see Chapter 20).
Mold growth on the surface or in air pockets inside the cheese can occur after removing the
packing material from cheese. The spores are generally present in the raw material or get in the
product during processing and before sealing. It is not possible to determine from the colonial
morphology whether they are mycotoxin producers. It is better not to consume cheese with heavy
growth.
Staphylococcus aureus, following contamination of milk after heating, can grow during processing Cheddar cheese and produce enterotoxins. The toxins remain in the cheese even after the
death of cells during curing. Food poisoning can occur from consuming such cheese (even if they
are heated in some preparations).
During curing, breakdown of remaining lactose in the curd continues. However, a large
change occurs in proteins and other nitrogenous compounds. By the actions of rennin (retained
in curd) and cellular exoproteinases and endoproteinases and peptidases, peptides of different
sizes and amino acids are released. Further breakdown of amino acids produces hydrogen sulfide,
methanethiol and related sulfur compounds, amines, and other products. Lipids also undergo
lipolysis, releasing fatty acids, including the C4–C8 fatty acids (which are present in milk fat).
Other reactions produce lactones, ketones, and thioesters. Some of the reactions are nonenzymatic. The typical Cheddar cheese flavor is the result of a delicate balance among the products
produced from carbohydrate, protein, and lipid breakdown during processing and curing. The
concentrations of these components change with curing time.
Some secondary microflora that survive heating or gain entrance later in the milk and curd
during processing have definite roles in the flavor of Cheddar cheese. These include some enterococci, lactobacilli, pediococci, micrococci, and some Gram-negative rods. They probably contribute to the typical intense flavor that could be missing in cheese made with defined starter strains
only. Some of these flora are known to produce several flavor compounds rather rapidly and at
higher concentrations (e.g., volatile fatty acids and H 2 S).
Genetics
Phenotypic characteristics, as described before for these species, should be considered. Lac + strains
capable of producing lactic acid rapidly at the initial stage are preferred. Also, strains with weak
proteinase (Pro + ) activity are desirable because they do not cause rapid proteolysis with the accumulation of some peptides and the appearance of bitter flavor in the products. In mixed starters,
they should not have an antagonistic effect. Also, the strains should preferably be resistant to
multiple phages.
Microbial Problems
Bitter flavor in Cheddar cheese, especially in the aged product, results from the accumulation of
bitter peptides that are approximately 1000–12,000 Da and rich in hydrophobic amino acids.
Starters capable of hydrolyzing proteins rapidly (fast starters) tend to produce bitter peptides more
than slow starters do. Their enzymes hydrolyze proteins quickly, releasing large amounts of peptides that are subsequently hydrolyzed slowly to smaller peptides and amino acid by peptidases,
resulting in the accumulation of peptides. Because they are hydrophobic, bitter peptides are generally hydrolyzed slowly, causing them to accumulate. Use of slow starters for protein breakdown at
a slow rate or treatment of cheese with peptidase, or both, are effective in reducing bitterness (also
see Chapter 20).
Mold growth on the surface or in air pockets inside the cheese can occur after removing the
packing material from cheese. The spores are generally present in the raw material or get in the
product during processing and before sealing. It is not possible to determine from the colonial
morphology whether they are mycotoxin producers. It is better not to consume cheese with heavy
growth.
Staphylococcus aureus, following contamination of milk after heating, can grow during processing Cheddar cheese and produce enterotoxins. The toxins remain in the cheese even after the
death of cells during curing. Food poisoning can occur from consuming such cheese (even if they
are heated in some preparations).