Bacterial Spoilage in Milk: Proteolysis, Gas Production And Ropiness

Any form of bacterial spoilage in milk is very significant because dairy products have a relatively limited keeping time and very high perishability. If you fail to apply sufficient preservation methods, these products can go bad really quick and result in massive losses.

The losses are not only in terms of quality but also monetary, as you may have to condemn the whole batch. There is no way you will sell substandard products to your customers for ethical reasons.

Again, defective products may cause cause food poisoning and/or adverse health effects on unsuspecting consumers.

Souring of fresh milk is a form of spoilage, especially if it curdles. However, if you intend to make fermented milk products, lactic acid fermentation is a very necessary technology for the process.

Milk is rich in nutrients such as lactose, citrate, butterfat (with fatty acids), among a plethora of other nitrogenous compounds. Given the fluidity of fresh milk, it exhibits very high water activity (a_w).

Which Bacteria are Responsible for Spoilage?

Other than the lactic acid bacteria (LABs), many other bacteria will produce acid to ferment milk if the conditions are not favorable for the LABs.

Therefore, eliminating LABs (if that were possible) will not assure you of safety from acid producing bacteria.

For instance,

• Coliforms will produce a mixture of acids, gases, and alcohols
• Some species of micrococci, micro-bacterium, and bacilli can produce acid in milk as well
• Clostridium spp. inhibits the growth of LABs and may produce butyric acid

Please note that insufficient heat treatment of milk will still lead to milk spoilage. If the heat destroys all the vegetative forms of bacteria BUT fails to destroy the Clostridium botulinum spores, the spores will still vegetate and cause butyric acid fermentation.

The defects/bacterial spoilage in milk:

Gas Production

Gas production is usually accompanied by acid formation, which is undesirable in milk and milk products.

Some of the most notorious gas formers include coliforms, yeasts, clostridium species, and gas forming bacillus that produce a mixture of carbon (IV) oxide and hydrogen gases.

To identify milk that has gas, check for foam at the top layer of the milk, ripping of the curd, and rapid frothy/stormy fermentation of the milk.

Proteolysis

This is the hydrolysis of milk proteins by microorganisms to produce peptides that gives the milk a bitter taste/flavor.

Storage of milk at low temperatures and destruction of lactic acid producers in the milk through heat treatment favor this process. Moulds and yeasts may also destroy any lactic acid formed in the milk and accelerate the process.

Proteolytic bacteria produce the following forms of bacterial spoilage in milk:

• Acid proteolysis, which involves production of acid and proteolysis
• Proteolysis that produces both acidity and alkalinity
• Sweet curdling, which results from the activity of the rennin-like enzymes that these bacteria produce at the early stages of proteolysis
• Slow proteolysis, which result from the activity of the bacterial endo-enzymes after autolysis

Acid proteolysis leads to production of a lot of whey and formation of shrunken curd. The bacteria further digests the curd, which changes the color of the curd from opaque to a little translucent. Some bacteria may completely dissolve the curd.

There are three major causes of acid proteolysis, namely:

• Micrococcus spp.: – these are very notorious and may even cause the proteolysis of freshly drawn milk as some of them inhabit the cow’s udder
• Streptococcus faecalis and Streptococcus liquifasciens: – very active proteolytic bacteria that may cause proteolysis of pasteurized milk
• Spores of some strains of Bacillus spp.: – especially the lactose fermenting and proteolytic strains such as Bacillus cereus, which can survive high temperature pasteurization and cause acid proteolysis.

Some bacteria that may not be able to ferment lactic acid may still cause proteolysis and this varies with different species of bacteria. Some strains may act on the casein directly and produce little proteolysis.

These three scenarios are very likely to occur following the action of these bacteria:

• They may produce very little or non-existent acidity making the milk alkaline
• Most of these bacteria cause sweet curdling of the milk before they digest the casein
• Others hydrolyze the protein extremely fast that no curdling is observed. This results into a clear liquid that lacks curd.

The most active proteolytic bacteria are found among the following species of bacteria.

Non-spore forming bacteria include:

• Micrococcus
• Alcaligenes
• Achromobacter
• Pseudomonas
• Proteus
• Flavobacterium

Spore forming bacteria include:

• Bacillus spp.
• Clostridium spp.

Most of these bacteria can grow and cause proteolysis and bitterness of milk that is held at chilling temperature. However, most of them are thermoduric (except some species of micrococci) and should not be present in pasteurized milk.

Proteolysis by the endo-enzymes after bacterial autolysis is slow and insignificant in milk. However, when long time is allowed for the process, it can be significant, especially in the curing of cheese.

Ropiness

Ropiness is a form of bacterial spoilage in milk that makes the milk highly viscous or sticky. Ropy milk has characteristic silk-like threads that may vary in length from a few inches to several feet.

You would test for this by dipping a pointed device (like a needle) on the surface of the milk (after incubating for 12-48 hours) and raising the needle to see if there is presence of a “rope.”

This defect affects milk, cream and whey. The effect is very significant in milk and cream meant for the market.

Ropiness can be classified as either bacterial or non-bacterial in nature

Bacterial Ropiness

Caused by a slimy capsular material produced by the bacterial cells (which is usually either gums or mucus).

You can further classify bacterial ropiness under these two categories:

• Surface Ropiness: – observed at the top of the milk and is caused by Alcalegenes viscolactis, which is majorly found in the soil and water.
• Ropiness observed throughout the milk: – caused by:

1. Some coliforms, (e.g. Enterobacter aerogenes, Enterobacter cloacae, and in rare curcumstances Escherichia coli.)
2. Some species of LABs, (e.g. Streptococcus lactis, Lactobacillus bulgaricus, Lactobacillus cereus, Streptococcus cremoris, and Lactobacillus plantarum). Most of these microorganisms grow in chains, which leads to the formation of the ropes in milk.
3. Other microbes like Micrococci and Bacilli.

Non-bacterial Ropiness

This form of bacterial spoilage in milk may occur as a result of:

1. Presence of mastitis in the milk, especially fibrin and leucocytes from the cow’s blood present in the milk.
2. Thick cream at the top of the milk
3. Casein film or lactalbumin that occur in milk during cooling