Bacterial Spoilage in Milk: Proteolysis, Gas Production And Ropiness

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.

Quality Management
24. Mar 2023
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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 other nitrogenous compounds. Given the fluidity of fresh milk, it exhibits very high water activity (aw) which provides a suitable environment for bacterial proliferation.

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:

1. 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.

2. 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:

  1. Acid proteolysis, which involves production of acid and proteolysis
  2. Proteolysis that produces both acidity and alkalinity
  3. Sweet curdling, which results from the activity of the rennin-like enzymes that these bacteria produce at the early stages of proteolysis
  4. 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.

The following 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.

3. 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.”

To test for #ropiness in milk: dip a pointed device on the surface & raise to check for 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

Common Defects in Fermented Milks

Fermented milks have a better keeping quality than fresh milk due to the low pH that do not favor growth of a wide range of bacteria. However, there still exist a variety of defects in fermented milks that can affect appearance, texture, or flavor.

To get a clear understanding of these defects, it is best that we classify the defects as follows:

  1. Defects resulting in changes of appearance
  2. Flavor and aroma defects
  3. Defects of consistency, body and texture, and viscosity

Some of the very common defects under these categories, their causes, and prevention methods include:

Appearance defects in fermented milks

Defect

Cause

Prevention

Wheying-off/whey separation

- Disturbance of the coagulum during incubation
- High inoculation rate leading to excessive acidification
- Incorporation of excessive air
- Too low total solids

- Avoid mechanical shaking during coagulation
- Proper incubation period
- Correct storage temperature
- Correct standardization

Stale/aged

- A film is formed on the surface due to freezing of the product
- The package is smeared over/damaged

- Avoid freezing of the product
- Ensure careful handling of the product during packaging and storage

Cream line

- Insufficient stirring of milk
- Insufficient or absence of homogenization

- Homogenize effectively
- Thoroughly and evenly stir the product to obtain a sufficiently homogenous product

Mould/yeast colonies formed on the surface

- Poor hygiene leading to mould and yeast growth

- Maintain hygiene during production and product handling

Inhomogenous appearance

- Insufficient stirring of the product after fermentation

- Thoroughly and evenly stir the product after fermentation

Gas formation

- Contamination by gas producing microorganisms

- Maintain hygiene during production and product handling

Discoloration

- Heating/low pH

- Use stable coloring agent

Flavour defects in fermented milks

Defect

Cause

Prevention

No coagulation/no sourness/slow coagulation

- Inhibitors in milk
- High inoculation temperature or low inoculation rate
- Starter culture strain imbalance cocci:rods

- Fresh, inhibitor-free milk
- Young and active starter culture
- Correct inoculation temperatures at 23-25°C
- 2-3% of starter culture

Bitter taste

- Too long storage
- Contamination with highly proteolytic bacteria or enzymes

- Normal storage (two weeks under cold storage)
- Avoid contaminants by ensuring high production hygiene

Yeast/fruity flavour

- Contamination by yeasts

- Ensure high standards of production hygiene as yeasts can withstand very high acid conditions

Rancid flavour

- Fat breakdown by lipolytic microrganisms
- Insufficient heat treatment of milk

- Maintain high production hygiene
- Ensure proper high temperature pasteurization of milk (85°C/30minutes or 90°C/5minutes or 121°C/5seconds)

High acid/very sour

- Too high acidification during and after incubation
- High inoculation rates and high storage temperatures

- Correct incubation period
- Cool down immediately after incubation

Consistency and viscosity defects in feremented milks

Defect

Cause

Prevention

Slimy/ropy/stringy

- Contamination with slime (exopolysaccharides) producing bacteria e.g. Leuconostocs and lactobacilli
- Too low incubation temperature

- Maintain proper starter culture ratio
- Use active starter culture
- Maintain high production and product handling hygiene

Sandy

- Contraction of the gel particles
- Early stirring of the coagulum
- Stirring coagulum at high temperatures above 38°C and holding at this temperature

- Avoid mechanical disturbance before coagulation
- Stir after cooling of the product

Gummy

- Unsuitable stabilizers
- Excessive addition of stabilizers
- Faulty incorporation of stabilizers

- Ensure correct amount of stabilizer and application procedure

Thin/liquid

- Too strong stirring of the gel
- Low total solids due to adulteration of the milk
- Too short cold storage period

- Gentle and thorough stirring of the gel
- Use unadulterated milk with normal total solids

Wheying-off

- High acidification before cooling
- Insufficient cooling
- High inoculation rate
- Low temperature pasteurization

- Proper incubation period
- Proper pasteurization temperatures (85°C/30minutes or 90°C/5minutes or 121°C/5seconds)
- Cool after the set incubation time

Soft/soupy

- Low protein content in milk as a result of adulteration
- Low inoculation rate
- Too short incubation period
- Mechanical shaking before coagulation is complete

- Standardize to proper total solids
- Use normal milk disturbance

Troubleshooting Appearance Defects in Fermented Dairy Products

Having understood the causes and prevention measures for the flavor defects, let us now look at the appearance defects of fermented milk products.

Poor quality and/or inadequate hydration of dairy powders, too-high or too-low pasteurization temperatures, low-homogenization pressures, and incorrect fermentation temperature are some of the factors that could contribute to the defects were are looking at in this post. These are the defects that you see on the product.

APPEARANCE DEFECTS OF DAIRY PRODUCTS

Causes

Prevention Measures

1. Syneresis/whey separation

 

Contamination of the milk with gas-forming micro-organisms

- Check delivered milk and culture for gas formers, aerobic spore formers (B. cereus)
- Only use fresh milk (<48h)

Aqueous yoghurt milk

- Control freezing point

Insufficient fat content in formula

- Increase fat content

Low milk solids non-fat content in formula

- Increase milk solids non-fat (MSNF)

Insufficient stabilization level and/or unsuitable stabilizer

- Add/ increase addition/ change stabilizer system

Insufficient heat treatment

- Pasteurize at 90-95C/5-10 minutes

Insufficient homogenization pressure

- Check homogenization pressure (> 200 bar)

Homogenization temperature too high

- Homogenization temperature 60-68C

Starter culture too fast

- Change to a slower fermenting starter culture
- Lower fermentation temperature

Coagulum disturbed at pH >4.6

- Avoid any movement of the coagulum while it is being formed
- Don’t agitate the coagulum before pH 4.5 is reached

The coagulum has not been agitated sufficiently after fermentation

- Increase agitation before/ during cooling

Excessive mechanical treatment

- Avoid agitating for too long or too vigorously when: - Breaking pH (stirred yoghurt), - Cooling, - Adding fruit preparation/ flavors, - Filling Apply gentle pumps and agitation devices

Unsuitable fruit preparation

- Use suitable fruit preparation:
- Optimized hydrocolloids,
- 35-50% fruit content
- 10-60% soluble solids (typically 35-40%SS)
- pH 3.8-4.0 (for twin pot yoghurt pH 3.2-3.5)

Post-acidification due to delayed or lengthy cooling (stirred yoghurt)

- Efficient cooling immediately after breaking the coagulum
- Use starter culture with lower post-acidification

Air incorporation during processing and filling

- Limit air intake during processing

pH too low on filling (set yoghurt)

- Avoid filling at pH <6.0

Temperature too low on filling (stirred yoghurt)

- Increase the filling temperature to22-25C

Excessive vibration of the finished product

- Allow the yoghurt to redevelop viscosity by keeping it undisturbed in cold storage for min, 24 hours
- Instruct handlers to avoid vibrations due to their negative influence on yoghurt quality

Inconstant temperatures (especially too high) during storage of the finished product

- Ensure constant cooling chain

2. Cream separation

 

Homogenization pressure too low

- Homogenization pressure should be >150 bar (150-250 bar)

3. Light and frothy

 

Incorporation of air due to excessive agitation of the milk after the addition of culture

- Avoid strong agitation as well as pumping after adding the culture
- Avoid the incorporation of air in any way

Contamination by gas-forming bacteria like coliforms or lactic yeasts

- Microbiological control of milk and equipment
- Increase disinfection and cleaning of equipment
- Increase pasteurization temperature/time

Frothy surface due to unsuitable filling device

- Replace pump seals (regular control) Repair/change filling machinery

4. Graininess

 

Improper formulation

- Decrease MSNF in formulation
- Change formulation to a more suitable starch type
- Optimize stabilizer system/ usage rate

Starter culture not uniformly distributed

- Agitate sufficiently when adding starter culture
- Make sure the culture is completely thawed before stopping agitation

Fermentation too fast

- Lower the fermentation temperature
- Decrease inoculation rate of starter culture Change to a slower starter culture with higher polysaccharide (EPS) formation

Breaking of coagulum at excessively high pH

- Break only at pH ≤4.5

Insufficient mechanical treatment after breaking up coagulum (stirred yoghurt)

- Use a smoothing valve in line

5. Color leaching

 

Difference in osmotic pressure too high in the fruit preparation and the white mass

- Choose a fruit preparation that is designed not to leach color (proper sweeteners)
- Substitute some or all the sucrose in the white mass with fructose

Fruit preparation insufficiently stabilized

- Use suitable fruit preparation (optimized hydrocolloids)

6. Yeasty/mouldy

 

Process milk or yoghurt contaminated with yeast/moulds: - contaminated ambient air/production rooms, - residues of rinse water in pumps and pipes, - filling devices or packaging material, - fruit preparation

- Ventilate production rooms sufficiently (laminar flow or filtrated air)
- Ensure that packaging material is sealed
- Correct storage of packaging material
- Use adequate cleaning agents/ disinfection of equipment and production room
- Control all incoming ingredients/ fruit preparations for yeast and mould counts
- Use protective starter cultures

 

Troubleshooting Texture Defects in Fermented Dairy Products

Texture defects in dairy products can be caused by lack of hydration time for added dairy protein ingredients, wrong usage level of stabilizers, low-homogenization pressures, too-high pasteurization temperatures, and/or too-high fermentation temperature.

Let us now look at the common texture defects of fermented dairy products and how to prevent them. They are some of the serious defects of fermented milk products you will ever encounter as a processor.

TEXTURE DEFECTS OF FERMENTED MILK PRODUCTS

Causes

Prevention Measures

1. Low Viscosity

 

Improper formulation

Increase milk solids non-fat (MSNF) by adding suitable milk proteins Add/ optimize stabilizer system

Low fat content

Increase fat content

Aqueous yoghurt milk

Control freezing point, change batch or compensate with addition of MSNF

Milk containing inhibitors

Examine raw milk for inhibitors

Insufficient heat treatment

Pasteurization 90-95 C/5-10 minutes

Insufficient homogenization pressure

Control homogenization pressure (>200 bar) Is the homogenizer working properly?

Improper starter culture

Change to a starter culture with higher polysaccharide (EPS) formation

Insufficient inoculation of starter culture

Increase culture dosage

Bacteriophage attack on starter culture

Change culture

Incorrect fermentation temperature

Depending on the starter culture, increase or reduce fermentation temperature

Disturbing the coagulum at pH>4.6

- Avoid any disturbance of the coagulum while its being formed
- Don’t agitate coagulum before pH 4.5 is reached

If the coagulum has been cooled before agitation, it becomes much more sensitive to mechanical treatment, and will quickly lose its viscosity

Stir the coagulum before cooling

Excessive mechanical treatment

- Avoid too long and too strong agitation at Breaking pH (stirred yoghurt)
- Cooling
- Addition of fruit preparation/flavors
- Filling - apply gentle pumps and agitation devices

Insufficiently concentrated flavorings/fruit base

- Use concentrated flavorings and fruit preparations
- Use stabilized fruit preparation with texturing effect in the yoghurt mass (stirred yoghurt)

Air incorporation during processing and filling

Limit air intake during processing

Too low temperature at filling (stirred yoghurt)

Increase the filling temperature to 22-25 C

Too fast cooling from filling temperature to final storage temperature

Increase cooling time of secondary cooling (from primary cooling 25C to final cooling 5C)

Excessive vibrations of the finished product

Allow the yoghurt to redevelop viscosity by keeping it undisturbed in cold storage for 24 hours

Unsuitable stabilizer blend (high gelatin content) when stored at high temperatures (> 15-20 C)

If the storage temperature are high, change to a more suitable stabilizer blend

2. Too firm/ hard curd

 

Improper formulation

Optimize stabilizer usage Reduce milk solids non fat

3. Soft/ smeary product

 

Contamination by residues of cleaning agents

Control cleaning of equipment (CIP device) Careful rinsing of the whole equipment with suitable water

Contamination by alkali-forming or protein-decomposing micro-organisms

Control fermentation equipment for possible raw milk contamination

Improper formulation

Increase milk solids non-fat (MSNF) by adding suitable milk proteins Add/ optimize stabilizer system

Too low incubation (set yoghurt) or inconstant temperature in incubation chamber

Control filling and incubation temperatures (37-43oC depending on culture)

Cooling at too high pH (set yoghurt)

Control pH and cool in time. Optimal cooling pH 4.65-4.80 depending on the culture

4. Stringy/Slimy/Ropy

 

Too long pre-storage of the raw milk at deep temperatures

Use adequate fresh milk

Contamination of the milk by slime-forming bacteria (influence of feeding)

Control raw milk. Acid cleaning and disinfection of equipment

Too high sugar content in formulation

Reduce sugar content or change to a starter culture that is less sensitive to high sugar /solids content

Degeneration in the bulk starter

Permanent control of bulk starter production and correct (and not too long) storage

Too low incubation temperature

Increase incubation temperature (37-43C depending on culture)

Improper culture

Change to a culture that produces less exo-polysaccharides

5. Lumpy

 

Too high gelatin content on formula

Use less gelatin in the stabilizer or reduce dosage of stabilizer

Insufficient agitation when breaking up coagulum (stirred yoghurt)

Increase mechanical treatment Use a smoothing valve in line.

Filling temperature too high (stirred yoghurt)

Fill at a lower temperature (22-24C)

Filling into cups at improper pH value (set yoghurt)

Filling of set yoghurt at pH > 5.7

6. Sandy/ gritty

 

Inadequate protein combination in formula

Change formula to a apply more adequate milk protein source

Excessive pasteurization

Pasteurization 90-95 C/5-10 minutes

Vibrations during incubation (set yoghurt)

Avoid application of any mechanical force during incubation

Irregular temperature during incubation

Control/monitor temperature during incubation

Agitating the gel above 38C followed by holding the temperature above 38C

Cool the yoghurt immediately after breaking the coagulum

Uneven agitation of the coagulum (stirred yoghurt)

Agitate the coagulum more evenly

7. Dry

 

Improper formulation (inadequate or too much starch or protein)

Optimize stabilizer blend Use adequate milk protein source

Improper hydration of protein

Ensure proper hydration (temperature/time) for dry ingredients used in formula

Insufficient homogenisation

Homogenization> 150 bar (150-250 bar)/ 60-68oC

8. Jelly/ gel-like

 

Too high stabilization

Add less stabilizer (especially if it contains gelatin)

Fruit preparation with gelling properties (set yoghurt, fruit on bottom)

Use a fruit preparation with no interfacial (to the white mass) interaction

Troubleshooting Texture Defects in Fermented Dairy Products

Texture defects in dairy products can be caused by lack of hydration time for added dairy protein ingredients, wrong usage level of stabilizers, low-homogenization pressures, too-high pasteurization temperatures, and/or too-high fermentation temperature.

Let us now look at the common texture defects of fermented dairy products and how to prevent them. They are some of the serious defects of fermented milk products you will ever encounter as a processor.

TEXTURE DEFECTS OF FERMENTED MILK PRODUCTS

Causes

Prevention Measures

1. Low Viscosity

 

Improper formulation

Increase milk solids non-fat (MSNF) by adding suitable milk proteins Add/ optimize stabilizer system

Low fat content

Increase fat content

Aqueous yoghurt milk

Control freezing point, change batch or compensate with addition of MSNF

Milk containing inhibitors

Examine raw milk for inhibitors

Insufficient heat treatment

Pasteurization 90-95 C/5-10 minutes

Insufficient homogenization pressure

Control homogenization pressure (>200 bar) Is the homogenizer working properly?

Improper starter culture

Change to a starter culture with higher polysaccharide (EPS) formation

Insufficient inoculation of starter culture

Increase culture dosage

Bacteriophage attack on starter culture

Change culture

Incorrect fermentation temperature

Depending on the starter culture, increase or reduce fermentation temperature

Disturbing the coagulum at pH>4.6

- Avoid any disturbance of the coagulum while its being formed
- Don’t agitate coagulum before pH 4.5 is reached

If the coagulum has been cooled before agitation, it becomes much more sensitive to mechanical treatment, and will quickly lose its viscosity

Stir the coagulum before cooling

Excessive mechanical treatment

- Avoid too long and too strong agitation at Breaking pH (stirred yoghurt)
- Cooling
- Addition of fruit preparation/flavors
- Filling - apply gentle pumps and agitation devices

Insufficiently concentrated flavorings/fruit base

- Use concentrated flavorings and fruit preparations
- Use stabilized fruit preparation with texturing effect in the yoghurt mass (stirred yoghurt)

Air incorporation during processing and filling

Limit air intake during processing

Too low temperature at filling (stirred yoghurt)

Increase the filling temperature to 22-25 C

Too fast cooling from filling temperature to final storage temperature

Increase cooling time of secondary cooling (from primary cooling 25C to final cooling 5C)

Excessive vibrations of the finished product

Allow the yoghurt to redevelop viscosity by keeping it undisturbed in cold storage for 24 hours

Unsuitable stabilizer blend (high gelatin content) when stored at high temperatures (> 15-20 C)

If the storage temperature are high, change to a more suitable stabilizer blend

2. Too firm/ hard curd

 

Improper formulation

Optimize stabilizer usage Reduce milk solids non fat

3. Soft/ smeary product

 

Contamination by residues of cleaning agents

Control cleaning of equipment (CIP device) Careful rinsing of the whole equipment with suitable water

Contamination by alkali-forming or protein-decomposing micro-organisms

Control fermentation equipment for possible raw milk contamination

Improper formulation

Increase milk solids non-fat (MSNF) by adding suitable milk proteins Add/ optimize stabilizer system

Too low incubation (set yoghurt) or inconstant temperature in incubation chamber

Control filling and incubation temperatures (37-43oC depending on culture)

Cooling at too high pH (set yoghurt)

Control pH and cool in time. Optimal cooling pH 4.65-4.80 depending on the culture

4. Stringy/Slimy/Ropy

 

Too long pre-storage of the raw milk at deep temperatures

Use adequate fresh milk

Contamination of the milk by slime-forming bacteria (influence of feeding)

Control raw milk. Acid cleaning and disinfection of equipment

Too high sugar content in formulation

Reduce sugar content or change to a starter culture that is less sensitive to high sugar /solids content

Degeneration in the bulk starter

Permanent control of bulk starter production and correct (and not too long) storage

Too low incubation temperature

Increase incubation temperature (37-43C depending on culture)

Improper culture

Change to a culture that produces less exo-polysaccharides

5. Lumpy

 

Too high gelatin content on formula

Use less gelatin in the stabilizer or reduce dosage of stabilizer

Insufficient agitation when breaking up coagulum (stirred yoghurt)

Increase mechanical treatment Use a smoothing valve in line.

Filling temperature too high (stirred yoghurt)

Fill at a lower temperature (22-24C)

Filling into cups at improper pH value (set yoghurt)

Filling of set yoghurt at pH > 5.7

6. Sandy/ gritty

 

Inadequate protein combination in formula

Change formula to a apply more adequate milk protein source

Excessive pasteurization

Pasteurization 90-95 C/5-10 minutes

Vibrations during incubation (set yoghurt)

Avoid application of any mechanical force during incubation

Irregular temperature during incubation

Control/monitor temperature during incubation

Agitating the gel above 38C followed by holding the temperature above 38C

Cool the yoghurt immediately after breaking the coagulum

Uneven agitation of the coagulum (stirred yoghurt)

Agitate the coagulum more evenly

7. Dry

 

Improper formulation (inadequate or too much starch or protein)

Optimize stabilizer blend Use adequate milk protein source

Improper hydration of protein

Ensure proper hydration (temperature/time) for dry ingredients used in formula

Insufficient homogenisation

Homogenization> 150 bar (150-250 bar)/ 60-68oC

8. Jelly/ gel-like

 

Too high stabilization

Add less stabilizer (especially if it contains gelatin)

Fruit preparation with gelling properties (set yoghurt, fruit on bottom)

Use a fruit preparation with no interfacial (to the white mass) interaction

 

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