Defects of fermented milks

The sensory defects of fermented milks may be divided into the following groups:

•  Defects resulting in changes of appearance,
•  Defects of flavour and aroma
•  Defects of consistency, body and texture, and viscosity.

The defects of dahi and yoghurt are discussed here:

i. Defects resulting in changes of appearance

The most important defects affecting the appearance of yoghurt are presented here under:

Natural Yoghurt

Defect	Cause of Defect
Settled, whey separation	Separation of whey due to over acidification/ mechanical shaking of gel/ low solids content/ admixture of air in stored yoghurt.
Fermented	Contamination by yeasts and coliform organisms
Unclean appearance	Mud particles, dirt, etc.
Formation of colonies or film on the surface	Growth of yeasts or moulds
Aged, not fresh appearance	Formation of film on the surface due to drying up crystal structures on the surface due to freezing/ packages smeared over/ packages damaged/ lacking care during transport
Cream line	None or insufficient homogenization
Condensed water inside of lid of package	Great fluctuations in temperatures and air pressure

Additional Defects for Fruit- and Flavoured Yoghurts

Atypical colour	Addition of fruit juice for colouring or artificial fruit colouring
Pale colour	Discolouration of fruit colour due to heating/ too pale colour of flavouring and fruit base
Un-homogenous appearance	Insufficient stirring/ fault during stirring

ii. Defects of Flavour and Aroma

The most important defects affecting the flavour and aroma of yoghurt are presented

here under:

Natural Yoghurt

Defect	Cause of Defect
Feed	Feed flavour of raw milk
Bitter	Too long storage of yoghurt/ high proteolytic activity of cultures/ growth of proteolytic contaminants
Burnt (cooked)	Too severe heat treatment of the milk
Unclean sour	Cultures contaminated by wild lactic acid bacteria, coliform organisms, etc.
Yeast, fruit	Contamination by yeasts
Stale, empty (no aroma)	One-sided growth of the streptococci/ too short time for fermentation or too low temperature of incubation/ low aroma production/ low total solids content of the milk/ too strong slime production.
Mealy, gluey	Excessive addition of milk powder/ too high evaporation
Cheesy	Contamination by proteolytic organisms (e.g. moulds)
Oxidized flavour	Effect of light, Metal catalysator, Oxidized flavour spontaneously occurring after 1-2 days of cold storage (without the effect of light and metal traces).
High acid (over-sour)	Too high acidification during incubation and after that
Low acid	Too low acidification during incubation/ inhibitory substances present in milk/ bacteriophage attacking the yoghurt culture.

Additional Defects for Fruit- and Flavoured Yoghurts

Artificial flavour	The use of too high concentrations of fruit base and flavours/ artificial flavours.
Too sweet	Excessive addition of sugar
Atypical, flat	Addition of foreign stuffs affecting flavour/ blunting the fruit acids by neutralization/ sever heat treatment of the fruit base and flavouring concentrate/ faults in the manufacture of fruit concentrate.

iii. Defects of consistency and viscosity

The most important defects affecting the consistency and viscosity of yoghurt are presented here under:

Defects in the consistency of Set Yoghurt

Defect	Cause of Defect
Whey separation, settled	High acidification of yoghurt before cooling/ containers too early moved/ insufficient cooling/ strong acid producing cultures/ low heating temperature of the milk.
Soupy, soft	Low protein content of the milk/ too small inoculum of culture/ short time of incubation/ mechanical shaking of the gel before the completed coagulation.
Spitted, cleavage	Splitting of a spherical segment of the gel due to mechanical shaking.

Defects in the Viscosity of Stirred Yoghurt

Sandy	Contraction of the gel particles: Severe heating of the milk/ homogenization of the milk at too high temperature-pressure combination/ too much evaporation of the milk/ excessive addition of milk powder and/ or uneven stirring/ shaking during incubation i.e. too early moving yoghurt from the production line/ stirring the gel above 38oC, followed by holding above 38oC.
Slimy	Slime-producing contaminants/ slime production by yoghurt culture/ addition of slime-producing strains of yoghurt culture/ too low temperature of incubation of the yoghurt culture.
Gummy, gluey, tough	The use of unsuitable stabilizers/ high quantities of added stabilizers/ faulty incorporation of stabilizers.
Phase separation on containers (below whey, above gel)	Admixture of air: strong cooling/ pumps; stirring
Liquid	Too strong stirring the gel/ low solids content of yoghurt/ insufficiently concentrated flavours and fruit base, insufficiently cooled/ short time of cold storage/ culture of low slime production.

iv. Common Defects in Dahi

Defect	Cause of Defect
Green/ yoghurt flavour	Acetaldehyde accumulation
Oxidized flavour	Copper contamination and/ or exposure to fluorescent light or sunlight.
Yeasty/ Cheesy	Contaminating yeast growth
Rancid	High lipolytic activity
Insufficient flavour	Poor quality starter, low citrate level in milk
Acidic/ sour.	Excessive level of starter culture
Weak body	Insufficient heat treatment of the mix, milk SNF too low.
Grainy texture	Acidity too high, skim milk powder not properly stabilized.
Chalky/ powdery texture	Excess amount of skim milk powder, poor quality of powder.
Wheying off	Excessive acidity development