Yogurt

A form of fermented milk produced in most countries where fresh milk is drunk.

Yogurt

• Yogurt is produced by the controlled fermentation of milk by two species of bacteria Lactobacillus bulgaricus and Lactotococcus thermophilus (also known as Streptococcus thermophilus)

Yogurt

• The sugar in milk (called lactose) is fermented to acid (lactic acid) and it is this that causes the characteristic curd to form.

Yogurt

• Streptococcus thermophilus brings the pH of the milk down to 5.5

• Lactobacillus bulgaricus converts lactose to lactic acid

Yogurt

• The two bacteria have a mutually stimulating effect on one another.

• Proteolytic enzymes from L. bulgaricus break down milk proteins into peptides.

Yogurt

• These peptides stimulate the growth of L. thermophilus which in turn produces formic acid and carbon dioxide

• These are growth stimulants for the L. bulgaricus.

Yogurt

• At the end of the incubation pH may fall to as low as 4.2 – 4.4

• The lactic acid coagulates the remaining milk proteins, causing the yoghurt to thicken.

Yogurt

• Acetaldehyde, a metabolic by product of both species, gives raw yoghurt its characteristic flavour.

Yogurt

• Incubation takes 12 hours at 32o C to reach the set point of natural yoghurt

• Sugar, colour and fruit pulp is often added to increase popularity of product.

Yogurt

• Yogurt is a nutritious dessert and its manufacture is an ideal way of using up Europe’s excess milk production

Yogurt

• Some yoghurts may receive additional heat treatment to kill off any living bacteria before or after packaging for the consumer

Fermentation Principles• Milk and LAB culture (Table 4.3)

– Cultured versus “directly set”• Functions

– Lactose– Precipitate casein (pH4.6)– Flavor compounds– Syneresis

• Separation of water from the coagulated milk• Considered unfavorable; to reduce,

– Increase milk solids– heat well above pateurization temp to denature whey proteins– Incorporate stabilizers– Using strains produce exopolysaccharide

• “Ropy” strain-EPS released or dis-attached• EPS remains attached to the cells surface

Yogurt Cultures

• S. thermophilus and Lb. delbrueckii subsp. bulgaricus• 1:1 ratio• Synergistic growth

– S. thermophilus growth first, use free aa and small peptides in milk, lower pH etc, preferred environment for Lb. delbrueckii subsp. bulgaricus

– Lb. delbrueckii subsp. bulgaricus proteolysis helps S. thermophilus to grow

– Lb. delbrueckii subsp. bulgaricus produce more acid, S. thermophilus will decrease

• So the cultures are grown separately, harvested and mixed

• Lactobacillus genus and Streptococcus thermophilus bacteria are usual starter organisms in yogurt.

• They are considered lactic acid bacteria (LAB).• Gram positive, non-spore forming rods or cock,

and aerotolerant anaerobes.

Introduction (cont.)• Streptococcus

thermophilus is a thermophile (grows at 45°C to 110°C). – Ferments lactose to lactic

acid.– Lactic acid chemically

alters casein, a milk protein, and forms curd.

Introduction (cont.)• Lactobacillus genus

bacteria are mesophiles.– Ferment remaining lactose

from S. thermophilus.

proteins peptides

proteins peptides

Lactobacillus thermophilus

proteins peptides

Lactobacillus thermophilusLactobacillus bulgaricus

Proteolytic enzymes

proteins peptides

Lactobacillus thermophilusLactobacillus bulgaricus

Proteolytic enzymes

formic acid + CO2

proteins peptides

Lactobacillus thermophilusLactobacillus bulgaricus

Proteolytic enzymes

formic acid + CO2

lactic acid

proteins peptides

Lactobacillus thermophilusLactobacillus bulgaricus

Proteolytic enzymes

formic acid + CO2

lactic acid acetaldehyde

Raw milk

Raw milk

Pasteurised Milk

pasteurisation

Raw milk

Pasteurised Milk

pasteurisation

Raw Yogurt

homogenisation

cooling

incubation

add starter

Raw milk

Pasteurised Milk

pasteurisation

Raw Yogurt

homogenisation

cooling

incubation

add starter

Processed Yogurt

(heat treatment)

packaging

(heat treatment)

add fruit/flavour