Kuliah Yogurt 2010
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Transcript of Kuliah Yogurt 2010
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Yogurt
A form of fermented milk produced in most countries where fresh milk is drunk.
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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)
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Yogurt
• The sugar in milk (called lactose) is fermented to acid (lactic acid) and it is this that causes the characteristic curd to form.
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Yogurt
• Streptococcus thermophilus brings the pH of the milk down to 5.5
• Lactobacillus bulgaricus converts lactose to lactic acid
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Yogurt
• The two bacteria have a mutually stimulating effect on one another.
• Proteolytic enzymes from L. bulgaricus break down milk proteins into peptides.
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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.
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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.
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Yogurt
• Acetaldehyde, a metabolic by product of both species, gives raw yoghurt its characteristic flavour.
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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.
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Yogurt
• Yogurt is a nutritious dessert and its manufacture is an ideal way of using up Europe’s excess milk production
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Yogurt
• Some yoghurts may receive additional heat treatment to kill off any living bacteria before or after packaging for the consumer
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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
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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
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• 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.
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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.
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Introduction (cont.)• Lactobacillus genus
bacteria are mesophiles.– Ferment remaining lactose
from S. thermophilus.
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proteins peptides
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proteins peptides
Lactobacillus thermophilus
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proteins peptides
Lactobacillus thermophilusLactobacillus bulgaricus
Proteolytic enzymes
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proteins peptides
Lactobacillus thermophilusLactobacillus bulgaricus
Proteolytic enzymes
formic acid + CO2
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proteins peptides
Lactobacillus thermophilusLactobacillus bulgaricus
Proteolytic enzymes
formic acid + CO2
lactic acid
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proteins peptides
Lactobacillus thermophilusLactobacillus bulgaricus
Proteolytic enzymes
formic acid + CO2
lactic acid acetaldehyde
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Raw milk
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Raw milk
Pasteurised Milk
pasteurisation
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Raw milk
Pasteurised Milk
pasteurisation
Raw Yogurt
homogenisation
cooling
incubation
add starter
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Raw milk
Pasteurised Milk
pasteurisation
Raw Yogurt
homogenisation
cooling
incubation
add starter
Processed Yogurt
(heat treatment)
packaging
(heat treatment)
add fruit/flavour