Week 7 Milk & Dairy Product Processing - Coggle Diagram
Week 7 Milk & Dairy Product Processing
Liquid Milk Processing: combination of the following unit operations namely clarification, separation(for low fat milk), pasteurisation and homogenization :check:
Pasteurised milk: subjected to thermal processing that ensure that every particle of milk is heated to specific temperature for specific time without recontamination during the heat treatment.
ensure milk is safe for human consumption by destroying all pathogen and undesirable enzymes
can have shelf life of 7, 10, 14 up to 16 days under chilled storage
Pasteurisation ~ Continuous processing using a High Temperature Short Time (HTST) pasteurizer
Cold raw milk at 4° C drawn into the regenerator section of pasteurizer and warmed to 57° C - 68° C
Raw milk forced through the heater section to heat milk to a temperature of at least 72° C
Milk flows through the holding tube where it is held for no less than 15 sec
Milk at end of holding tube pass into flow diversion device (FDD).
Homogenization ~ Mechanical treatment of fat globules by passing milk under high pressure through tiny orifice to reduce creaming of fat globules. Pasteurisation breakdown protein complex so tends to cluster fat globule
After homogenization and cooling, milk is packaged into paper cartons/ plastic opaque containers etc. Packaged milk is then delivered in refrigerated trucks.
varying fat content
whole milk (3.4%)
low fat (2% or 1%)
skim milk (<0.1%)
Vitamins (vitamin A & D) may be added to both full fat or reduced fat milk.
Dry Milk Products
water content ranges between 1.5 – 5% which prevent microbial growth. The drying process of milk not only increase shelf life, it reduces the transportation and storage space of final product.
Whole milk powder ~ Produced from standardised milk. Milk pasteurised at 80 - 85℃ to inactive lipolytic enzymes
Skimmed milk powder ~ Depending on heat treatment intensity can be typically divided into Low-heat powder (Whey Protein Nitrogen Index WPNI > 6), Medium-heat powder (WPNI 5 – 6), High heat powder (WPNI <1.4)
Instant milk powder ~ High wettability and solubility. Powder agglomerated into larger particles by partially rehydrating the dried milk powder particles followed by removal of water through drying resulting in an increased amount of air incorporated between the powder particles.
Spray drying of milk powder includes primarily the following unit operations evaporation, size reduction (atomization), dehydration, and separation.
milk is first concentrated using evaporation (48 to 52% total solids) prior to dehydration in the spray tower.
Evaporation (usually under vacuum) is important:
facilitate powder separation
increase particle size (more solids per droplet)
increase particle density
Process: Done in 2 phases
Pre-treated milk is evaporated to 48 -52% DM
Concentrated milk is pumped to atomizer whereby it is broken into small droplets (40 - 150 𝜇m). These droplets come into contact with stream of hot air ( 150 – 210 ℃) and lose moisture very rapidly while suspending in the drying hot air. (Residence time typically 10-20 sec depending on volume rate of air flow and volume of chamber).
Dry powder separated from moist air in cyclone system. Dense powder particles forced down into discharge containers and collected while moist air directed to exhaust pipes.
Air conveying ducts for dry powder may sometimes be connected to cooling system to admit cold air for transportation of product.
Depending on products, dry powder may be subjected to instantizing (aggregation/ agglomeration process) that prevent powder particles from sticking together (lumpy) during rehydration.
heat damage to products are slight
exposure time to high temperature of dry material is very short
“evaporative cooling effect” during the critical drying period. The typical surface temperature of the milk particles during the constant drying zone is around 45 - 50℃.
Other conditions to be controlled
Flow rate of liquid feed (pump speed/pressure)
Air flow rate (Fan speed/ location of baffles)
Particle size (Speed/pressure of atomizer)
Outlet temperature (Flow rate of feed, air flow rate)
Changes in milk powder during storage:
Milk fat will oxidise in whole milk powder (resolved by antioxidant or filling in inert gas)
Chemical reaction in milk powder takes place slowly if stored under room temperature, away from light and maintain low moisture content
Effects on quality
Rehydration ~ rate and extent of rehydration is used as indicator of food quality.
Nutrition/Flavour/Aroma ~ high loss under high temperature and high air velocity
Texture ~ formation of hard skin
Fermented Milk Product
Yogurt: fermented product made by inoculating pasteurised milk or dried milk/non-fat milk solids have been added before pasteurisation with at least starter culture Lactobacillus bulgaricus and one or more of the following bacteria, namely, Streptococcus thermophilus, Lactobacillus acidophilus and Bacterium yoghurtii.
Set type ~ incubated and cooled in package
Stirred type ~ incubated in tanks and cooled prior to packaged
Drinking type ~ similar to stirred type, coagulum broken down to liquid prior to packaging
Frozen yogurt ~ incubated in tanks and frozen like ice cream
fermentation: part of lactose is converted to lactic acid with production of other substances (e.g. carbon dioxide, acetaldehyde, acetic acid, diacetyl) during the conversion process.
Lactic acid (low pH) produced act as preservative effect on milk since it prevents microbial growth.
Suitable for lactose intolerant people whose digestive system is lack of lactase system.
Milk (whole milk or skimmed milk)
Low bacteria count,
No enzyme/substance to decrease development of starter culture,
free from antibiotics
Starter culture (Streptococcus thermophilus and Lactobacillus bulgaricus )
Stabilisers (Gelatin, CMC, hydrocolloids)
Flavourings, Colouring (artificial or natural)
Other dairy ingredients (Non-fat dry milk, whey, lactose)
Milk is clarified, separated followed by standardized to desired fat or dry solids content
Pre-treated milk is blended with other ingredients (sweetener, stabiliser etc.) [not applies to drinking yoghurt]
Mixture is pasteurised for 30 min at 85° C or 10 min at 95° C Severe treatment since need to ensure starter culture can grow, need to denature and coagulate whey protein **
Pasteurized mixture is homogenized
ensure mixing of ingredients, prevent creaming and whey off during incubation / storage
Homogenized mix is cooled to an optimum growth temperature of starter culture
Factors affecting the quality of yogurt
Choice of milk ~ Low bacterial count and absent of inhibitory substances (antibiotics, residues of cleaning solutions, bacteriophages) for growth of yogurt culture
Milk standardisation ~ Typically 0.5 – 3.5% fat, min. MSNF is 8.2% increase of total DM content in particular whey proteins and casein which will result in firmer yoghurt coagulum.
Milk additives ~ E.g. sugar however >10% sugar to milk before incubation has adverse effect on fermentation, E.g. stabilisers (0.1 – 0.5%) such as gelatin, starch, hydrocolloids increase viscosity and prevent whey separation
Homogenisation ~ Improves stability and consistency. Generally 20 – 25MPa and 65 - 70℃
Deaeration ~ Low air content to minimise fouling in heat treatment, improve stability and viscosity, remove volatile off-flavour