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Summary Dairy Science and Technology
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DAIRY SCIENCE AND TECHNOLOGYLECTURE 1 – INTRODUCTION
Milk composition: 86% water, 4.6% lactose, 4.5% fat, 3.5% protein, 1% vitamins and minerals
Characteristics of the dairy industry:
- Milk is liquid and homogeneous, so transport and storage are simple.
- Milk properties vary based on source, season, storage conditions, duration of storage, so processes
might need to be adapted to reduce this variation. Variation can be largely reduced already when
the supply of milk is constant during the year, and when it is collected in large tanks.
- Raw milk is usually delivered to the dairy industry throughout the year, but quantities may vary
depending on the season (in some countries there is no supply in certain periods), so in some
moments dairy factories cannot work at full capacity.
- Milk is highly perishable, so control of hygiene and storage is necessary as it can contain pathogens.
Unit operations can be grouped as: Transfer of momentum (pumping, flow), Heat transfer (heating,
cooling), Mixing (homogenization, stirring), Phase separation (centrifugation, skimming), Molecular
separation (membrane processes, evaporation, drying), Physical transformations (gel formation due to
renneting, churning during butter making), Microbial and enzymatic transformation (fermented products,
cheese ripening), Stabilization (pasteurization, sterilization, cooling, freezing)
Separation technologies Centrifugation = gravity separation. Evaporation and drying = phase separation.
Membrane filtration = filtration. Ion exchange = chromatography. Electrodialysis = current induced
separation.
Manufacturer and consumers mainly determine the objectives of milk processing (and governments):
- Product safety for consumers pathogenic bacteria and toxins must be avoided. treating raw milk
to kill pathogens, prevent recontamination, transformation of milk into a product unsuited for
pathogen growth (e.g. fermented products).
- Product quality nutritional value, eating quality, shelf life, emotional value, usage properties.
Producing a consistent end product is challenging because milk is variable. Preventive measures,
hygiene, quality checkpoints are necessary to ensure good quality.
- Process quality safe and convenient for the staff, and should not cause environmental problems.
- Costs processing costs should be kept as low as possible, including price of raw materials,
energy, equipment, labour. All milk components should be valorized to minimize waste.
Two routes of milk cracking via cheese (produce cheese and cheese whey as by product) and via butter
(produce butter, and casein and acid whey as by product). Both routes also result in separation of cream.
LECTURE 2
HEAT TREATMENT
Objectives
- Product safety for consumers heat treatment allows killing of pathogens
- Shelf life extension spoilage organisms and spores are killed, so shelf life is longer. It also allows
inactivation of enzymes that can cause spoilage or off-flavors.
- Creation of specific product properties e.g. increased heat stability, stimulating growth of
starter bacteria, denaturation of whey proteins
, - No creaming during storage (homogenization)
Heat treatment can be: Continuous vs. in batch; post-heating packaging vs. in-container heating. Most
industries use continuous operations because of efficiency, economics and energy recovery. In in-container
heating there is a higher heat load, but there is no risk of product recontamination.
Heat intensity = heating time + temperature different combinations will generate different effects on the
product. You have to use a combination that balances the desired and undesired effects.
Processes of different intensity
Heating processes classify based on intensity:
- Thermisation usually 10s at 68°C or 20s at 65°C. Effects:
o Extend shelf life of raw milk
o Reduce the number of psychotrophic bacteria (can grow at low temp.), which can produce
heat-resistant lipases and proteases
o Almost no irreversible changes in milk
- Low pasteurization e.g. 15s at 72°C. The effectiveness of the process can be verified by the
presence of alkaline phosphatase; if this enzyme is not present in milk anymore, pasteurization
was effective (because it inactivates it). Effects:
o Kill pathogens e.g. M. tuberculosis and C. burnettii are used as indicators. Log N against
time indicates the reduction of bacterial count during heating at a specific temperature.
o Yeasts, moulds, most vegetative bacteria killed , but not spores and heat resistant bacteria.
o Some enzymes are inactivated. Lipoprotein-lipase is of main concern. Lipolysis can lead to
rancid flavor due to free FA
o No alteration of flavor, almost no denaturation of serum proteins, bacteriostatic properties
of milk remain intact.
- High pasteurization e.g. 20s at 85°C. Effects:
o Inactivation of enzyme lactoperoxydase
o Nearly all vegetative organisms are killed, but not spores
o Most enzymes inactivated, but milk proteinase and bacterial lipases and proteinases are
not.
o Bacteriostatic properties of milk are destroyed, partly denaturation of serum proteins,
cooked flavor, no big changes in nutritional value.
- Sterilization e.g. 30min at 110°C, 1s at 145°C (latter is UHT – ultra high temperature). It also kills
spores. Two treatments:
o Traditional in-bottle sterilization inactivates all milk enzymes but not all bacterial ones. It
causes Maillard reactions (browning), sterilized milk flavor, changes in proteins.
o UHT cannot inactivate all enzymes (plasmin and bacterial enzymes), most serum proteins
are not affected, and only weak cooked flavor. Some thermophilic bacteria might resist.
Methods of heating
Selection of a heating process:
- Time-temperature during heating should be at optimum. Desirable reactions should be obtained
fast, while undesirable reactions should not occur fast.
, - Desirable combination of time and temperature, e.g. short heating times require direct systems,
while low heating rates require indirect systems
- Selection of heating process should take into account properties of the liquid. The main property is
the heat transfer rate which depends on the thermal conductivity and the viscosity. Highly viscous
products or with high fat content have poor heat exchange and have higher fouling rate.
- Combination with other processes may be possible, e.g. homogenization
- Costs should be minimized. A way to reduce them is to use heating processes that allow the
regeneration and reuse of heat.
- Effect of process on the air content of the product should also be considered, especially O2
content because it affects bacterial growth and oxidation (off-flavor).
Traditional equipment:
- Holder pasteurization liquids can be heated and cooled in batch process using relatively low
temperatures. This was generally used for in-bottle pasteurization of consumption milk. There is
usually steam or hot water circulating to heat milk, and cold water to cool it.
o Pros: simple and flexibility
o Cons: warming and cooling take long, difficult regeneration of heat, difficult connection to
other processes, high risk of recontamination.
- Autoclaving liquid is heated in a hermetically sealed container
o Pros: prevents recontamination of the product
o Cons: long warming and cooling times, large temperature differences inside the bottle
(uneven) so possible browning or off-flavor formation. Agitating or rotating the containers
during treatment can help make temperature more homogeneous in the product
Modern equipment:
Indirect systems (indirect contact between product and heating agent, the heating agent heats a surface):
- Plate heat exchangers (mostly used) made of a pack of steel plates into a frame. Each frame
can include different sections where different stages of treatment occur, e.g. preheating, final
heating, and cooling.
o Since there is a large heating surface per L of product to be heated, there is a small
difference in temperature between the heating agent and the product. This leads to less
fouling.
o Warming and cooling is fast.
o Turbulent flow enhances heat transfer and reduces fouling. Countercurrent flow of
product and heating medium ensures max. energy efficiency and min. fouling.
o Heat can be regenerated. When milk enters the heat exchanger, it is heated by the
outgoing milk that is already warm. At the same time the outgoing milk is cooled down by
the incoming milk. Then the incoming milk is further heated with hot water or steam, and
then cooled with cold water.
o In between the different sections of the pasteurizer, the milk can go out for other
treatments such as homogenization and centrifugation, and then go back in.
o There is risk of fractures and pinholes that can cause leakage of raw milk into pasteurized
milk. This can be prevented by increasing pressure, so pasturized milk would leak in raw
milk.
o Plate heaters are unsuitable for heating high viscous products. You would need too high
pressures to increase their speed enough.
- Tubular heat exchangers they have tubes instead of plates to exchange heat. They also have
different sections.
, o there is a smaller heating surface per L of product than for plate exchangers, so there is a
larger difference in temp. between product and heating agent which can increase fouling
to limit fouling and enhance heat transfer, high flow rates are used (high pressure
needed).
o Fouling is less significant than in plate exchangers due to a smaller surface contact area,
which increases the production run time.
o Can be applied for UHT, viscous products, high-fouling products and products with
particles.
- Scraped-surface heat exchangers a cylinder through which the product is pumped in counter
current flow. Rotary mechanical scraper blades are used to remove the product from the surface
and create turbulence to improve heat transfer.
o Applied for viscous products
o Heat damage on the surface might occur because of the large temperature difference
between product and heating surface.
o Regeneration of heat not possible.
Direct systems (direct contact between product and heating agent):
- Steam injection steam at pressure higher than the product is injected into the product (steam in
product). It can cause disruption of fat globules and some protein coagulation; this can be solved
with homogenization.
- Steam infusion product is sprayed through an infusion chamber filled with steam under
pressure at desired temperature (product in steam)
- Direct systems allow to heat the product much faster than indirect systems; this allows to avoid big
chemical changes in the product. However there is less heat generation.
- After heating the product, it enters a vacuum vessel where flash cooling occurs, i.e. milk temp. falls
fast to temperature before heating (70-80). The resulting energy makes some water evaporate; the
quantity of water evaporating should be equal to the amount of steam that was absorbed in the
product.
- The holding time of milk above 80°C is quite short, and insufficient for plasmin inactivation
(proteinase). Therefore holding times are often increased, or holding temperature is increased.
- Direct systems have less fouling problems
Considerations for microbial inactivation
- Heat resistance may vary within one strain
- Heat treatment at short times may increase CFU
- Heat treatment may germinate spores
- Microorganisms are protected against heat in high fat products like cream
- Heat exchangers may form bio-films, and you have to control leakage
PASTEURIZED MILK, STERILIZED MILK AND CREAM
Fresh/pasteurized milk distributed in cold chain; UHT/sterilized milk at ambient temperature.
Pasteurized milk
Objectives
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