How Evaporation Works in the Production of Condensed Milk

Condensed milk – whether sweetened or unsweetened (evaporated) – is one of the most widely consumed concentrated dairy products worldwide. But have you ever wondered how manufacturers turn regular liquid milk into that thick, shelf-stable product? The answer lies in evaporation, a carefully controlled process that removes water from milk while keeping its nutritional value, flavour, and texture intact. Let’s break down exactly how this process works, what equipment is involved, and why every detail matters.

What is evaporation in milk processing? 🔗

In the context of dairy manufacturing, evaporation refers to the removal of water as vapour from liquid milk using heat, without attempting to separate the individual components of that vapour. The goal is to concentrate the remaining milk solids – proteins, fats, lactose, and minerals – into a smaller volume. This concentrated form extends shelf life, reduces transportation costs, and creates products with unique culinary properties.

Evaporation is technically classified as a unit operation involving simultaneous heat and mass transfer. Heat energy (usually from steam) passes through a metallic surface and warms the milk, causing water molecules to escape as vapour. The component where this heat exchange happens is called the calandria or heating element.

Why vacuum is essential 🔗

Here’s the critical part: milk cannot simply be boiled at atmospheric pressure. At sea level, water boils at 100°C – a temperature that would damage heat-sensitive milk proteins, cause browning through the Maillard reaction, and create undesirable cooked flavours. To avoid this, evaporation in condensed milk production is carried out under vacuum.

By reducing the pressure inside the evaporation chamber, water boils at much lower temperatures – typically between 40°C and 70°C. This is the same basic physics you see at high altitudes, where water boils below 100°C due to lower atmospheric pressure. In a vacuum evaporator, this principle is applied deliberately to protect milk quality.

Vacuum evaporation also helps remove trapped air from milk, which improves curd stability and reduces syneresis (liquid separation) during storage. The result is a concentrated product with a clean, fresh taste – free from the off-flavours that high-temperature processing would cause.

The temperature-pressure relationship 🔗

The relationship between pressure and boiling point is governed by well-established thermodynamic principles. In industrial dairy evaporators, the vacuum is maintained at pressures of roughly 160-320 hPa, which corresponds to boiling temperatures of about 55-70°C. This range is low enough to protect milk components but high enough to allow efficient evaporation rates.

How heat transfer works in evaporators 🔗

Efficient heat transfer is the backbone of any evaporation system. In dairy evaporators, heat is typically supplied by condensing steam on one side of a metallic surface (usually stainless steel tubes or plates), while milk flows on the other side. The heat moves through the metal wall by conduction, and then transfers to the milk through convection.

As milk near the heated surface warms up, it becomes less dense and rises. Cooler milk flows in to replace it. These natural circulation patterns help distribute heat evenly. However, the most efficient evaporation occurs when milk is spread as a thin film over the heated surface. A thin film maximises the surface area in contact with heat, minimises the distance heat must travel through the liquid, and reduces the time the milk spends at elevated temperatures.

This thin-film principle is the foundation of modern evaporator design and directly influences the quality of the final product.

Vapour-liquid separation 🔗

Once water evaporates from milk, it needs to be efficiently removed from the system. If the water vapour isn’t separated properly, it can condense back into the product and reduce the concentration level. Two primary separation methods are used:

Centrifugal separation: As the milk-vapour mixture passes through curved paths or rotating chambers, heavier liquid droplets are thrown outward while lighter vapour continues upward. This prevents milk solids from being lost along with the water vapour.

Gravity-based separation: In some designs, large separation chambers allow vapour to rise naturally while the concentrated milk collects at the bottom. Internal baffles and deflectors help prevent liquid droplets from being carried away with the vapour stream.

Effective separation is critical – it determines both the efficiency of the process and the final concentration of the product.

Types of evaporators used in condensed milk production 🔗

Over the decades, several evaporator designs have been developed for dairy processing. Each type has distinct advantages depending on the production scale, product characteristics, and energy requirements.

Falling-film evaporators 🔗

This is the most commonly used evaporator type in the dairy industry today. In a falling-film evaporator, milk is pumped to the top of vertical heated tubes and distributed evenly across them. The milk flows downward as a thin film on the inner surface of the tubes while steam condenses on the outer surface, providing heat.

The key advantage of this design is the very short residence time – the milk spends only seconds in contact with the heated surface. This minimises thermal damage and preserves the nutritional and sensory qualities of the milk. Tube lengths in these systems can extend up to 20 metres. Falling-film evaporators handle low-to-moderate viscosity liquids exceptionally well and are the standard choice for concentrating dairy products like whey, skim milk, cream, and whole milk.

Rising-film evaporators 🔗

In a rising-film (or climbing-film) evaporator, the liquid enters at the bottom of vertical heated tubes. As the milk heats up, vapour bubbles form and expand, creating a lifting effect that pushes the liquid film upward through the tubes. This rising action produces good heat transfer because the thin film allows rapid heat penetration.

However, rising-film evaporators have a notable drawback: the hydrostatic head (the weight of the liquid column) at the bottom of the tubes raises the boiling point, which can cause temperature-sensitivity issues. They also tend to have longer residence times – around 15 to 30 minutes – and often require recirculation to achieve the desired concentration. For these reasons, falling-film evaporators have largely replaced rising-film designs in modern dairy plants.

Forced-circulation evaporators 🔗

When milk reaches higher concentration levels and becomes viscous, natural circulation may not be sufficient. Forced-circulation evaporators use pumps to push the liquid through the heating elements at high velocities – typically between 1.2 and 3 m/s. This prevents fouling (build-up of deposits on heated surfaces) and ensures uniform heating even with thick, concentrated products.

In many dairy operations, forced-circulation evaporators serve as finishers – they handle the final stage of concentration after falling-film evaporators have done the bulk of the water removal. For example, skim milk might be pre-concentrated to 40% solids in a falling-film unit and then brought to its final concentration of around 50% solids in a forced-circulation finisher.

Circulation (vacuum pan) evaporators 🔗

Circulation evaporators are used when only a low degree of concentration is needed or when processing small batch volumes. The milk enters a vacuum chamber tangentially at high velocity, forms a thin rotating layer on the wall surface, and loses some of its water content. These systems are used, for example, in yoghurt production to concentrate milk from 13% to about 14.5-16% solids. They also serve a secondary function of deaeration, removing off-flavours from the product.

Multiple-effect evaporation: the key to energy efficiency 🔗

Evaporation requires a great deal of energy – converting liquid water to vapour is inherently energy-intensive. In a single-effect evaporator, one kilogram of steam removes roughly one kilogram of water from the product. For large-scale condensed milk production, this would be extremely costly.

The solution is the multiple-effect evaporator (MEE). In an MEE system, vapour generated in one stage is used as the heating medium for the next stage, which operates at a lower pressure (and therefore a lower boiling point). This cascading reuse of energy dramatically reduces steam consumption.

Modern dairy plants typically use four to seven effects. A six-effect evaporator with a thermocompressor can evaporate up to 12 kg of water per kg of steam consumed – a remarkable level of efficiency. In comparison, a single-effect system uses one kg of steam per kg of water removed. A four-effect evaporator can save roughly 25% of energy compared to a single-effect unit, with even greater savings as more effects are added.

Thermal vapour recompression (TVR) 🔗

To further improve energy efficiency, many dairy evaporators are equipped with thermocompressors. A thermocompressor uses high-pressure steam (600-1,000 kPa) to compress and reuse some of the vapour generated during evaporation. The resulting mixture of steam and vapour is at a higher temperature and pressure, making it suitable as a heating medium again. This technology, when combined with a multiple-effect system, optimises the overall energy balance significantly.

Mechanical vapour recompression (MVR) 🔗

MVR takes a different approach. Instead of using steam, it uses an electrically driven compressor (fan) to compress all the vapour from an evaporator effect, raising its temperature by 3-7°C so it can be returned to the heating side. During normal production, an MVR system requires little to no additional thermal energy – some systems achieve up to 98% steam savings. MVR is particularly popular in pre-concentrators, where milk and whey are concentrated to 32-40% solids.

Achieving the right concentration 🔗

The target concentration depends on the product being made. For evaporated milk (unsweetened condensed milk), the milk is typically concentrated to 30-40% total solids. The U.S. FDA requires evaporated milk to contain at least 6.5% milk fat, 16.5% non-fat milk solids, and 23% total milk solids. For sweetened condensed milk, the requirements are at least 28% total milk solids and 8% milk fat by weight, with a sugar ratio of 62.5-64.5% to ensure preservation without excessive crystallisation.

India’s Food Safety and Standards Act (FSSA), 2006, also sets legal standards for the composition of condensed and evaporated milks manufactured in the country.

How far the concentration can be pushed depends on milk properties – particularly viscosity and heat sensitivity. Skim milk can typically be concentrated to around 50% solids, while whole milk can reach approximately 52%.

Quality control during evaporation 🔗

Maintaining product quality throughout the evaporation process requires careful attention to several factors:

Temperature control: Evaporation temperatures should not exceed 65-70°C. Below 65°C, heat-resistant bacteria and spores can thrive, while temperatures above 70°C risk thermal damage to milk proteins. This narrow operating window must be maintained consistently.

Raw material quality: The initial quality of the incoming milk directly affects the final product. Milk should be fresh, free from off-flavours, and have low bacterial counts. Seasonal variations in milk composition, feed quality, and stage of lactation can all affect how milk responds to heat treatment.

Heat stability: The ability of milk to withstand intensive heat processing depends on its acidity and mineral salt balance. Stabilising salts such as disodium hydrogen phosphate are often added to adjust the heat stability of the concentrate and prevent coagulation during subsequent sterilisation steps.

Fouling prevention: Over time, milk proteins and minerals can deposit on heated surfaces, reducing heat transfer efficiency. Regular cleaning-in-place (CIP) procedures and proper evaporator design (such as high liquid velocities in forced-circulation systems) help manage this challenge.

Modern advances in evaporation technology 🔗

Evaporation technology continues to evolve. Some notable recent developments include:

Advanced tube coatings: Hydrophilic and hydrophobic coatings on evaporator tubes can increase heat transfer efficiency by 15-25% and reduce fouling by over 40%, extending the time between cleaning cycles.

AI-driven process control: Artificial intelligence and machine learning algorithms are being used to dynamically adjust evaporator parameters in real time. These systems can reduce energy waste by 20-30% and detect process anomalies before they cause equipment failures.

Inline sensors: Near-infrared (NIR) spectroscopy can monitor milk solids concentration in real time during evaporation, keeping total solids within the target range of 45-55% for consistent product quality.

Hybrid evaporators: Combining features of film-type and forced-circulation units, hybrid vacuum evaporators are recognised as the most energy-efficient option for many dairy applications.

Why understanding evaporation matters 🔗

Evaporation is not just a step in a manufacturing process – it’s the step that defines condensed milk as a product. Every decision made during evaporation – the vacuum level, the evaporator type, the number of effects, the concentration target – directly shapes the taste, texture, nutritional value, and shelf life of the final product. For dairy science students and professionals, a solid understanding of evaporation principles is essential for optimising production, reducing costs, and ensuring consistent quality.

What do you think? Given the rising energy costs and growing emphasis on sustainability, how might future evaporator designs further reduce energy consumption in the dairy industry? And could emerging technologies like AI-driven controls eventually make the evaporation process fully automated?

References

1. https://ebooks.inflibnet.ac.in/ftp04/chapter/technology-of-concentrated-milk-products-evaporated-milk-sweetened-condensed-milk/
2. https://dairyprocessinghandbook.tetrapak.com/chapter/evaporators
3. https://foodtechprocess.com/blog/post/46_vacuum-evaporation-and-concentration-of-liquid-products
4. https://www.lcicorp.com/en/Knowledge-Center/selecting-evaporators-for-process-applications
5. https://www.andritz.com/separation-en/insights/evaporators-guide
6. https://link.springer.com/article/10.1007/s12393-023-09350-6
7. https://www.raygmindia.com/multiple-effect-evaporators-in-food-processing.php
8. https://www.cft-group.com/technology/evaporators/
9. https://www.gemak.co.uk/dairy-science/evaporated-condensed-milk
10. https://invexoil.com/evaporator-innovations/