L9: DSCI 229-Drying

Concentration and Drying of Dairy Products

Overview of Concentration and Drying

• Concept Introduction: This lecture delves into the fundamental principles and applications of concentration and drying processes in dairy products. These methods are crucial for reducing water content, thereby extending shelf life, decreasing transportation costs, and creating new product forms. The focus is on evaporative techniques and their resulting products, highlighting the transformation of liquid milk into stable, commercially viable forms.

• Key Product Examples: The discussion will primarily focus on two significant concentrated dairy products:

  • Condensed milk: This product typically contains added sugar (sucrose) and is characterized by its thick consistency and extended shelf life, mainly due to the high sugar content.

  • Unsweetened condensed milk: In the U.S., this is commonly known as evaporated milk. It undergoes heat sterilization after concentration, which provides its stability without the addition of sugar.

Evaporation Process

• Evaporation Definition: Evaporation is a thermal process designed to remove a significant portion of water from liquid milk, typically 50-60%, to produce evaporated milk. This process increases the total solids content, leading to a more concentrated product.

• Typical Steps Involved

  1. Standardization: This initial step involves adjusting the fat and solids-not-fat (SNF) ratio to meet specific product specifications, ensuring consistent composition. For instance, the fat content might be standardized to $(7.9\%)$ to $(8.2\%)$.

  2. Preheating: Milk is quickly heated to activate enzymes and reduce microbial load. Common temperatures range from $90-95^\circ\text{C}$ for $5-10$ minutes. This step also prevents heat coagulation during subsequent sterilization and helps in flavor development.

  3. Evaporation: Milk is concentrated under vacuum conditions in multi-effect evaporators. Operating under vacuum lowers the boiling point of water, reducing energy consumption and minimizing heat damage to the milk components. Water is removed until the total solids reach about $(25-30\%)$.

  4. Homogenization: After evaporation, the concentrated milk is homogenized at high pressure ($10-25$ MPa or $1500-3600$ psi) to reduce the size of fat globules ($0.5-1.0$ micron range), preventing fat separation (creaming) during storage. This results in a smoother, more uniform product.

  5. Final Standardization: The concentrated milk is adjusted precisely to the target total solids content, often involving the addition of water or milk solids if necessary.

  6. Fortification: Essential nutrients, such as Vitamin D, are often added at this stage to enhance the nutritional value of the product as evaporated milk is often used as a baby food ingredient.

  7. Sterilization: This critical thermal processing step ensures commercial sterility of the product, eliminating pathogenic and spoilage microorganisms, thereby enabling long-term shelf stability without refrigeration.

Sterilization

• Importance of Sterilization: Sterilization is paramount for extending the shelf life of evaporated milk to approximately one year or even longer under proper storage conditions. It renders the product shelf-stable at ambient temperatures.

• Canning Process: The concentrated and standardized evaporated milk is typically filled hot into pre-sterilized metal cans and then hermetically sealed.

• Retorting: The sealed cans filled with evaporated milk are subjected to high-temperature, high-pressure processing in specialized equipment called retorts (industrial pressure cookers). Conditions typically involve heating to temperatures around $120-121^\circ\text{C}$ ( $248-250^\circ\text{F}$ ) for $10-20$ minutes. This intense heat treatment effectively destroys spores of thermophilic bacteria, ensuring food safety and preventing spoilage, surpassing the boiling point of water at atmospheric pressure. An alternative, especially for larger batches or different packaging, is aseptic processing, where the product and packaging are sterilized separately and then filled in a sterile environment.

Condensed Milk

• Composition: Sweetened condensed milk is primarily characterized by the addition of sucrose, which acts as both a sweetener and a primary preservative. The typical composition in the U.S. includes approximately $45\%$ sucrose, along with about $28\%$ milk solids and $27\%$ water.

• Sugar Concentration:

  • The high sugar content is crucial for preservation. On an aqueous basis, about $167$ grams of sucrose is dissolved for every $100$ grams of water, leading to a very high $62.5\%$ sugar concentration in the aqueous phase. This high osmotic pressure is the main mechanism of preservation.

• Effect on Bacterial Growth:

  • The exceptionally high sugar concentration significantly lowers the water activity ($A_w$) of the product to approximately $0.75-0.76$. This reduced water availability effectively inhibits the growth of most spoilage bacteria, yeasts, and molds, making the product shelf-stable without further sterilization or refrigeration once opened.

• Crystallization Challenges: One significant challenge in the production and storage of sweetened condensed milk is the potential for lactose crystallization. Lactose, a natural sugar in milk, becomes supersaturated during concentration. If not properly controlled, lactose can crystallize into large, gritty structures, leading to an unpleasant sandy or coarse texture that is highly undesirable for consumers.

  • Desired crystal size: To prevent textural defects, the goal is to form very fine lactose crystals, typically below $10$ microns in diameter, which remain imperceptible to the palate.

• Crystallization Control Methods: The formation of small, desirable lactose crystals is achieved through specific post-evaporation processing steps:

  • Rapid cooling: Immediately after evaporation and sugar addition, the condensed milk is rapidly cooled from, for example, $60^\circ\text{C}$ to $15^\circ\text{C}$.

  • Vigorous agitation: During rapid cooling, the product is continuously and vigorously agitated. This agitation, combined with controlled cooling, promotes the rapid nucleation of many small lactose crystals while minimizing the growth of individual crystals, ensuring a smooth texture. Often, a small amount of finely ground lactose "seed" is added to further promote controlled crystallization.

Packaging Innovations

• Modern packaging for condensed milk, such as flexible pouches or tubes resembling a toothpaste tube, has been introduced to enhance convenience, ease of dispensing, and minimize waste compared to traditional cans.

Water Activity ($A_w$)

• Definition: Water activity ($A<em>w$) is a critical thermodynamic parameter that quantifies the availability of free (unbound) water in a product for chemical reactions, enzyme activity, and microbial growth. It is defined as the ratio of the vapor pressure of water in a food system to the vapor pressure of pure water at the same temperature ($P/P</em>0$).

  • $A_w = 1.0$: Indicates pure or freely available water (e.g., fresh milk, fruits), supporting rapid microbial growth.

  • Lower $A_w$ values: Indicate reduced water availability due to binding with solutes (like sugars, salts) or physical adsorption.

  • Practical Threshold: Maintaining $A<em>w$ below $0.8$ (or more specifically, $0.75$ for many dairy products) is generally sufficient to effectively halt the growth of most pathogenic bacteria and many spoilage microorganisms, thereby preserving the product. For example, most bacteria cease growth below $A</em>w$ $0.91$, yeasts below $A<em>w$ $0.88$, and molds below $A</em>w$ $0.80$.

Drying Methods in Dairy Processing

• Drying Definition: Drying is a mass transfer process involving the removal of moisture from products through evaporation, resulting in a solid form. In dairy, this transforms liquid milk into highly stable products like milk powder, which typically contains a very low moisture content of about $1.5-5\%$. This low moisture content significantly prolongs shelf life, reduces weight and volume for storage and transport, and allows for versatile applications.

• Drying Methods: The primary industrial techniques for milk drying include spray drying and roller drying. Freeze drying is another method, but it is typically reserved for high-value products due to its higher cost and energy consumption, mainly used when maximum nutrient and flavor retention is critical.

Freeze Drying

• Process Overview: Freeze drying (lyophilization) is a gentle drying process that preserves the integrity of heat-sensitive compounds. It involves three main steps:

  1. Freezing: The product is first frozen to a very low temperature (e.g., $-30^\circ\text{C}$ to $-50^\circ\text{C}$) to solidify all the water.

  2. Primary Drying (Sublimation): Under high vacuum and with carefully controlled heat, the ice directly sublimates into water vapor, bypassing the liquid phase.

  3. Secondary Drying (Desorption): Any remaining unfrozen water molecules (bound water) are removed at slightly higher temperatures under vacuum.

  • Result: This process produces a highly porous product with excellent reconstitution properties and superior retention of heat-sensitive nutrients, flavors, and biological activity. It is commonly used for preserving delicate items like astronaut ice cream, instant coffee, and camping foods, as well as pharmaceuticals.

Roller Drying

• Process Description: Roller drying, also known as drum drying, involves applying a thin film of concentrated milk onto the surface of large, internally steam-heated rotating drums.

  • Mechanism: The drums, heated to temperatures around $150^\circ\text{C}$ to $170^\circ\text{C}$, rapidly transfer heat to the milk film. Water rapidly flashes off as steam within a few seconds. The dried milk forms a thin sheet on the drum surface, which is then continuously scraped off by knives.

  • Result: The scraped product typically forms flaky milk powder or shards. While effective, the high direct heat contact results in greater protein denaturation, leading to altered functional properties (e.g., reduced solubility) and a more cooked or caramelized flavor which is common in products like orange juice concentrate where it provides a specific flavor profile. Due to the high heat, roller-dried milk powder is less soluble than spray-dried variants.

Spray Drying

• Process Steps: Spray drying is the most common method for producing milk powder due to its efficiency and ability to produce high-quality, soluble powders. It involves several key stages:

  1. Atomization: Concentrated milk (typically $40-50\%$ solids) is fed into an atomizer, which disintegrates the liquid into extremely fine droplets (e.g., $50-200$ microns in diameter). Common atomizers include rotary atomizers (spinning disc) or **pressure nozzles).

  2. Drying Chamber: These fine droplets are immediately introduced into a large drying chamber, where they come into contact with a stream of hot air (e.g., Inlet air temperatures: $180-220^\circ\text{C}$) for a very short duration (a few seconds). Water rapidly evaporates from the surface of each droplet.

  3. Powder Collection: As the moisture evaporates, the droplets transform into solid powder particles. The powdered milk is then separated from the spent moist air, typically using cyclones (centrifugal separators that collect heavier powder particles) and sometimes bag filters or scrubbers (for fine particle capture and air purification, respectively).

• Single Stage Design for Basic Powder Production: In a basic single-stage spray dryer, the milk is atomized into a hot air chamber. Most of the drying occurs here, and the powder is collected from the bottom of the chamber and cyclones. This yields a fine, non-instant powder.

  • Benefits: Produces a fine, spherical milk powder with good solubility, generally less protein denaturation compared to roller drying due to shorter drying times and lower product temperatures.

Multi-Stage Design for Instant Powder Creation

• Instantization Process: To improve the rehydration properties of milk powder, multi-stage spray dryers are employed to create "instant" powder. This involves an additional drying and agglomeration step, usually in a fluidized bed dryer, integrated into or post-spray drying.

• Agglomeration Mechanics:

  1. Fine powder particles exiting the primary spray dryer stage are routed to a fluidized bed.

  2. In the fluidized bed, these fine particles are wetted (e.g., with steam or water spray) just enough to make their surfaces sticky.

  3. As the particles collide, they stick together, forming larger, porous aggregates or agglomerates.

  4. These agglomerates are then gently dried in the same fluidized bed with cooler air.

  • Result: The agglomerated powder has a larger particle size, reduced bulk density, and crucially, an internal porous structure with capillary channels. These channels allow water to penetrate rapidly and uniformly into the particle matrix upon rehydration, making the powder "instant" (dissolving quickly and completely without clumping).

Visual Comparisons of Powders

• The distinct manufacturing methods lead to visually different products:

  • Flaky milk powder: Characterized by irregular, flat, or shard-like particles, typically resulting from roller drying.

  • Spherical milk powder: Uniform, often hollow, globular particles, characteristic of spray drying. This visual difference helps in identifying the drying process used.

Transition to Other Dairy Products

• The discussion will transition to other dairy products, specifically focusing on butter. This will include examining the regulatory definitions of butter, which often specify minimum fat content (e.g., $80\%$ milk fat in the U.S.), and its general composition.

Humor and Engagement

• Teacher shares a joke: "Milk is the fastest liquid on Earth because it past your eyes before you even see it!" - A playful pun related to the concept of pasteurization (past-your-eyes), adding a light moment to the lecture.

Review Questions

• True/False Question: Roller drying produces flaky milk powders – True. This is due to the scraping action that removes the dried film from the drum.

• Fill in the Blank: The step in which milk is finely dispersed into droplets in spray drying is called Atomization. This is the initial and crucial step for efficient drying.

• Equipment for Instantization: A Fluidized bed dryer is the key equipment used to transform fine spray-dried powder into instant agglomerated powder.

Conclusion

• Final discussions centered on the characteristics and regulatory definitions of butter. This includes its composition, legal requirements for labeling (e.g., minimum milk fat content and absence of non-dairy fats), and its overall significance as a dairy product.