Dairy and Food Engineering

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This section covers thermodynamic cycles for refrigeration, refrigeration system types, the functional requirements of cold storage for different perishable products, cooling load calculation, and classification of refrigerants.

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Thermodynamic Cycles for Refrigeration The vapour compression cycle is the most common refrigeration cycle, consisting of four main processes: compression (compressor raises refrigerant pressure/temperature as a superheated vapour), condensation (heat rejected to ambient/cooling medium as vapour condenses to liquid at high pressure), expansion (throttling through an expansion valve/capillary, reducing pressure/temperature), and evaporation (refrigerant absorbs heat from the cold space, vaporizing at low pressure) — the cycle repeating continuously.

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The vapour absorption cycle replaces the mechanical compressor with a thermally-driven absorption/generation process (using a refrigerant-absorbent pair, e.g. ammonia-water or water-lithium bromide), useful where waste heat or an alternative heat source (rather than electricity/mechanical work) is available to drive the cycle.

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Refrigeration System Types Vapour compression systems: the dominant system type for most agricultural/food cold storage applications, using an electrically driven compressor; simpler control, widely available equipment and spares.

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Vapour absorption systems: less common for typical farm-scale cold storage, but relevant where heat sources like biogas, solar thermal, or waste heat are available in place of reliable electricity.

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Functional Requirements and Cooling Load Functional requirements for cold storage vary by product: different perishables require specific temperature and relative humidity ranges to maximize storage life while minimizing quality loss (chilling injury for some products if too cold, excessive moisture loss/wilting if too dry, condensation/decay if too humid); adequate air circulation is needed for uniform temperature distribution.

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Calculation of cooling load for a cold store sums:

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(1) product load (heat to be removed from the product to bring it to storage temperature, plus its respiration heat if it is a living, respiring commodity such as fresh fruit/vegetables), (2) transmission load (heat gain through walls/roof/floor by conduction, based on insulation U-value, area, and temperature difference), (3) infiltration/air-change load (heat gain from outside air entering via door openings/leakage), and (4) internal loads (lighting, fan motors, personnel, defrost heat) — the compressor/refrigeration system is then sized to remove this total load within the desired pull-down time.

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Principle and design of a cold storage system includes selecting appropriate insulation thickness (to limit transmission load and condensation risk), evaporator/condenser sizing matched to the calculated load, and a control system maintaining the required temperature/humidity set-points.

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Classification of Refrigerants Refrigerant Class Characteristics CFCs (chlorofluorocarbons) Historically common (e.g.

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R-12), but ozone-depleting; now largely phased out under international agreements HCFCs (hydrochlorofluorocarbons) Lower ozone-depletion potential than CFCs but still ozone-depleting (e.g.

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R-22); being phased out HFCs (hydrofluorocarbons) Non-ozone-depleting (e.g.

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R-134a), but many have high global warming potential (GWP); increasingly regulated/phased down Natural/environment-friendly refrigerants Ammonia (NH3, efficient, toxic/flammable at high concentration, common in large industrial systems), CO2 (non-toxic, non-flammable, but requires high operating pressure), and hydrocarbons (e.g. propane/isobutane, efficient, flammable, used in small systems) — increasingly favored for low ozone-depletion and low global warming potential

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This section covers the use of engineering properties of bio-materials in agricultural processing design, and the concept, objectives, methods, and need of quality control in food industries.

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Engineering Properties of Bio-materials in Processing Design Property Category Relevance to Processing Equipment Design Physical properties Size, shape, volume, density, and surface area of grains/produce — govern sizing of hoppers, conveyors, sieves/graders, and packaging Mechanical properties Hardness, compressive/shear strength, coefficient of friction, and rupture/crushing force — govern design of milling/crushing/cutting equipment and determine safe handling limits to avoid mechanical damage/bruising Thermal properties Specific heat, thermal conductivity, and thermal diffusivity — govern design of drying, heating, cooling, and refrigeration equipment (rate of heat transfer into/out of the product) Rheological properties Flow behaviour of semi-solid/liquid food materials (viscosity, consistency) — relevant to pumping, mixing, and processing equipment design for products like milk, juice, and pastes Aerodynamic/frictional properties Terminal velocity and drag characteristics of grains/seeds — relevant to pneumatic conveying and cleaning/grading equipment (e.g. air-screen cleaners) design Understanding these engineering properties of bio-materials (which vary by crop, variety, moisture content, and maturity) is essential for correctly sizing and designing equipment that handles, processes, or stores agricultural produce, ensuring efficient operation while minimizing product damage.

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Quality Control in Food Industries Concept: quality control (QC) is the systematic set of activities used to ensure that food products consistently meet defined quality and safety standards, spanning raw material inspection, in-process monitoring, and finished product testing.

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Objectives: ensuring food safety (preventing contamination/adulteration), maintaining consistent product quality (taste, texture, appearance, nutritional value), meeting regulatory/standard requirements, and protecting consumer health and brand/producer reputation.

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Methods: physical/sensory inspection, chemical analysis (composition, contaminant testing), microbiological testing (pathogen/spoilage organism detection), and systematic quality management frameworks such as HACCP (Hazard Analysis and Critical Control Points) — identifying and controlling critical points in the production process where food safety hazards could occur.

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Need: protects public health, ensures compliance with national/international food standards (important for both domestic consumer protection and export market access), reduces spoilage/wastage, and builds consumer trust and market competitiveness.

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This section covers grain drying needs, methods, and theory, grain pressure theory, types of dryers and their performance/energy utilization, seed processing equipment, grain storage structures, and moisture content determination.

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Needs, Methods and Theory Need for drying: freshly harvested grain typically has moisture content too high for safe long-term storage; drying reduces moisture to a safe storage level, preventing mould/fungal growth, insect infestation, biochemical deterioration, and germination during storage.

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Drying theory: drying proceeds through moisture movement from the interior of the grain to its surface (governed by internal diffusion) and evaporation of moisture from the surface into the surrounding air (governed by the surrounding air's temperature, humidity, and velocity); drying rate typically shows an initial constant-rate period (surface moisture removal limited by external conditions) followed by a falling-rate period (removal limited by internal moisture diffusion) as drying proceeds.

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Equilibrium moisture content (EMC): the moisture content a grain reaches when in equilibrium with the surrounding air's temperature and relative humidity (net moisture exchange becomes zero) — drying cannot proceed below the EMC for the given air conditions.

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Grain Pressure Theory Grain pressure theory describes the lateral and vertical pressures exerted by stored bulk grain on the walls and floor of a storage structure (bin/silo), important for structural design of storage structures; unlike a liquid, grain pressure does not increase linearly and indefinitely with depth — instead, due to internal friction between grain particles and friction against the bin wall, the pressure increase tapers off with depth (some of the vertical load being transferred to the walls via friction, described classically by relationships such as Janssen's theory for granular material pressure in bins).

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Types of Dryers, Performance and Energy Utilization Dryer Type Description Sun/natural drying Grain spread in thin layers under open sun; low cost, but slow, weather-dependent, and prone to contamination/quality loss Batch dryer (bin/box) A fixed batch of grain is dried at a time by forcing heated (or ambient) air through it; simple, moderate cost, suited to small/medium scale Continuous-flow dryer Grain flows continuously through the dryer (e.g. gravity or mechanically moved) while heated air passes through, suited to larger, higher-throughput operations Fluidized-bed dryer Air velocity suspends (fluidizes) grain particles, giving very rapid, uniform heat/mass transfer; higher cost/complexity, used for specialized high-capacity applications Dryer performance is evaluated by drying rate/capacity, uniformity of final moisture content across the batch/flow, and energy consumption per unit of moisture removed; energy utilization is improved by proper insulation, air recirculation where feasible, and matching airflow/temperature to the grain's actual drying requirement (avoiding over-drying or excessive temperatures that could cause grain damage/cracking).

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Seed Processing Equipment, Grain Storage, and Moisture Determination Seed processing equipment: cleaners (removing chaff/foreign material, often air-screen type), graders (sorting by size/density/shape for uniformity), and seed treaters (applying protective treatments) — important for producing high-quality planting seed.

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Grain storage structures: range from traditional structures (mud bins, bamboo/wooden structures) to modern metal/concrete silos; importance lies in protecting stored grain from moisture ingress, pests (rodents/insects), and mechanical damage, preserving both quantity (reducing storage losses) and quality (viability, food safety) of the stored grain.

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Moisture content determination: the standard reference method is the oven-drying (gravimetric) method (comparing wet and dry weight of a sample); rapid field/commercial methods use electronic moisture meters (based on electrical resistance or dielectric/capacitance properties of the grain, calibrated against the oven method).

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This section covers milling, hulling, and expelling processes and their equipment for major crops — rice, wheat, maize, sugarcane, legumes, and oilseeds.

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Rice Milling Rice milling removes the husk (outer covering) and bran layers from paddy (rough rice) to produce edible milled (white) rice, typically involving: husking/dehusking (rubber-roll sheller or under-runner disc sheller separates husk from brown rice), separation (paddy separator removes any unhusked paddy remaining after the husker), whitening/polishing (abrasive or friction-type whitener removes the bran layer to produce white rice, with a polisher further improving surface finish), and grading/sorting (by size, removing broken grains).

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Milling recovery (the percentage of milled rice obtained from paddy) and the proportion of broken grains are key performance/quality indicators, influenced by paddy variety, moisture content at milling, and machine settings.

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Wheat and Maize Milling Wheat milling (flour milling): wheat grain is cleaned, conditioned (tempering — adding water and resting to toughen bran/soften endosperm), then passed through a series of roller mills (break rolls to open the grain and separate bran/germ from endosperm, followed by reduction rolls to grind endosperm into flour) with sieving (sifters) between stages to separate flour, semolina/middlings, and bran/germ fractions.

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Maize milling: can be dry-milled (producing maize flour/grits/meal via degermination, grinding, and sieving, without adding water) or wet-milled (steeping in water to soften the kernel before separating starch, germ, fibre, and gluten fractions, mainly for industrial starch/oil production).

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Sugarcane, Legume, and Oilseed Processing Sugarcane processing: cane is crushed/milled between heavy rollers (or by diffusion) to extract juice, which is then clarified, evaporated, and crystallized to produce raw/refined sugar (or, for gur/jaggery production, simply boiled and concentrated without full refining).

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Legume (pulse) milling (dehulling): pulses are typically pre-treated (e.g. with oil/water conditioning) to loosen the seed coat (husk), then dehulled (removing the husk) and split into the two cotyledons (dal), followed by polishing and grading.

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Oilseed expelling: oilseeds are typically cleaned, sometimes pre-heated/cooked (to improve oil release and expeller efficiency), then passed through a mechanical screw expeller (ghani/oil press), which applies high pressure to squeeze oil from the seed, with the residual solid material (oil cake) recovered as a valuable byproduct (animal feed/fertilizer); solvent extraction (using a chemical solvent to extract residual oil from the cake) may follow for higher oil recovery in larger-scale industrial processing.

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This section covers the working principle and unit operations of equipment for receiving, pasteurization, sterilization, homogenization, filling, and packaging of dairy products, and estimation of refrigeration size requirements in dairy and food products.

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Milk Receiving and Initial Processing Milk receiving: incoming milk is weighed/measured, sampled for quality testing (fat content, adulteration, microbial load), and typically cooled/chilled promptly to slow bacterial growth if not processed immediately; clarification/filtration removes physical debris/sediment before further processing.

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Pasteurization, Sterilization and Homogenization Process Working Principle Pasteurization Milk is heated to a specific temperature for a defined holding time (e.g. common combinations such as a lower temperature for a longer time, or a higher temperature for a much shorter time) sufficient to destroy pathogenic microorganisms and most spoilage organisms, then rapidly cooled — extends shelf life while retaining most nutritional/sensory qualities (does not achieve sterility) Sterilization (UHT) Milk is heated to a much higher temperature for a very short time (ultra-high temperature treatment) sufficient to destroy essentially all microorganisms including heat-resistant spores, enabling long shelf-life storage at ambient temperature when packaged aseptically Homogenization Milk is forced under high pressure through a narrow valve/orifice, breaking fat globules into much smaller, uniformly sized particles that remain evenly dispersed rather than separating/creaming to the top, giving a more uniform texture/appearance Filling and Packaging Filling and packaging equipment dispenses processed milk/dairy products into consumer or bulk packaging (pouches, bottles, cartons, tetra-packs) under hygienic (often aseptic, for long-life products) conditions, with accurate volume/weight filling and secure sealing to maintain product safety and shelf life.

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Estimation of Refrigeration Size Requirement Estimating the refrigeration capacity needed for dairy/food processing and storage involves calculating the total cooling load: the product cooling load (heat to be removed to bring milk/product from receiving temperature down to storage/processing temperature, based on the product's specific heat and mass flow rate), transmission load through insulated surfaces, and other loads (equipment heat, personnel, air infiltration) similar in principle to the cold storage cooling load calculation (see Section 8.1), sized to meet the required cooling within the process's time constraints (e.g. rapid milk chilling immediately after receiving).

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This final section of the chapter covers the economic aspects of food processing across farm crops, animal products, fruits and vegetables; material handling systems and devices; and freezing, freeze-drying, membrane separation, and irradiation processes.

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Economic Aspects of Food Processing Economic evaluation of food processing (for farm crops, animal products, fruits and vegetables) weighs the added value from processing (extended shelf life, higher market price, reduced post-harvest losses, new product forms) against the capital and operating costs of processing equipment/facilities, considering the scale of operation, raw material availability/seasonality, and market demand for processed versus fresh products.

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Processing can substantially reduce post-harvest losses (which are often high for perishable fruits/vegetables/animal products in the absence of adequate processing/storage infrastructure), improving farmer income and food security.

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Material Handling Systems and Devices System / Device Typical Use Belt conveyor Continuous horizontal/inclined transport of bulk or packaged materials over moderate distances Screw/auger conveyor Transport of bulk granular/powdery materials (grain, flour) within an enclosed trough, also useful for metered/controlled feeding Bucket elevator Vertical lifting of bulk granular materials (grain) using buckets mounted on a moving belt/chain Pneumatic conveying Transport of grain/powder using air flow through ducting, suited to dusty/fine materials and flexible routing Forklifts/pallet trucks Movement of palletized/packaged goods within a processing facility or warehouse Freezing and Freezing Equipment Freezing preserves food by lowering its temperature well below freezing point, converting most of the water content to ice, which greatly slows microbial growth and enzymatic/chemical deterioration; freezing equipment includes plate freezers (product pressed between refrigerated plates), blast freezers (rapid freezing via high-velocity cold air), and cryogenic freezers (using liquid nitrogen/CO2 for very rapid freezing) — faster freezing generally produces smaller ice crystals, causing less cellular damage and better product quality upon thawing.

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Freeze Drying, Membrane Separation and Irradiation Freeze drying (lyophilization): food is first frozen, then subjected to a vacuum, allowing the frozen water to sublime directly from solid ice to vapour without passing through the liquid state — produces a dried product with excellent retention of shape, structure, nutritional value, and flavor compared to conventional heat drying, though at a higher processing cost.

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Membrane separation (e.g. ultrafiltration, reverse osmosis): uses semi-permeable membranes to separate components of a liquid food (e.g. concentrating milk proteins, clarifying/concentrating juice) based on molecular size, without applying heat — preserving heat-sensitive nutritional/flavor qualities.

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Irradiation: exposes food to controlled doses of ionizing radiation (e.g. gamma rays) to destroy microorganisms/pests/pathogens and extend shelf life, without significantly raising the product's temperature (a 'cold' preservation process); subject to specific regulatory approval and labeling requirements in most countries due to consumer perception/safety considerations, even though the process itself does not make food radioactive.