CHE 564: Food Process Engineering
Estimated study time: 10 minutes
Table of contents
Sources and References
Primary texts — Singh, R.P. and Heldman, D.R., Introduction to Food Engineering, 5th ed., Academic Press, 2014; Fellows, P.J., Food Processing Technology: Principles and Practice, 4th ed., Woodhead, 2017.
Supplementary texts — Toledo, R.T., Fundamentals of Food Process Engineering, 3rd ed., Springer, 2007; Saravacos, G.D. and Maroulis, Z.B., Food Process Engineering Operations, CRC Press, 2011.
Online resources — MIT OCW 10.40/10.50 process engineering materials applied to food case studies; FAO technical manuals; USDA FSIS public documents; Codex Alimentarius food safety standards; IFT open-access reviews.
Chapter 1: Characteristics of Food Systems
1.1 Food as a Process Substrate
Foods are heterogeneous, multiphase, biologically active materials. Composition spans water (1–95%), proteins, carbohydrates, lipids, minerals, vitamins, flavor and color compounds. Structure ranges from dilute liquids (milk, juice) through concentrated dispersions (yogurt, ketchup) to solid matrices (bread, cheese, meat). Processing must preserve nutritional value, sensory quality, and safety while delivering shelf life and convenience.
1.2 Unit Operations in Food
Thermal: pasteurization, sterilization, blanching, cooking, evaporation, drying, freezing, thawing. Mechanical: size reduction, mixing, separation, extrusion, forming. Mass transfer: extraction, membrane processing, crystallization. Biochemical: fermentation, enzyme reactions. Many unit operations serve dual roles (e.g., drying for preservation and structure).
1.3 Quality and Safety Constraints
Food engineering operates within tight constraints: pathogen reduction (5-log or 6-log decimal reduction), preservation of heat-labile vitamins (C, B1, folate), retention of sensory attributes (color, flavor, texture), and allergen/contaminant control. Every process is the optimization of these sometimes-competing goals subject to capital, energy, and water budgets.
Chapter 2: Thermal Processing
2.1 Microbial Inactivation Kinetics
Thermal destruction of microorganisms follows approximately first-order kinetics:
\[ \log\frac{N}{N_0} = -\frac{t}{D(T)}, \]where \( D(T) \) is the decimal reduction time. Temperature dependence:
\[ \log\frac{D(T_1)}{D(T_2)} = \frac{T_2 - T_1}{z}, \]with \( z \) typically 10 °C for mesophilic bacterial spores. The reference lethality:
\[ F_0 = \int_0^t 10^{(T-121.1)/z} dt, \]gives an equivalent time at 121.1 °C for Clostridium botulinum spore control; \( F_0 = 3 \) min is the minimum “botulinum cook” for low-acid canned foods.
2.2 Pasteurization vs. Sterilization
Pasteurization reduces pathogens and spoilage organisms to safe levels without sterilization: HTST milk (72 °C, 15 s), UHT (135+ °C, few seconds), LTLT (63 °C, 30 min). Commercial sterility: sufficient to prevent spoilage and pathogen growth under normal storage; absolute sterility not required. For low-acid foods (pH > 4.6) C. botulinum governs; for acid foods yeasts, molds, and thermoduric bacteria dominate.
2.3 Heat Transfer in Containers
Conduction heating (viscous pastes, solid packs): temperature history computed from Fourier equation with convective boundary. Ball’s formula or numerical integration yields process time. Convection heating (brine, syrup, liquid foods) is faster; mixed-mode (particulate in liquid) requires careful analysis—the slowest-heating point governs.
2.4 Retort and Continuous Systems
Still retorts, rotary retorts, hydrostatic cookers, continuous agitating retorts, scraped-surface heat exchangers for viscous. Aseptic processing: sterilize product and container separately, fill in sterile environment. Permits thin-walled packaging (cartons, pouches) not suitable for retort.
Chapter 3: Low-Temperature Preservation
3.1 Refrigeration
Chilling slows microbial growth and enzyme activity. Q\(_{10}\) of 2-3 typical: a 10 °C drop halves (or thirds) reaction rates. Critical design considerations: product core temperature history, chilled vs partly frozen (superchilling at -1 to -3 °C for extended shelf life of fish and meat).
3.2 Freezing
Water crystallization removes liquid water, arresting most spoilage. Freezing curve: rapid initial cooling, plateau at initial freezing point (typically -1 to -3 °C), subcooling. Fraction of water frozen follows Raoult-type depression:
\[ X_{ice} = 1 - \frac{T_f'}{T}, \]approximately, with \( T_f' \) the initial freezing point.
3.3 Freezing Time
Plank’s equation for a slab of thickness \( L \):
\[ t_f = \frac{\lambda \rho}{T_f - T_\infty}\left(\frac{L}{2h} + \frac{L^2}{8k}\right). \]Refinements (Pham, Cleland-Earle) incorporate sensible heat. Rapid freezing (blast, cryogenic, plate, fluidized-bed IQF) produces small ice crystals preserving texture; slow freezing damages cell walls.
3.4 Thawing and Storage
Thawing is slower than freezing (driving force smaller at elevated temperatures); microbial risk rises near 0 °C. Cold-chain integrity (HACCP), frozen-storage stability (drip loss, oxidation, sublimation causing freezer burn), and packaging (moisture/oxygen barriers) close the loop.
Chapter 4: Concentration and Separation
4.1 Evaporation
Water removal by boiling. Driving force \( T_{steam} - T_{boil} \). Boiling point rise from dissolved solids depresses efficiency. Multiple-effect evaporators reuse vapor: 3-effect typical for dairy/tomato; energy consumption ≈ steam/effect. Mechanical vapor recompression (MVR) further reduces energy by compressing vapor for reuse as heating medium.
Design equation:
\[ Q = UA \Delta T, \quad \dot m_{vap} = Q/\lambda. \]Fouling, viscous heating, and heat-sensitive components (falling-film evaporators preferred) shape practical design.
4.2 Membrane Processing
Reverse osmosis (NaCl retention, pressures 20-80 bar), nanofiltration (divalent salt retention), ultrafiltration (protein retention, e.g., whey protein concentration), microfiltration (bacteria removal, milk). Energy consumption often lower than thermal concentration, but flux decline from fouling (concentration polarization, cake formation) requires periodic cleaning.
4.3 Extraction
Solid-liquid extraction of oils (hexane extraction of soybean oil), flavors (supercritical CO\(_2\) for hops, coffee decaffeination), caffeine (hot water, CO\(_2\)), and sugars (diffusion from sugar beet slices). Solvent selection balances efficiency, residue, and regulatory acceptability.
4.4 Crystallization
Sugar refining (sucrose from cane/beet juice via multi-stage evaporative crystallization), lactose from whey, salt, fats (winterization, interesterification). Control of nucleation vs growth determines crystal size distribution, affecting downstream centrifugation and sensory texture.
Chapter 5: Formulation, Texture, and Quality
5.1 Water Activity and Stability
Water activity \( a_w = p/p^0 \) at equilibrium. Microbial growth limits: bacteria generally \( a_w > 0.90 \); yeasts \( > 0.87 \); molds \( > 0.70 \); no microbial growth below 0.60. Intermediate-moisture foods (jams, dried fruit) rely on humectants (sugar, salt, glycerol) to reduce \( a_w \) below critical values without drying to powders.
5.2 Sorption Isotherms
Moisture-equilibrium relationships (GAB, BET, Oswin models) describe water uptake vs RH. Type II sigmoidal curves typify polymers (starch, proteins). Hysteresis between adsorption and desorption branches is common. These isotherms govern packaging design, glass-transition phenomena, and caking in powders.
5.3 Rheology and Texture
Yogurt, mayonnaise, ketchup, and peanut butter exhibit yield stress and shear thinning; flow models (Herschel-Bulkley, Casson) guide design of mixers, pumps, and dispensers. Texture instrumental measurement (TPA, creep tests) correlates imperfectly with sensory perception; joint use of instrumental and panel data is standard in product development.
5.4 Effects on Organoleptic Quality
Processing inevitably alters flavor (Maillard, caramelization, off-flavors from lipid oxidation), color (pigment degradation, browning), nutrition (vitamin loss, protein denaturation), and structure (starch gelatinization, protein aggregation). Process design optimizes delivered quality: high-temperature short-time tends to preserve better than long hold times at moderate temperatures.
Chapter 6: Regulatory and Hygienic Design
6.1 HACCP and Food Safety Plans
Hazard Analysis and Critical Control Points: identify hazards (biological, chemical, physical), determine critical control points (CCP), establish critical limits, monitor, corrective actions, verification, documentation. Codex and FSMA (US) formalize HACCP-based preventive control.
6.2 Hygienic Design
Surfaces that cannot be cleaned cannot be sanitized. EHEDG, 3-A Sanitary Standards: smooth, accessible surfaces; no dead legs; self-draining; materials (SS316L, EPDM gaskets) compatible with cleaning agents. Clean-in-place (CIP) cycles: prewash, caustic wash, intermediate rinse, acid wash, final rinse, sanitize.
6.3 Packaging
Functional requirements: barrier (O\(_2\), H\(_2\)O, aroma, light, pests), mechanical (drops, vibration, compression), information (labels, nutrition), sustainability (recyclability, compostability). Materials: glass, metal, plastics (PET, HDPE, LDPE, PP, PLA), paperboard. Modified atmosphere (MAP) extends shelf life (high CO\(_2\) inhibits many spoilage bacteria).
6.4 Sustainability in Food Processing
Water use, energy, packaging waste, and food loss. Utilities optimization (pinch analysis, heat pumps), valorization of byproducts (whey, peels, bran) into co-products, and minimal processing technologies (high-pressure, pulsed electric field, cold plasma) reduce environmental footprint while meeting consumer expectations for minimally processed foods.
