23-Chem-B4 Biochemical Engineering · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B4, Biochemical Engineering — May 2018. 3 hours, Closed-Book Exam (any non-communicating Casio or Sharp calculator permitted). Per the exam notes, FIVE (5) questions constitute a complete paper and all five must be answered; most require a short-essay-format answer, and clarity/organization of the answer are explicitly marked.
Reference texts: Shuler & Kargi, Bioprocess Engineering: Basic Concepts, 2nd ed.; Bailey & Ollis, Biochemical Engineering Fundamentals, 2nd ed.; Madigan et al., Brock Biology of Microorganisms, 13th ed.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Definition. Immobilization is the physical or chemical confinement of an enzyme (or a whole cell) to a defined region or support — a solid particle, a gel matrix, or a membrane-bound compartment — such that it retains catalytic activity while being spatially separated from the bulk liquid phase, so it can be reused, retained inside a reactor, and easily separated from the product stream.
(a) Adsorption. The enzyme attaches to a solid support (activated carbon, alumina, ion-exchange resin) via weak physical or ionic forces. Simple, inexpensive, and minimally disruptive to the enzyme's native structure, but the binding is reversible — the enzyme can desorb (leach) under shear, pH, or ionic-strength changes during operation.
(b) Covalent binding. The enzyme is chemically bonded to a functionalized support surface (e.g. via a carbodiimide or glutaraldehyde linkage to a hydroxyl/amine-bearing carrier). The bond is strong and essentially irreversible, giving excellent operational stability and no leaching, but the coupling chemistry can attack residues in or near the active site, reducing activity, and the support cannot be regenerated once the enzyme's usable life ends.
(c) Entrapment. The enzyme (or whole cell) is physically trapped inside the three-dimensional network of a gel or polymer matrix (calcium alginate, polyacrylamide, κ-carrageenan) formed around it. No chemical modification of the enzyme is needed (preserves native activity well) and it suits whole-cell immobilization particularly well, but substrate and product must diffuse through the matrix to reach the enzyme, adding an internal mass-transfer resistance (exactly the Thiele-modulus/effectiveness-factor limitation quantified in Question 1(iii)).
(d) Encapsulation (membrane confinement). The enzyme is held in free solution behind a semi-permeable membrane (a microcapsule or hollow-fibre device) that lets small substrate/product molecules pass but retains the (larger) enzyme. This avoids any direct chemical or physical contact between enzyme and support, preserving activity well, but the membrane itself is an added mass-transfer barrier and can foul or rupture in service.
| Advantages | Limitations |
|---|---|
| Enzyme is reused across many batches/continuous operation, greatly lowering enzyme cost per unit product | Immobilization itself can reduce specific activity (steric/diffusional/chemical effects, depending on method) |
| Easy separation of enzyme from product stream (no downstream enzyme-removal step) | Internal/external mass-transfer resistance can make the immobilized system markedly slower than free enzyme at the same loading (Question 1) |
| Often improved operational/thermal stability vs. free enzyme in solution | Added capital/process cost of the support, coupling chemistry, or membrane |
| Enables continuous packed-bed or fluidized-bed reactor operation | Support fouling, bead attrition, or membrane rupture over extended operation |
The best-known industrial application is immobilized glucose isomerase, used at massive scale to convert glucose (from enzymatically hydrolyzed corn starch) to fructose in the production of high-fructose corn syrup (HFCS). The enzyme is immobilized (typically by adsorption/cross-linking onto a particulate support or whole dead-cell entrapment) and packed into large fixed-bed reactors; the immobilized form is reused for months of continuous operation, which is what makes continuous HFCS production economically viable at the scale the food industry requires — a free-enzyme (single-use) process at that throughput would be prohibitively expensive in enzyme cost alone.
