Experimental and theoretical investigation of the mechanisms of kānuka wood smoke formation for food smoking : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Bioprocess Engineering at Massey University, Palmerston North, New Zealand

Abstract

The consistent production of smoked foods with a tunable aroma profile presents an industrial challenge, often relying on the subjective skill of artisan operators rather than precise process control. While temperature is known to be a key factor, the underlying mechanisms governing smoke chemistry and heat dynamics are less understood. This thesis aimed to deconstruct the thermochemical processes of kānuka wood pyrolysis to establish a framework for predictable and controllable smoke generation. The research objectives were to: (1) determine the intrinsic decomposition kinetics of kānuka wood; (2) characterise the temperature-dependent evolution of its volatile aroma profile; (3) quantify the influence of process parameters on its specific enthalpy of reaction; and (4) develop a predictive model to simulate and optimise the process.

Thermogravimetric Analysis (TGA) was used to determine the decomposition kinetics of kānuka’s principal components (hemicellulose, cellulose and lignin). The specific volatile organic compounds (VOCs) comprising the smoke’s aroma profile were identified and quantified across a temperature range of 200-520 °C using Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS). The heats of primary and secondary reactions were measured using TGA-differential Scanning Calorimetry (TGA-DSC). Py-EGA/MS was also used to study the formation kinetics of key aroma compounds. Finally, these experimental insights were integrated into a one-dimensional, transient model that solves coupled equations for heat transfer, mass transfer and reaction kinetics to simulate pyrolysis in a thermally thick bed of wood.

The experimental results revealed that smoke composition is highly temperature-dependent, with carbohydrate-derived compounds (e.g., fufural) dominating at 200-350 °C and lignin-derived phenolics (e.g., guaiacol, syringol) surging above 400 °C. Importantly, TGA-DSC analysis demonstrated that small particle sizes (<90 µm) and high samples masses promote exothermic secondary reactions due to low bed permeability, leading to increased char yield and potential thermal runaways. This finding identified bed permeability, governed by particle size, as the critical parameter for controlling local temperature and by extension the smoke’s aroma. The numerical model confirmed these relationships, showing that slower heating rates, lower final temperature and deeper beds bias the aroma proxy towards compounds formed at 200-300 °C, while faster heating rates and shallower beds shift it towards the 300-400 °C range.

In conclusion, this research provides the first comprehensive thermochemical characterization of kānuka wood for smoking applications. It establishes a novel conceptual framework that moves beyond simple temperature control to identify bed permeability and secondary reaction management as the key to process consistency. By clarifying the fundamental mechanisms and providing a predictive model, this thesis provides the scientific basis to help transform food smoking from an operator dependent art into a predictable, tune-able and repeatable science.