Abstract
The purpose of this thesis was to investigate the possibility of using pulsed electric fields (PEF) as a pre-treatment for sous vide processing to improve the quality of tough meat cuts, such as brisket. The effect of PEF on the microstructure of connective tissue was investigated as well as the impact of the changes on the thermal stability of connective tissue and the physicochemical properties of meat such as collagen solubility and quality attributes such as texture, cooking loss, colour, lipid stability. Additionally, the effect of ageing after PEF which is a common practice in the meat industry on meat quality was investigated in combination with PEF and sous vide processing.
The impact of PEF on the thermal stability of connective tissue, was investigated using a novel model system, in which connective tissue obtained from beef deep and superficial pectoral muscle (brisket) was exposed to PEF at combinations of electric field strength (1.0 and 1.5 kV/cm) and specific energy (50 and 100 kJ/kg) within an agar matrix at electrical conductivities (1-7 mS/cm) representing the electrical conductivity found in brisket. Differential scanning calorimetry showed that PEF treatment significantly (p<0.05) decreased the denaturation temperature of connective tissue compared to non-PEF treated samples. Increasing the electric field strength and the specific energy increased the Ringer soluble collagen fraction. PEF treated samples showed higher solubilisation compared to the non-PEF treated samples at both 60°C and 70°C. Scanning electron microscopy (SEM) examination of PEF treated (at 1.5 kV/cm and 100 kJ/kg) and non-PEF treated samples revealed that PEF appeared to increase the porosity of the connective tissue structure.
Process optimisation for PEF treated briskets (at electric field strength 1.5 kV/cm; specific energy of 90-100 kJ/kg) subjected to various sous vide temperature and time combinations was determined using a multiple regression model. Sous vide temperature had a linear significant (p<0.05) effect in reducing the shear force of PEF treated meat while a quadratic effect of time significantly (p<0.05) reduced the hardness. SEM examination revealed that PEF treated meat showed evidence of pore formation in connective tissue and polarized sensitive optical coherence topography (PS-OCT) revealed that the collagen matrix of PEF treated meat had a reduced birefringence capacity compared to non-PEF treated meat, which indicated a breakdown of the collagen.
To optimise the processing parameters of PEF pre-treatment to reduce the sous vide processing time, the effects of electric field strengths (0.7, 1.0, or 1.5 kV/cm; specific energy of 90-100 kJ/kg) and sous vide processing time at 60°C (12, 18, or 24 h) on each meat quality parameter (Textural properties, cooking loss, total myoglobin content etc.) was modeled using a multiple regression model. Increasing the electric field strength and/or prolonging sous vide time significantly increased the collagen solubilisation of the sous vide brisket. However, PEF treatment at 0.7 kV/cm was found to be the optimal electric field strength required to reduce the meat shear force, and hardness after less than 24 h of sous vide processing. PEF treatment prior to sous vide processing had no effect on lipid oxidation, colour stability or cooking loss of the sous vide processed brisket.
The impact of PEF treatment at electrical field strengths of either non-PEF, 0.7, or 1.5 kV/cm (at specific energy of 90-100 kJ/kg) followed by post-PEF ageing for 0, 3, 7, and 14 d and sous vide processing at various time intervals (12 and 24 h) at 60°C on different meat quality parameters were considered. PEF treatment had much greater effect than the effect of ageing time, and sous vide processing time to reduce the hardness. Longer ageing significantly (p<0.05) increased the lipid and colour oxidation in sous vide processed meat regardless of the pre-treatment. Sous vide processing reduced the total microbial and lactic acid bacterial number of meat to below the detection limit (< 1 log CFU/g) regardless of the pre-treatment. Under certain PEF and sous vide treatments condition, a significantly (p<0.05) higher soluble peptide concentration of 14 d aged meat was observed. After a 30 min peptic digestion, 0.7 kV/cm PEF treated meat processed by sous vide for 12 h without ageing, had a significantly higher soluble peptide content than that of non-PEF treated meat. In the pepsin soluble fraction, the bands of small molecular weight peptides (≤15 kDa) appeared to be more intense in PEF treated meat processed by sous vide after 14 d ageing compared to others at 30 or 60 min peptic digestion. In the pellet fraction, a band >160 kDa was not visible in non-PEF treated meat sous vide processed for 24 h without ageing suggesting its degradation.
The findings of this thesis suggest that prior PEF processing will improve the tenderness and decrease the cooking time of collagen-rich, meat cuts such as briskets without having any deleterious effects on other meat quality attributes. Thereby providing meat companies with a means of PEF as a pre-treatment for sous vide processing to improve the quality of the final product while enhancing the process efficiency.