The colour and oxidative stability of cooked pork

Hay, Theresa Kathleen Clare. (2002). The colour and oxidative stability of cooked pork PhD Thesis, School of Land, Crop and Food Sciences, The University of Queensland.

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Author Hay, Theresa Kathleen Clare.
Thesis Title The colour and oxidative stability of cooked pork
School, Centre or Institute School of Land, Crop and Food Sciences
Institution The University of Queensland
Publication date 2002
Thesis type PhD Thesis
Supervisor Heather Bruce
Bruce D'Arcy
Maria Lanari
Total pages 248
Collection year 2002
Language eng
Subjects L
300406 Animal Growth and Development
630105 Pigs
Formatted abstract
If food safety is assured and the price acceptable, the eating quality of meat then becomes the most important consideration to consumers. Tenderness, juiciness and flavour are characteristics that contribute to the eating quality of pork. During this study, a kinetic mathematical model was developed that will provide cooking guidelines to ensure pork is juicy and tender. Flavour aspects of pork were also explored during this work specifically, the effects of dietary components in pig feed on the shelf life and flavour of pork.

Tenderness, juiciness and flavour are characteristics that contribute to the eating quality of pork and cooking conditions can have a marked effect on these characteristics. Juiciness declines and tenderness is reduced as the final cooking temperature is increased. The eating quality of pork is further jeopardised by the tendency of consumers to overcook pork that appears pink or red. Pork products that contain a high proportion of the red, oxidative muscle fibres have a high possibility of being overcooked, as these products may appear undercooked by observation of colour, even after prolonged cooking.

For the first part of this work, kinetically based cooking guidelines for pork and beef muscles were developed. Two approaches were used to develop kinetic models to describe the variation of internal colour during cooking of muscles. Firstly, a simple model with first order kinetics was fitted to the isothermal colour / time data of pork and beef loins cooked to three end point temperatures (65°, 80° or 95° C). The colour changes during cooking were indicated by the CIELAB a*, b* and hue angle for each muscle. The activation energy of the systems was then predicted from the Arrhenius relationship. The models developed for beef and pork loin were then validated on independent data of muscle cooked at 65° C, and were found to fit quite well. The validation set also contained colour data of pork knuckle. The pork knuckle colour data was not well fitted by either the beef or pork loin models. It was concluded that beef and pork loin and pork knuckle require independent models to accurately predict the colour changes during cooking.

The second approach to modelling used a Runge-Kutta-Felberg routine to solve the non-isothermal partial differential equation that has both meat time and temperature as dependent variables. Models were developed to fit the validation data set of beef and pork loin and pork knuckle cooked to an endpoint of 65° C, and the frequency factor and activation energy of the systems were estimated from the model predictions. Although there was a high level of variation in the data, particularly that of the pork knuckle, the predicted colour values showed good agreement with the observed data.

In addition to juiciness and tenderness, flavour is an important aspect of eating quality. The flavour and composition of pork can be manipulated by controlling the dietary components consumed by the pig. Increasing the concentration of polyunsaturated fatty acids (PUFAs) of pork can enhance its healthy image, as increasing the n-3 / n-6 fatty acid ratio in the human diet may reduce the risk of coronary heat disease (CHD). Pork containing high levels of n-3 PUFAs has been produced through manipulating the diet of the pig. However, the oxidation potential increases as the degree of unsaturation in the lipid increases. In addition, pig diets contain high levels of iron, zinc and copper, which are added as growth stimulants. These metals have been shown to catalyse lipid oxidation. Lipid oxidation in pork has been linked to off-flavours and therefore has the potential to reduce consumer confidence in the eating quality of pork. The potential market for cooked stored pork products is increasing rapidly as consumers embrace the ease and speed of using home meal replacements or convenience meals. Unfortunately, cooked reheated meat products often develop an unpleasant flavour called warmed-over flavour (WOF). WOF is a stale flavour that has been described as 'cardboard-like' or 'painty', and appears in meat products reheated less than 48 hours after cooking, and has been linked to the oxidation of the highly unsaturated phospholipids of cell membranes. As a result, WOF is recognised as a major limitation to the inclusion of meat in HMR.

The second part of this study investigated the effect of pig dietary components on the flavour of cooked, stored pork, in particular the interaction of dietary fat saturation and metal ion concentration on the incidence of WOF in cooked pork. Pigs were assigned to either low or high PUFAs based diets (tallow or tuna stearin at 3%) containing either low or high metal ion concentration (60 or 150 ppm Fe2+ , 4 or 250 ppm Cu2+ and 45 or 150 ppm Zn2+). Muscle from these animals was cooked and then assessed by chemical tests and a trained panel after 0, 1, 2 and 4 day of storage (4°C). Pork from pigs fed tallow had a significantly (P<0.05) increased cooked pork meat-like flavour, a desirable attribute. Pork from pigs fed tuna stearin had significantly increased scores for fish-like flavour and rubber-like aroma, attributes associated with lipid oxidation. Pork from pigs fed high levels of polyunsaturated fats displayed higher scores for some WOF attributes than pork from pigs fed saturated fats. There was no significant interaction between PUFAs and metal ion concentration in the pig diets as assessed by sensory scores of the cooked, stored pork.

Chemical analysis of the cooked stored pork included the standard thiobarbituric acid reactive substances (TEARS) test and a new solid phase microextraction (SPME) method to determine the level of oxidation in the cooked stored samples. The SPME method was developed during this study for comparison of treatment samples and involved the extraction and analysis of hexanal from the headspace of cooked pork samples. The results from this study suggest that hexanal may not be a good oxidation indicator to discriminate between pork from pigs fed tallow and tuna diets. From the chemical analysis of the cooked stored pork samples it was concluded that the inclusion of a high level of metal ions did not increase oxidation in pigs fed tallow or tuna stearin. However, the TEARS results revealed an interesting interaction. The TEARS results of the loin showed that samples from pigs fed high PUFAs and low metal ions had significantly higher TEARS scores that pigs fed high PUFA and high metals. This unexpected result is discussed in relation to current literature regarding the complex relationship of dietary trace metals, their cellular stores and both the pro-and anti-oxidant potential of these metals.
Keyword Pork
Cooking (Pork)

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
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Created: Fri, 24 Aug 2007, 18:01:17 EST