with increased blemishes and lacerations on pigs due to fighting, reduced carcass yield (even though food is offered), as well as dark firm dry (DFD) meat (Warriss 2003). At temperatures greater than 10◦C, showering serves to clean pigs, reduce aggression and fighting, and results in improved meat quality (Knowles et al. 1998). Holding times of no more than 30 minutes are optimal under high ambient temperatures (35◦C) as it reduces the incidence of PSE meat (Warriss 2003). Generally with respect to pig quality, a lairage time of 1 – 3 hours is regarded as optimal in the UK (Warriss 2003, Boes et al. 2001).
Pigs are electrically stunned in the head, rendering them unconscious, during which time the animal is shackled and hung up by its hind legs. A knife is then inserted into the mid-line of the neck within thirty seconds of stunning and the anterior vena cava is severed. Death results from exsanguination (Borch et al. 1996).
Scalding
After bleeding is complete, the carcass is immersed in a tank of water between 58 and 65◦C for 6 – 10 minutes (Borch et al. 1996), in a process known as scalding. This immer- sion loosens the hair and bristle in the outer layer of the skin, facilitating dehairing which occurs in a subsequent processing stage. Several pigs are scalded together in the same batch of water, thus resulting in the accumulation of dirt, faeces, ingesta and bacteria in the tank (Bolton et al. 2003). There was no addition of water to the scald tank at any of the plants visited during operations, so the effect of water replacement was not included in the model.
Dehairing
Dehairing is accomplished when one or more carcasses are placed in a rotating drum with in-built metal paddles that scrape the hair and outer layer of the skin off the carcass.
Warm water is sprayed on the carcasses and equipment to remove hair and debris. During this process faecal matter may be spread onto the carcass surface and machine. In this case, the machinery becomes contaminated with bacteria and may serve as a source of contamination for other carcasses in the processing chain.
2.12 Overview of Production processes 43 Singeing
The carcass is singed using a blow torch, where the flames remove any remaining hair and tighten the skin. This increases the external temperature of the skin to up to 100◦C and has been reported to be very effective in reducing bacterial counts on the surface of the carcass (Pearce et al. 2004, Berends et al. 1997, Borch et al. 1996, Sorqvist & Danielsson- Tham 1990). The time taken to complete this process varies between carcasses and is also dependent on the operator’s judgement; but is usually between 10 – 15 seconds. In some abattoirs ‘polishing’ occurs, where rotating rubber brushes are used to remove the burnt skin from the carcass.
Evisceration, trimming and halving
Evisceration involves removal of the entrails. This procedure is thought to be the main cause of carcass contamination (Bonardi, Brindani, Pizzin, Lucidi, D’Incau, Liebana &
Morabito 2003, Bonardi, Pizzin, Lucidi, Brindani, Paterlini & Tagliabue 2003, Hald et al.
2003, Berends et al. 1997) as it is estimated that 55 – 90% of carcass contamination occurs from this procedure (Berends et al. 1997). The critical operations are circumcising of the rectum, which can be done manually or mechanically, followed by cutting the carcass from the anus to the breastbone to allow manual removal of the intestinal tract and pluck set (trachea, lungs, gullet, heart and liver). Removal of the intestines carries the risk of puncturing of the intestine resulting in faecal contamination of the carcass and spread of zoonotic pathogens from the intestine to the carcass. Trimming is the manual removal of visibly contaminated regions on the carcass (Borch et al. 1996). Halving is the process of cutting the carcass into two halves with the aid of an automated saw.
Chilling/Storage
The final processing stage is the chilling of the carcass. Carcasses are chilled to a temper- ature of approximately 4 – 7◦C using blast and/or conventional chillers. This is a critical step for both meat quality and bacterial contamination. Carcasses chilled too rapidly can experience contraction of the muscles, reducing the quality and tenderness of any subse- quent meat produced. Chilling can also minimise some bacterial proliferation, however, pathogen growth is very variable and influenced by the type of organism, temperature and
pH (Borch et al. 1996).
At present in New Zealand there are 11 abattoirs that slaughter pigs. Six of these are located in the North Island and five in the South Island. In this thesis three abattoirs were visited, two of which were in the North Island. One abattoir in the North Island slaughters pigs on two days in the week, except for the month of December in which increased market demand led to slaughtering of pigs on 3 days in the week. It is estimated that a total of 15,000 pigs are slaughter per week. The number of pigs slaughtered daily in abattoirs in New Zealand can vary from 150 to over 1,000 pigs. The procedures described above are undertaken in all visited plants. There is a procedure called polishing, that results in the removal of any black rind from singeing. This procedure was not modelled as the methods used in New Zealand to execute this procedure are varied, ranging from the manual use of nylon hand brushes to large automated equipment with rubber flails and this procedure is sometimes absent. There appeared to be no standardisation with which the procedure is executed in abattoirs visited throughout New Zealand.
2.12.2 Further processing processes
The production of pork chops is evaluated in this thesis as it is one of the most commonly purchased and consumed pork products in New Zealand. Pork chops can be produced in some abattoirs and/or retail outlets by cutting the longissimus dorsi muscles into desired sizes.