Food Focus November 2008

Freezing of chicken: the pros and cons

Although freezing can reduce Campylobacter levels and potentially contribute to reducing foodborne risks, a number of practical issues need to be considered. Freezing also impacts on our food choices. NZFSA has been investigating the pros and cons

In New Zealand campylobacteriosis is the most frequently reported gastrointestinal illness and over half the cases have been attributed to consumption of chicken.

NZFSA works closely with the industry and has had a detailed strategy in place for over two years to control Campylobacter in poultry. A multi-pronged approach has been taken, with ongoing improvements, including hygiene, on farms and at processing premises. NZFSA has also commissioned several reports from Environmental Science & Research (ESR) to investigate the effectiveness of freezing chicken as a possible intervention to reduce the levels of Campylobacter. The pros and cons of this control measure are summarised in Table 1.

The facts around freezing

Fresh chicken can be frozen at two main stages before it makes it to the dining table. It can be commercially frozen, and sold to consumers in a frozen state, or it can be frozen by the consumer in the home. Separate ESR reports were commissioned to investigate each.

The reports identified differences in how well different freezing processes killed off microorganisms. Slower freezing was more effective because it prolonged the exposure of microorganisms to osmotic shock and dehydration – the two factors that will cause Campylobacter to die during freezing. Therefore, the way chicken is frozen and the length of time it is held at effective killing temperatures are both important factors. Another factor is how well the domestic freezer keeps chicken at the right temperature once the consumer has taken it home.

On the commercial front, ESR used a commercial processing premises with freezing at –30°C. The poultry industry favours the use of blast freezers as these form smaller ice crystals and maintain the quality of the product by preventing tissue damage and excessive drip during thawing. Significant but variable reductions in Campylobacter numbers were observed over time. Maximum reduction in numbers was achieved after four weeks at –30°C.

On the domestic front, ESR simulated domestic freezing and frozen storage for up to 10 weeks. The freezing temperature applied was approximately –18°C. They found Campylobacter numbers were significantly reduced. However, the largest reductions didn’t occur until chicken was stored frozen for six weeks or longer.

Are domestic freezers adequate?

ESR found fridge-freezers are the most common type in homes (70.3%), with twice as many bottom-loading freezer compartments than top-loading freezers.

The surveyed freezers operated at a mean temperature of –16.56°C. Top sections of freezers were 2–2.5°C warmer than the middle or bottom sections, and full freezers were 1–2°C warmer than less-full ones. The currently accepted safe range for domestic freezing temperatures is –18 to –20°C but only 27.5% of surveyed freezers operated at or below this range.

After the freeze

Chicken takes about 10 hours to thaw at room temperature. This is not recommended as pathogens can grow quickly if the poultry is left out too long.

Thawing in the refrigerator is recommended to minimise pathogen growth.

What does this mean?

The NZFSA-commissioned trials provided scientific evidence of a significant decline in Campylobacter numbers following freezing under simulated commercial and domestic conditions. However, this was not totally effective and does not mitigate potential problems around thawing and undercooking. (It is also important to note that the trials were performed in experimental conditions and may not reflect results that might be achieved in the home.) Nevertheless, the scientific work indicates that freezing can provide a potential control option.

Most New Zealand consumers reportedly prefer to purchase fresh poultry meat and this is a risk management consideration – around 70% of consumers purchase half or more of their poultry meat fresh. However, nearly 70% of those consumers also freeze over half of this fresh poultry meat in the home.

Freezing is one possible control measure for Campylobacter that could be applied either by the industry or the consumer in conjunction with other control measures throughout the food chain. The selection of measures will depend on the particular processing circumstance and the goal of enabling consumers to choose whether they want to buy fresh or frozen poultry meat. In all cases, care must be taken to prevent cross-contamination and to ensure the meat is thoroughly cooked.

Table 1: Pros and cons of freezing

New Zealand’s monthly campylobacteriosis cases

NZFSA’s Campylobacter strategy

NZFSA’s risk management strategy aims to reduce Campylobacter levels in chicken meat through scientifically robust interventions at appropriate points in the food chain. The strategy spells out an ongoing work programme, which is reviewed each year in response to results achieved and new information.

The trend for campylobacteriosis cases this year is looking promising, with much lower rates than previous years. Campylobacteriosis is known to show seasonal variation with higher rates usual in spring and summer.

About NZFSA’s Campylobacter Risk Management Strategy