The researchers, based at Ohio State University in the US and Abant Izzet Baysal University in Turkey published their findings in the journal Food Control.
The authors concluded that the addition of plant essential oil reduces HPP process severity and lowers costs while remaining effective in terms of inactivating L. monocytogenes and L. innocua in aryan – a product popular in Turkey consisting of yoghurt, water and salt (added at a maximum level of 1g 100 g-1 to impart flavour).
Multiple listeriosis outbreaks have been linked to contaminated cheese and other dairy products, note the authors, and they stress that there is no reported research in the literature investigating the survival of L. monocytogenes in yogurt-based drinks using pressure treatments.
Non-thermal food preservation technologies, high pressure processing (HPP) in particular, have drawn considerable attention among scientists and the food industry due to the fact that they allow minimal food processing and fewer preservatives.
Despite the high costs involved, HPP is being used commercially in the both the EU and the US, and has been increasingly and successfully applied to various foods such as dairy, meat, seafood, poultry products, vegetables and vegetable products, fruit products as well as acidified products.
The authors said they aimed to evaluate efficacy of pressure treatment both alone and in combination with mint essential oil on inactivation of L. monocytogenes and L. innocua inoculated in ayran samples, using a control for reference purposes.
Colour, pH, water activity, and serum protein separation were also measured, said the scientists, adding that both Weibull distribution and log-logistic models informed the methodology.
Ayran samples were inoculated with either L. monocytogenes or L. innocua cultures at the level of ca. 105-106 CFU mL-1. The inoculated samples (100 mL) were aseptically
packaged into sterile pouches. After the removal of air bubbles, the pouches were heat sealed. The sample pouches were then placed in a high barrier film bag and heat-sealed.
The sample pouches, said the team, were subsequently placed inside a larger high barrier film bag and vacuum packed at -97 kPa.
An additional set of experiments was also carried out to evaluate efficacy of combining mint essential oils and pressure treatment. The mint essential oil was added at the concentrations of 0.05 and 0.1 mL 100 mL-1 to the inoculated ayran samples. These samples were also vacuum packaged and subsequently pressure treated, added the team.
HPP of ayran samples at 600 MPa for treatment time of 300 s reduced L. monocytogenes and L. innocua by more than 5-log units at ambient temperature, found the researchers.
The scientists reported that the addition of mint essential oil further enhanced inactivation of both bacteria by more than 1 log cfu 34 mL-1. Combination of mint essential oil with HPP provided a reduction in pressure treatment severity by 100-300 MPa or by 210 s to achieve the same amount of inactivation relative to HPP alone.
And pressure treatment alone or HPP combined with mint essential oil, they noted, did not cause significant changes in pH, water activity, colour and serum protein separation, they added.
The authors urge further research to determine the impacts of HPP combined with different plant essential oils on the inactivation of spoilage as well as pathogenic microorganisms inoculated into different foods taking into account the impact on physicochemical and sensory properties.
Source: Food Control
Published online ahead of print: 10.1016/j.foodcont.2011.03.005
Title: Inactivation of Listeria monocytogenes and Listeria innocua in Yogurt Drink Applying Combination of High Pressure Processing and Mint Essential Oils
Authors: G Evrendilek, V. M. Balasubramaniamb