Pasteurisation: Oilseeds, nuts and almonds

Agronomic foods are often naturally contaminated with harmless and pathogenic microorganisms. In most cases, agronomic goods are freshly processed, or appropriately processed to preserve and increase shelf stability. Common preservation techniques include heat pasteurisation or sterilisation, irradiation, disinfestations with gaseous substances etc. In particular, the two latter techniques are rarely used nowadays due to legal restrictions as well as safety and nutritional concerns.

In 2001 and 2004, incidents of salmonella infections caused by raw almond consumption were reported in the US and in Canada. In order to secure and improve the safety of almond consumption, the USDA has enacted a rule that all almonds used in domestic markets have to be treated with a process realising a 4 log reduction of the corresponding salmonella species. This rule came into effect from 1 September 2007 and affects the complete Californian almond harvest dedicated to the markets in the US, in Mexico and Canada.

Because almonds, as well as tree nuts and other oil seeds, are susceptible to quality changes during processing, the safe inactivation of salmonella requires new pasteurisation techniques able to preserve the original quality. In particular, the moisture content of raw almonds must not increase significantly during or after the treatment reducing shelf stability and impairing appearance. In addition, almonds tend to loosen their brown skins if treated in a humid atmosphere, impairing quality massively. In case of treatment with moist heat, the modification of flavours such as the generation of cooked flavour notes has to be avoided. Conventional ‘moist heat’ treatment systems tend to alter the flavour and typically generate a ‘cooked’ or ‘boiled’ note in the almond – this has to be avoided as well.

Pasteurisation of raw oilseeds, almonds and nuts

Conditions of high water activity are essential in order to inactivate microorganisms efficiently with heat. Common pasteurisation and sterili­sation techniques as in the case of dairy or canning utilise conditions of high water activity close to one. In a dry atmosphere and in dry products, vegetative microorganisms are inactivated at a strongly reduced rate. For this reason, pasteurisation of dry goods, especially those with low moisture content, is preferably carried out under conditions in which water condenses on the product’s surface. As an example, spices are pasteurised by the injection of overheated steam in a pressure vessel, followed by a sudden expansion. Microorganisms are released from the spice surface due to the pressure drop and the cells burst. The combined action of humid heat, forced cell release from surface and cell burst due to the pressure drop effectively inactivates microorganisms.

Such a process cannot be applied to pasteurise oil seeds, almonds and nuts for two reasons. First, the application of steam at elevated pressures leads to a considerable condensation and increase in moisture content, which even after a separate drying process reduces resulting product quality. Furthermore, harsh mechanical action such as a sudden pressure drop degrades the integrity of the kernels and leads to unacceptable product damages. In the case of raw almonds, the brown skin is impaired and released from the almond, degrading the quality of the product. Other processes utilising steam as energy loaded heat source are carried out under elevated or normal pressure without pressure drop. However, also under such conditions, the increase in moisture content at temperatures above 100°C is obvious, altering flavour quality and impairing integrity of the product considerably.

Pasteurisation of oilseeds in a moist atmosphere

Water activity influences pasteurisation kinetics and heat resistance of microorganisms. Several scientific research works investigated the influence of substrate water activity on inactivation of bacteria with dry heat. An increasing heat resistance of Salmonella at decreasing water activities has been reported. This effect was even more pronounced when the bacteria were fixated to the surface of a rawhide product and dry heat was directly applied to the rawhide.

With actual pasteurisation systems, the application of steam is used. At normal pressure (i.e. 100°C), inactivation of Salmonella will be log 2.05 – 3.83 after 35 seconds by steam treatment. By increasing the treatment time up to 65 seconds, the log reduction increases further to 4.00 – 5.72, but the moisture uptake progressed considerably, so the treatment duration has to be limited to 35 seconds. However, this indicates that the application of humid heat improves heat inactivation considerably due to several reasons: first, the heat capacity of water vapour is very high and even exceeds the heat capacity of water at the same temperature; Secondly, if the surface temperature is below dew point temperature of the atmosphere, water vapour condenses on the product surface and penetrates in cavities and crevices. Finally, surface condensation increases water activity on the surface and therefore reduce heat resistance of microorganisms.

Consequently, an ideal pasteurisation process for oilseeds and almonds or nuts includes the application of moist heat under conditions, at which the condensation of moisture on the almond surface is limited to an extent, at which a high water activity is established and a sufficient heat transfer is realised to achieve the necessary micro­biological inactivation.

Product flavour and properties are susceptive to changes when heated to increased temperatures. Therefore, it is essential to carry out such treatment at the lowest possible pressure in order to reduce evaporation temp­erature. Furthermore, in order to avoid excessive moisture condensation on the product surface during the pasteurisation treatment, it is essential to realise an appropriate product temperature before pasteurising. These physical conditions are realised with the CCP process.

CCP – controlled condensation pasteurisation of oilseeds and almonds or nuts

The steam pasteuriser CCP consists of a pressure vessel with a central mixing system. The pressure vessel has to be insulated to avoid energy loss and condensation during the pasteurising process.

First, the oilseeds are getting an appropriate product temperature in a separate process. Then, the oilseeds are filled into the vessel, equilibrated and pasteurised in a humid atmosphere. After pasteurisation, small quantities of residual moisture are removed in vacuum at the same temperature. Finally, the oilseeds are cooled and packed or further processed.

In order to investigate the pasteurisation conditions and efficiency as well as for vali­dating the process, micro challenge tests were carried out. Therefore, oilseeds were inoculated with Enterococcus faecium bacteria which serve as a surrogate for Salmonella SE PT 30. Approximately 30 grams of the inoculated oilseeds were placed in polyamide grids. The grids were preheated and placed at different locations in the CCP pasteuriser together with bulk oilseeds. After pasteurisation and vacuum drying, the grids were collected and the inactivation of Enterococcus faecium was measured.

The analysis of the heat-treated oilseeds showed a reduction of the bacterial count for Enterococcus faecium of more than 5 logs, when the pasteurisation duration was five minutes or more. At more than five minutes treatment time, the reduction of 5 log units was achieved in every single grid independently of location or treatment day.

The CCP pasteurisation temperature itself is a function of the pressure conditions during pasteurisation. An increase in pasteurisation pressure would increase the evaporation and condensation temperature and thereby the pasteurisation temperature. Consequently, the inactivation rate can be increased by increasing the pasteurisation pressure and pasteurisation temperature.

After CCP pasteurisation, the almonds were pasteurised without any changes in appearance or colour (Figure 1), and no skin loosening was observed. Furthermore, the increase in moisture content was less than 0.5 – one per cent and therefore very little. In additional tests, the oxidative stability of the pasteurised almonds was assessed by measuring the formation of primary and secondary oxidation reaction products (i.e. conjugated double bonds and Hexanal, respectively) during subsequent storage, and no significant or relevant differences were observed as compared to non- pasteurised almonds.

In March 2008, an application file was handed to the TERP (Technical Expert Review Panel) of the ABC (Almond Board of California, Modest, CA) applying to validate the process as suitable for almond pasteurisation securing a 5 log reduction of Salmonella SE PT 30.

The CCP pasteurisation process as presented can be appropriately applied to any type of particulate goods, such as tree nuts, oil seeds, grains and powder components. At overpressure conditions, the CCP process is able to sterilise particulate goods inactivating thermophilic endospores.

Process features

Oilseed, almond and nut pasteurisation

» Optimal process management for the best possible repeating accuracy of the process parameters

» Applicable to new as well as to existing plants

» Largest flexibility possible in order to be able to respond to the different product characteristics

Construction

» Central mixing agitator in robust steel construction. This allows only little maintenance

» No complicated seal instalments at the shaft crossover of the agitation hopper

» Optimal emptying of the agitation hopper

Capacities and dimensions

The oilseed, almond and nut pasteurisation-plant CCP by Bühler Barth is available in different capacities and dimensions which allow an exact capacity adjustment on eventually existing plants.

For more information on the system, please contact:

Jürgen Fischer, Member of the Executive Board of Bühler Barth AG in Freiberg / Germany
Tel: +49 (0)7141 705 201
Fax: +49 (0)7141 705 100
E-mail: [email protected]
Bühler Barth AG
Daimlerstr. 6
D-71691 Freiberg/Neckar
T +49 7141 705-0
F +49 7141 705-100
[email protected]
www.gw-barth.de