Learn about steam pasteurization of almonds and other nuts: What it is, how it works, and why it works.

One pasteurization system combines steam with fluidization, then drying and cooling via a shaking mechanism.
Heat is lethal to spoilage microorganisms, but each species has its own particular heat tolerance. During a thermal destruction process such as pasteurization, the rate of destruction is logarithmic, as is the microorganisms' rate of growth. Thus, bacteria subjected to heat are killed at a rate that is proportional to the number of organisms present.

The effectiveness of the process depends both on the temperature of exposure and the time required at this temperature to accomplish the desired rate of destruction. Thermal calculations thus involve the need for knowledge of:

  • The concentration of microorganisms to be destroyed.

  • The acceptable concentration of microorganisms that can remain behind (spoilage organisms, for example, but not pathogens).

  • The thermal resistance of the target microorganisms (the most heat-tolerant ones).

  • The temperature/time relationship required for destruction of the target organisms.

    The extent of the pasteurization treatment required is determined by the heat resistance of the most heat-resistant enzyme, microorganism or pathogen in the food. For example, milk pasteurization historically was based on Mycobacterium tuberculosis and Coxiella burnetti. But, with the recognition of each new pathogen, the required time/temperature relationships are continuously reexamined.

    For almond processors, the identified pathogen is Salmonella Enteritidis Phange Type 30 (SE PT30). The almond industry has determined that a certain temperature and time period is required to destroy this pathogen. A 5 log reduction, or a 100,000 count reduction, is the goal of any treatment of SE PT30.

    Chemical, Radiation or Steam

    Three technologies can be used to pasteurize almonds to destroy salmonella: chemical, radiation (irradiation) and steam.

    The chemical method, which also is called PPO, is accepted in the United States but few other countries worldwide for pasteurizing nuts for human consumption.

    Radiation, or irradiation, using gamma rays or other isotopes, is accepted in some countries but banned in others. Because the long-term effects of food irradiation are not known, many countries forbid importation of any food being treated in this manner.

    Steam pasteurization involves heating a product to a specific temperature for a specific period of time; hence, its most common method is called high temperature short time (HTST) treatment. The processes of steam treating for pasteurization and sterilization is widely used and accepted by all countries in the world.

    Dry heat also can be used to pasteurize almonds but a higher temperature and longer processing time period are needed for the same killing effect as steam.

    The key to steam pasteurizing almonds is in the condensation power of the steam. During pasteurization, the steam is heated to super saturation and then allowed to condense on the cool surface of the almond. It is this process that kills the microorganisms.

    An analogy that may make this process clearer is a person in a dry or wet steam sauna. The temperature that a person can withstand in a dry sauna is much higher than the temperature he or she can withstand in a wet sauna. When in the steam environment, the steam condenses on the skin. It is very hot, and a person can only withstand it for a short time.

    While almond pasteurization seems simple -- just add steam -- in practice, it is more complicated than that. In order to be sure the steam condenses on all surfaces of the nut, you must add the second part of the process: fluidization by a gas medium. Steam can be used as the gas medium but you need pressure and velocity.

    One system combines steam with fluidization, then drying and cooling via a shaking mechanism. The almonds are fed into the steam pasteurizer, where superheated steam is released in a specially designed pressure area. The steam then “fluidizes” the nuts with the assistance of the shaking mechanism via a specially designed distribution plate. It is with this condensation and consequent hot moisture that the pathogen SE PT30 is “killed” and the 5 log reduction occurs. Accurate time and temperature are necessary.

    Any two nuts on a flat conveyor belt could have contact points. It is at these points that the steam could possibly not condense, creating the potential for salmonella to still exist in packaged almonds. The shaking and fluidization method ensures that all surfaces are exposed and treated equally.

    Treatment times are critical for 5 log or above kills. The almond processing time window is short. In testing, it was determined that steam treating the almonds past a certain time changed the “natural” aspects of the almond. The germ is killed and the almond is no longer and able to germinate. In addition, too long of a steaming time leads to skin damage and swelling of the almond. Too little, you do not achieve the 5 log kill rates.

    Integrated controls provide complete control over the process and can include PLC datalogging technology.

    Pasteurizing Can Add Value

    The process of drying almonds to an exact moisture level is a complex issue. Advanced controls, instruments and algorithms are necessary to control the process accurately. However, if controlled effectively, the drying process can help you realize additional profits.

    Suppose this year’s almond crop has come in with less moisture than usual. Steam treating of the almonds can add up to 5 percent moisture if desired. Based on a $2 per pound selling price, each 1 percent increase in moisture level adds $40 per ton profit. Adding 2.5 percent moisture yields an additional $100 per ton. A fluid bed dryer/cooler can accurately dry the almonds to a desired moisture level by controlling the amount of air and heat.

    A fluid bed dryer/cooler can accurately dry the almonds to a desired moisture level by controlling the amount of air and heat.

    Equipment Wish List

    The equipment for handling steam must be robust and safe. Because steam is at high pressure, great care must be undertaken in the design of the equipment so that damage to human life and other equipment does not occur.

  • The equipment design should meet local, state and national safety regulations.

  • If your processing needs require it, it should be designed for continuous operation.

    The steam pasteurizer must allow for the pressure and fluidization of the almonds as well as withstand forces. It should be sanitary and able to be cleaned in place.

    Consider selecting fully contained air-handling units equipped with HEPA filters that are 99.99 percent efficient at filtering out particulates. Other features to look for are 304L stainless steel housings, easy access doors, advanced instrumentation, and automatic heat exchangers and high-pressure/high-volume fans.

    Integrated controls provide complete control over the process and can include datalogging technology. This ensures repeatability, and thus, the validation necessary for FDA and/or USDA acceptance. PH

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