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Dwell Time / Lag Time

“Dwell Time” is typically defined as the time between closing of the first can in a retort load to the start of steam-on in the retort.  Most firms use 2 hours as a maximum dwell time and any product that exceeds this time limit as a deviation.   The question often comes up as to where this 2 hour limit came from.   The answer is as follows:

 First of all, it is important to remember that the US Food and Drug Administration (FDA) has no requirements for dwell time in its low-acid canned food regulation, 21 CFR Part 113.  

 USDA, however, does mention a maximum of 2 hour dwell time (9 CFR Part 318(Meat)/ 381(Poultry).301(f)(2)which states:

 “The maximum time lapse between closing and initiation of thermal processing shall be 2 hours. However, the Administrator may specify a shorter period of time when considered necessary to ensure product safety and stability. A longer period of time between closing and the initiation of thermal processing may be permitted by the Administrator.”  

Note that USDA allows for a “longer period of time” if you can provide reasoning for such extension.    Therefore, any dwell time imposed at most canneries is self-imposed and is not based on any FDA or USDA requirement. 

 Secondly, it is important to note that there are three (3) issues to consider in a canning operation when it comes to “extended dwell time”:

1. PRODUCT INITIAL TEMPERATURE MAY DROP BELOW ALLOWABLE LIMIT IN THERMAL PROCESS

 The first concern is that the temperature of the product may fall below the minimum initial temperature (IT) specified by the process schedule BEFORE the retort process begins.   In this case, the product IT would be lower than allowed in the scheduled process and, therefore, is a safety issue.  Our concern here is whether or not the product initial temperature is being recorded at the proper point in the operation (determined shortly before the retort process begins).  More specifically, the product IT should be taken as follows..

    • The first can into the retort should be set aside and clearly marked as the IT can .
    • Once the retort is loaded and process is about to start, the operator should take the initial temperature of the IT Can by shaking its contents and then inserting a calibrated thermometer into the product cold-spot.
    • The IT measured at that point should be used as the Product Initial Temperature entered into control system prior to starting the process.
    • The control system will then use that IT to determine the process time.
    • The IT Can should then be discarded. Care should be taken NOT to mix the UN-RETORTED IT CAN with already retorted products!

 If the IT is taken immediately before the start of the process and entered into the control system, the effect of dwell time on the product initial temperature should be taken into account and will be handled accordingly (either by applying the proper process time or by flagging the process as a deviation from the minimum IT allowed).  If you are complying with the canning regulations that require product IT be taken prior to each process batch,  this should not be an issue that will be related to Dwell Time.

2. PRODUCT CHARACTERISTICS MAY CHANGE, AFFECTING ADEQUACY OF THERMAL PROCESS—

If the product’s characteristics can possibly change as the product sits for extended periods of time before retorting, the thermal process designed may not be adequate.  For example, if a starch begins to thicken while sitting for longer than normal during “extended dwell-time”, the product may become more thick and viscous compared to the thickness of the product when it was originally Heat Penetration tested.    This could change the heat penetration rate of the product and could result in the thermal process not being adequate.   You should determine if your products are formulated in a way where this would be a concern.. and if so, you would want to establish a maximum time before the product characteristics change, effecting the heat penetration of the product.    

3. INCIPIENT SPOILAGE MAY OCCUR-

 “Incipient spoilage” is generally defined as spoilage that occurs (usually due to non-pathogenic microorganisms) between packing and thermal processing.   The two most common causes of incipient spoilage result from (1) delays on the production line, usually after product has been hot-filled or run through an exhaust box, then seamed, but not processed; and (2) excessive hot holding at temperatures conducive to thermophilic spoilage.  The mesophilic organisms that cause incipient spoilage are usually destroyed by a hot-fill where they are “pasteurized”.   But thermophilic organisms become a concern in some cases.   Both the FDA and the USDA are primarily concerned about products at temperatures in the approximate temperature range of 80-130°F for long periods of time.  

Remember that monitoring and controlling dwell-time is NOT a requirement of FDA.  And while the USDA calls for 2 hours maximum, they do allow longer times if justified.   Likewise, Dwell Time is not listed as a critical factor in any thermal processes issued by TechniCAL.  

If you would like to discuss this in more detail or if we may be of any further assistance regarding this issue, please do not hesitate to contact us.

Scott Cabes- President- TechniCAL scabes@tcal.com

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