Heat treatment equipment for salmonella control in animal feeds

The requirement for Salmonella control in animal feeds encouraged numerous equipment manufacturers to develop a solution. Today, we typically recognize 3 heat treatment equipment generations which are still available on the market for new projects. This article takes you through the differences and advantages of each equipment generation, concluding that the 3rd generation is at the top of the pyramid today.

Paul Alderliefste
Area Manager
CPM Europe

We have seen various concepts working the principle of increased product temperatures at increased retention times for Salmonella control during the past decennia. The heat treatment of feed stuffs has been generally considered to be a natural and quite effective approach to eliminate the heat sensitive Salmonella strains. Saturated steam is typically the preferred energy source to heat up the product.

We observe that out of all practiced heat treatment concepts, a limited group is still available and applied for new projects at our feed mills today.

Three heat treatment equipment generations:
• 1st generation continuous conditioners
• 2nd generation inclined conditioners and retention vessels
• 3rd generation conditioners with retention screw conveyors

All three generations of heat treatment equipment are working a continuous process. This means that during production, a stationary situation is obtained where all process parameters stay constant. This is typically demonstrated by uninterrupted horizontal lines at trending screens of the applied process controllers.

The formerly used heat treatment batch systems are typically phased out due to the process start-stop sequences at every intermediate batch.

DESIGN CRITERIA FOR RELIABLE SALMONELLA CONTROL SOLUTIONS
The widely accepted design criteria for reliable Salmonella control by heat treatment became available through Salmonella control auditing parties. Underneath, we limit ourselves to the actual production line and describe the characteristics of the 3 heat treatment equipment generations.

Illustration 1: Typical flow diagram heat treatment installation for animal feeds

Product Heating phase
• Set point temperature obtained prior to product retention time phase
• Automated response at temperature deviation
• Good self-cleaning out properties

Product Retention Time phase
• Adjustable between 2 and 6 min.
• First in, first out (FIFO) product flow
• Constant product temperature
• Excellent self-cleaning out properties

Optional Product Pelleting phase
• Roller slip management to prevent die chokes and process interruption
• Excellent self-cleaning out properties

Product Cooling phase
• Prevent recontamination by unfiltered aspiration system
• Prevent air leaking into the cooler at flange connections
• Excellent self-cleaning out properties

Product fines retour management
• Prevent fine particles produced by aspiration systems or screens to blend into the finished product
• Good self-cleaning out properties

1st GENERATION HEAT TREATMENT EQUIPMENT BY CONTINUOUS CONDITIONERS
We consider the use of typical continuous conditioners as the 1st generation heat treatment equipment. The filling percentage of product inside subject conditioners is typically approx. 30%; however the filling is not controlled effectively. By increasing the size of the conditioners or stacking them, the average particle retention time can be increased between approx. 60 to 90 seconds, depending on the applied capacities.

We observe that it takes a bit of time to heat up all particles by steam to the required setpoint temperature. Therefore, the actual available heat treatment retention time is reduced.

More disturbing is the absence of a First In, First Out product flow. The variation of actual product particle retention time is estimated between approx. 20 and 120 seconds. The wish for increased product filling percentages to increase retention times contradicts with the requirement for good self-cleaning properties at the end of the batch.

This makes the 1st generation heat treatment equipment suitable for an Enterobacteria and Salmonella reduction of log3 (1000:1) and is typically applied for swine- and pig feeds.

Illustration 2: Example 1st generation heat treatment equipment by stacked conditioners. The diagram illustrates the typical product temperature during the retention time in the conditioner

2nd GENERATION HEAT TREATMENT EQUIPMENT BY INCLINED CONDITIONERS
We consider the use of inclined continuous conditioners as the 2nd generation heat treatment equipment. The filling percentage of product into subject inclined conditioners is typically controlled close to 100%. This means that the average particle retention time can be increased between approx. 120 to 180 seconds depending on the applied capacities.

However, the steam is absorbed quickly on the product under a steam inlet after which the blending of hot particles has to take place. The consequence is that various steam inlets are required to heat up the material gradually in steps. Therefore, the actual heat treatment retention time at setpoint temperature is reduced.

Even more disturbing is the absence of an effective First In, First Out product flow. The variation of actual product particle retention time has been evaluated and is estimated between approx. 40 and 240 seconds where the setpoint temperature is achieved at approx. in the middle of the conditioner.

This makes subject 2nd generation heat treatment equipment suitable for an Enterobacteria and Salmonella reduction of log3 (1000:1) and log4 (10.000:1) and is typically applied for swine- , pig and layer feeds.

Illustration 3: Example 2nd generation heat treatment equipment by inclined conditioners. The diagram illustrates the typical product temperature during the retention time in the conditioner

2nd GENERATION HEAT TREATMENT EQUIPMENT BY CONDITIONERS AND RETENTION VESSELS
The application of conditioners in combination with retention vessels are also categorized within the 2nd generation of heat treatment equipment. At this concept, the product heating phase and the retention phase are well separated process steps. The special feeding and discharge solutions applied inside the retention vessels provide an effective First In, First Out product flow. However, the concept allows for low filling percentages between approx. 20% and 30% only. The large volume of the vessels compensates for this matter, but leaves significant internal vessel surface area unexposed from product. The consequence is that the applied process parameters have to anticipate for a slight temperature drop during the product retention phase.

Pre-heating of the vessel before production start is required and can take up to an hour. Drying of the vessel with hot air after production is required to prevent condensation. The product retention time can typically be adjusted between 2 and 6 minutes depending on the applied capacities.

This makes subject 2nd generation heat treatment equipment suitable for an Enterobacteria and Salmonella reduction of log4 (10.000:1) and log5 (100.000:1) and is typically applied for layer- and layer parent feeds.

Illustration 4: Example 2nd generation heat treatment equipment by conditioners and retention vessels. The diagram illustrates the typical product temperature during the retention time in the system.

3rd GENERATION HEAT TREATMENT EQUIPMENT BY CONTINUOUS
CONDITIONERS AND RETENTION SCREW CONVEYORS
The use of hygienic conditioners and retention screw conveyors are considered to be the 3rd generation heat treatment equipment. The product heating phase and the retention phase are well separated process steps.

Illustration 6: Correlation Time, Temperature and Salmonella reduction at steam conditioned feed.

The hygienic conditioner is discharging at setpoint product temperature into the retention screw conveyor only. An automatic response pauses the product discharge into the retention screw conveyor at temperature deviations by means of a hot start function. The hygienic screw conveyor can be filled up to approx. 80% with product and provides the required First In, First Out product flow. The product retention time is adjustable between 2 and 6 minutes depending on the applied capacities.

Because of the typically applied active wall and flange heating at the hygienic screw conveyor, no product temperature deviation occurs during the retention time. Consequently, no condensation takes place which results into excellent self-cleaning out properties of the system.

This makes subject 3rd generation heat treatment equipment suitable for an Enterobacteria and Salmonella reduction from log5 (100.000:1) and up. It became the preferred solution for layer parent- and layer grandparent feeds today.

Illustration 5: Example 3rd generation heat treatment equipment by hygienic conditioners and retention screw conveyor. The diagram illustrates the typical product temperature during the retention time in the system.

PELLET MILL ROLL SLIP MANAGEMENT TO PREVENT DIE CHOKES AND HEAT TREATMENT PROCESS INTERRUPTION
Each of the described heat treatment equipment generations applies saturated steam as an energy source to heat up the product. The advantage of condensing steam is that the heat energy transfer goes quickly into the product particles and penetrates through the Salmonella cell boundaries. Studies have shown that Salmonella strains are much more resilient towards any form of dry heat like for example generated by friction.

The consequence of heating by steam is a moisture increase in the feed which can be more difficult to handle by a Pellet Mill and cause a die choke. However, the application of roll speed measurement at the Pellet Mill results effectively into an anti-blocking system. An automated response to reducing roller speeds, or increasing roll slip, is for example a reduced capacity at which the steam addition is proportionally reduced as well in order to keep the heat treatment temperature stable. The product retention time is then temporarily proportionally increased for an immediate effect on the Pellet Mil. The heat treatment is continuous and stable by automated monitoring the roll speed and thus acceptance of the material by the Pellet Mill.

Illustration 7: Pellet Mill die choke and thus heat treatment interruption can be prevented by an automated response to roll speed measurement

About Paul Alderliefste
Paul Alderliefste has been working for CPM for more than 30 years. He developed a worldwide recognized expertise in hygienic feed production while being involved in the development of heat treatment equipment since the absolute beginning. In close cooperation with leading grandparent stock suppliers worldwide, he contributed to the development of the latest 3rd generation heat treatment equipment which set the standard for salmonella control at grandparent feeds today. In his current capacity of CPM Area Manager, he maintains a close contact with the stakeholders in biosecurity for animal feeds.