Corin Holmes, Operations Manager for Jenike & Johanson, explains what needs to be done to improve food safety across Australia.
Food Safety Australia and New Zealand (FSANZ) places obligations on food businesses to produce safe and edible food. The associated standards seek to lower incidents of food illnesses by ensuring that unsafe food is removed from distribution, sale, and consumption. Issuing food recalls is one means of doing so, often classified by contamination problems such as undeclared allergen, microbial, foreign matter, or chemical contamination.
So far, in 2020, Product Safety Australia has listed 74 food and drink recalls. Recently the United States Food and Drug Administration (FDA) released a document entitled New Era of Smarter Food Safety – FDA’s Blueprint for the Future. The 18-page document envisions a framework that will enable food to be traced to its source in seconds utilising analytical techniques to enable real time alerts of contamination prior to consumption.
Interestingly, the blueprint concludes: “to be successful, it’s equally important for food companies and technology firms, as well as government agencies and consumers, to join in this effort.”
The problem
Many food products are manufactured from dry ingredients or sold in dry granular or powdered form. The potential for flow related problems, ranging from mild irritations to serious consequences, can arise from how ingredients are stored, transferred, or fed into or out of the production process. Problems related to solids flow in food processing plants may include:
- Segregation of blended materials (for those containing a range of particle sizes)
- Damage of friable products
- Interrupted product flow (eg. rate limitation)
- Uncontrolled product flow (eg. flooding)
- Fluctuations in bulk density over time
- Bin vibrations or structural damage
- Spoilage, caking, or microbial growth.
Food storage bins are seldom filled and then fully discharged. Instead, they are often operated on an intermittent basis with variations in fill and discharge rates. Variables influencing this may include production rate and frequency, production schedules, surge capacity, replenishment of incoming feedstocks, consumption rate, surge/storage capacity, and the ability to empty storage vessel completely. The later directly relates to understanding material flow and the FDA’s call to action in context of food recalls.
Understanding flow
There is a rational approach to designing processes and equipment to reliably handle bulk solids. Andrew Jenike’s research at the University of Utah forms the basis of modern solids flow engineering. There are measurable physical properties of bulk solids that should be used for design of storage bins. These properties, referred to as flow properties, are measured in accordance with American Society for Testing and Materials (ASTM) and Australian Standards. When flow properties of food ingredients are defined and properly applied to design, a reliable handling system can be achieved. However, when a designer is unaware of this approach, ignores or even misapplies the flow properties data in the design process, the resulting flow problems could be deadly.
Understanding flow patterns
As identified by Jenike, there are two primary flow patterns that can develop in a bin – funnel flow and mass flow as depicted in Figure 1. In funnel flow, an active flow channel forms above the outlet, with non-flowing material at the periphery. As the level of material in the vessel decreases, layers of the non-flowing material may or may not slide into the flowing channel, which can result in the formation of stable ratholes. With many materials operating in funnel flow can create serious problems with product quality or process reliability. The first-in last-out flow sequence can even cause some materials to cake, segregate, or spoil which can have serious economic and safety or health consequences. Ultimately it could even result, as in the case of food manufacturing, in a potential for recall – hopefully before someone consumes it.
Stagnant regions can occur at many points in a process, including conveyors, feeders, transfer chutes, and processing equipment such as product scrubbers, mills or dryers. The stagnant material may be visible, such as the build-up that might be seen in a transfer chute or a mill. In other cases, it may not be possible to see the stagnant material because of the design of the equipment. For example, screw conveyors and feeders frequently leave a stagnant layer of material between the flight tip and the trough. If not designed properly, hoppers can have stagnant material along the walls that may not be evident from the top surface.
In mass flow, by contrast, all of the material is in motion whenever any is withdrawn from the hopper. Material from the centre as well as the periphery moves toward the outlet. With mass flow, the material flow and bulk density are uniform and reliable; there are no stagnant regions, so material doesn’t cake or spoil. The first-in first-out flow sequence minimises provides sufficient residence so fine powders such as flour can deaerate. Mass flow containers are suitable for fine powders, cohesive (non-free flowing) bulk materials, and materials that tend to degrade/spoil when stored for extended periods of time without movement.
Understanding flow problems
Solids handling is an integral part of many food manufacturing processes. Whether the end product is a solid or liquid, chances are that powdered or granular materials are handled at some point in the process. Handling problems can cause an undesirable change resulting in a significant impact on product quality and food safety.
Two of the most common flow (handling) problems experienced in an improperly designed bin are no-flow and erratic flow. No-flow from a bin can be due to either arching or ratholing, as depicted in Figure 2. Arching occurs when an obstruction in the shape of an arch forms above the outlet of a hopper and prevents any further discharge.
It can be an interlocking arch, where the particles mechanically lock to form the obstruction. An interlocking arch occurs when the particles are large compared to the outlet size of the hopper. This is often seen with large particles, particularly those that are irregularly shaped such as breakfast cereals, corn chips or leafy vegetables that may form interlocking structures making discharge difficult.
Alternatively, an arch may form as a result of the materials cohesive characteristics. A cohesive arch occurs when particle-to-particle bonds form, allowing the material to pack together to form an obstruction. Some materials, such as brown sugar, cocoa powder or raisins are prone to bridging as a result of their cohesive strength.
Ratholing occurs in a silo when flow takes place in a channel located above the outlet. If the material being handled has sufficient cohesive strength, the stagnant material outside of this channel will be stagnant. Once the flow channel has emptied, all flow from the silo stops even though full material discharge has not occurred.
Other problems that can occur are related to fine powders, such as flour, powdered sugar and baking powder. These and other fine powders may exhibit solids-air interactions that make them prone to flooding, fluidisation or a reduced rate. Blended soup mixes and breakfast cereals are often prone to de-mixing (segregation) during storage and transfer. These bulk solids follow fundamental behaviours that, when understood, can be used to design equipment that will function reliably, eliminate stagnant regions and minimise product degradation.
The affect to food safety
Flow problems can lead to myriad safety problems such as spoilage, contamination and even dust explosions. If a material such as flour is placed in a hopper with stagnant regions, microbial growth could occur leading to the development of salmonella. If hoppers do not discharge completely and instead leave a layer of material in the bin, then cross-contamination can occur between batches or cause companies to expend resources cleaning out each hopper before switching batches. If fine powders, such as icing sugar, flood through a hopper then dust explosions could occur. These safety problems can lead to product recalls or loss of life.
Adverse product changes might include moisture absorption, caking, breakage, and/or microbial growth in the food product. Microbial growth can include pathogenic microbes that are capable of causing disease or illness. E. Coli, salmonella, and listeria outbreaks can cause illness and, in some cases, loss of life. Many factors affect the growth of microorganisms in food processing environments including the source of contamination, moisture, nutrients, pH, temperature, presence or absence of inhibitors, interactions between microorganisms in a population, and time.
Understanding the material flow properties and applying them correctly to design in order to avoid and correct flow problems common to the handling of bulk solid food materials is aligned with the FDA’s call to action and is synergetic with FSANZ’s obligation.
Another factor that can be influenced by equipment and process design is time. Time is an important factor in the growth of microorganisms since the growth rate of mature microorganisms, where conditions are not limiting, is typically exponential. Thus, where growth conditions may occur, it is important to limit the duration of exposure. In some processes, a product may pass through a temperature/moisture condition that is well suited for microbial growth. If the product has the potential to enter the process with some level of contamination it is particularly important to limit the residence time at these conditions in order to avoid excessive populations of potentially harmful microorganisms.
Even in less-than-ideal growth conditions, populations can increase. This is particularly true for in-process materials that may not be as shelf-stable as finished products. Thus, it is essential to maintain positive control of movement of the product through the process. When handling powdered, or other dry materials in bulk form, maintaining this positive control through the process can be difficult. In order to accomplish this, stagnant regions and build-up, where microbial growth can flourish, must be eliminated.
Solids handling problems are common when handling food products in bulk form, and these problems can have significant impacts on plant operating costs as well as product quality. With the constant concern for food safety, proper bin design for dry bulk materials should be undertaken. Stagnant regions should be eliminated and a first-in, first-out flow sequence should be ensured. This can only be accomplished by measuring key flow properties of the materials and using the properties in a systematic design process to ensure that the storage bin will operate as intended to minimise any negative impact on product quality.
Well-known design methods which have been validated through more than 35 years of application in the food processing industry can be employed to ensure reliable flow, maximising plant performance and ensure product quality and safety, after all, we owe it to ourselves to “join in the effort”.