Success for Recipes

Tony Hasting discusses process and equipment design for food safety

THIS is the second article in a series examining the role that chemical engineers play in food safety, this time looking at process and equipment design.

Of the main types of contamination described in the first article (All You Can Eat, issue 948), that most closely associated with process design is microbiology, with specific processes having been developed to either kill (inactivate) the organisms of concern or minimise their growth rate through the supply chain. The other sources – allergens, foreign bodies, and chemicals – are mainly controlled by effective management practices. Process design usually starts with defining the final product or products, packaging, and required shelf life. It also needs to reflect a balance between capital and running costs as well as product quality, but safety must always be the top priority.

Food is processed to convert raw materials into desirable and safe products, with an acceptable shelf life. The food industry uses many of the traditional chemical engineering unit operations but those most relevant to food safety are based around:

• heating the product to temperatures that will inactivate the organisms of concern – for example, pasteurisation and sterilisation;
• cooling the product to inhibit or prevent microbiological growth – for example, cooling, chilling, and freezing;
• removing water from the product to extend shelf life by reducing water activity – for example, evaporation and drying; and
• packaging – enclosing the product to maintain safety and quality throughout its distribution, storage, sale, and consumer use – for example active packaging that incorporates components into the packaging that prolong shelf life and maintain safety and quality, and aseptic packaging.

The industry is increasingly influenced by consumer demands. In the soft drinks industry, pasteurised non-carbonated products are susceptible to microbial growth, and a range of additives such as benzoates, sorbates, sulfites and dimethyldicarbonate (DMDC) are permitted for industry use.¹ A combination of consumer resistance to additives as well as an increasing concern over preservative-resistant yeasts in these products resulted in a decision by some manufacturers to remove these chemicals, whilst maintaining the shelf life. This required changing the filler to a far more complex aseptic system. This involved chemically sterilising and then rinsing the bottle and caps with sterile water prior to filling. The filler and capper also needed to be sterilised prior to production and a sterile air overpressure was maintained during production to the critical parts of the line. This added significant complexity and cost to the operation but allowed a “clean” label, free from additives.

Pasteurisation and sterilisation both use heat to inactivate micro-organisms, the main difference being in the temperatures used and the potential shelf life achievable. Pasteurisation is widely used, particularly targeting pathogens in low-acid foods such as milk and ice cream as well as yeasts and moulds in beverages. The process will not result in a sterile product and surviving spoilage organisms will be present. These are then controlled by chilling (in the case of milk and cream), and freezing of ice cream.

Sterilisation aims to inactivate all microorganisms by using higher temperatures, typically 120–150^oC and when combined with aseptic filling will provide a shelf life of several months. This is widely used in countries whose size and climate makes chill distribution less practical.

Microorganisms grow over a limited temperature range above which there is no further growth and at temperatures above 60^oC vegetative organisms start to die, for example salmonella bacteria found in raw meat, eggs, and unpasteurised milk. Spores are more heat resistant and require temperatures above 100^oC to inactivate them. Inactivation is a combination of time and temperature with temperature being the dominant parameter. It is based on first-order reaction kinetics such that a plot of log (number of survivors) versus time will be a straight line.² As microbial numbers are generally large, scientific notation is used. From this the decimal reduction time, D, the time in minutes at a given temperature for the surviving population to be reduced by 1 log, ie 90%, can be determined (see Figure 1). Pasteurisation is typically designed to achieve a 5-6 log reduction of the most heat-resistant organism that could realistically be expected in the product.

The D value also changes with temperature and a plot of log D versus temperature also results in a straight line. The temperature change equivalent to a 1 log reduction in D is termed z and is a measure of the heat resistance of the organism: the higher the value, the more resistant the organism. Typical z values are 7–12^oC for bacterial spores and 4–8^oC for yeasts and vegetative bacteria.

This enables the rate of inactivation of the organism as a function of temperature to be estimated, the lethal rate L, compared to a reference temperature:

Lethal rate L = 10^{((T – Tref)/z)}

T_ref is the reference temperature, usually 121.1^oC for sterilisation processes, and 60-80^oC for pasteurisation processes.

The total process delivered, often defined as F, is therefore lethal rate multiplied by the time, in minutes held at temperature T^oC. An F value of 4 means the process is equivalent to 4 minutes at 121.1^oC. Product quality losses also occur during the heating process, including nutrient destruction, and loss of organoleptic quality, colour, texture, and flavour. The z values for such processes are higher than for microorganism inactivation², hence a higher-temperature, shorter-time process will usually benefit product quality.

This approach was developed during the early part of the 20th century, particularly in support of the rapidly-expanding canning industry where such temperatures were typical. It has been shown to be effective despite its simplicity. Modern-day continuous sterilisation of liquid products uses higher temperatures and shorter holding times to achieve a higher quality product whilst ensuring an adequate thermal process is delivered.

Whatever the process and time-temperature combination selected, consideration must be given to the type of process selected: