From raw materials through to processing and packaging, many factors influence the shelf life of chilled liquid foods. Our experts share insights into how filling and packaging can help you optimise your products’ time on the shelf.
Shelf life refers to the period of time during which a product retains the desired sensory, chemical, physical, and microbiological characteristics, as well as an acceptable quality for consumption from a safety point of view. Chilled liquid foods are products that contain microorganisms that can grow and multiply, and where microbial spoilage is the predominant factor limiting their shelf life – to delay this spoilage, the product must be kept chilled (defined as being between 0°C and 8°C). The design and operation of your production process must ensure that your end product is safe to consume, and will also impact its potential shelf life.
To learn more about how Tetra Pak’s expertise supports the processing of chilled liquid food products for extended shelf life, please consult our Dairy Processing Handbook, download the white paper Extending the Shelf Life of Chilled Milk and read more about Pasteurized & ESL milk.
The shelf life of chilled liquid foods, and what can happen if a product is not kept chilled or exceeds its expiration date, vary from product to product. But some potential changes in the product can be:
- Pathogenic: If a product is removed from the cold chain for too long, it may be subject to potential pathogenic issues and would no longer be safe for human consumption.
- Microbiological: The growth of microorganisms, such as various bacteria, resulting in changes in taste, smell, or appearance.
- Physical: Cream rising to the surface of milk, or a water layer forming on the surface of yoghurt, are two examples of physical changes.
- Biochemical: for example, the enzymatic degradation of milk components, giving rise to a bitter or rancid taste.
- Chemical: oxidation in orange juice causing a brownish colour is an unwanted chemical reaction.
Not all chilled liquid foods behave the same when it comes to shelf life. Raw material and processing methods play a significant role, as does the product's acidity. The shelf life of low-acid products, such as milk and cream, is most often limited by bacterial growth, while the shelf life of high-acid products – such as yoghurt and juice – is microbiologically more sensitive to yeast and mould, or changes in physical, biochemical, or chemical characteristics.
So, beyond the challenges of processing chilled liquid foods, what do you need to be aware of when considering filling and packaging solutions to optimise your product’s shelf life?
Temperature plays a critical role in determining a product's shelf life, although its precise effect varies between different chilled liquid foods. Yoghurt, for example, is a shear-sensitive product that is often filled at temperatures around 20°C to protect product viscosity and then cooled. However, in many cases, chilled liquid foods should not be handled, filled, or stored above 8°C, and a reduction to 6°C or even 4°C can drastically reduce the rate of microbiological growth – the colder, the better.
Within your chosen processing parameters, equipment, and specific product, there are still many ways you can optimise the shelf life of your chilled liquid foods. Here are the aspects you need to consider.
- Fill at low temperatures. Cooling a product after it has been packed can negatively impact shelf life as the microbiological growth rate is higher at elevated temperatures.
- Good hygiene. A key step to achieving a long shelf life is to avoid contamination during the filling stage by using filling machines with effective hygienic technologies. You should ensure that incoming air passes through adequate HEPA filters to prevent airborne contamination. Operation of the filling machine should also ensure that HEPA filters are cleaned – and replaced – according to the relevant regulations.
- Remove oxygen from the inside of the package by flushing it with nitrogen gas. In general, less oxygen leads to a longer shelf life, as oxygen leads to oxidation, which causes changes in colour, taste, and the loss of vitamin C.
- With gas-producing products such as kefir, flush the package’s head space with a mixture of nitrogen and CO2 to lower the risk of package bulging. And by decreasing the oxygen level in the head space this way, you also lower the risk of it interacting with the product’s culture in a negative way.
Packaging’s most critical role is to protect what’s inside, and different chilled liquid foods require different packaging solutions to achieve the desired shelf life. Spraying the inside of packages with peroxide and exposing them to UV radiation, or using new innovations such as e-Beam technology, are recommended to eliminate any remaining microorganisms.
With the package itself, extra barriers might be required depending on the precise characteristics of the product being packaged. For example, an additional polymer oxygen barrier in the packaging sleeve is often necessary for chilled oxygen-sensitive products, such as juice. All milk products are light-sensitive, requiring a light barrier to prevent nutritional and sensory degradation.
The package opening must also be compatible with the rest of the package in terms of integrity and surface disinfection or sterilisation capabilities. The integrity of the package is important for avoiding post-packaging bacterial contamination that might occur in sealed packages, even without any visible leakage – critical areas are seals, creases, and openings.
And the mechanical stability – that is to say, sturdiness – of a package intended for products with a long shelf life is also important, since it will be stored and distributed in cold and humid conditions for extended periods. Secondary packaging should be made of materials permeable to moisture to prevent the formation of a highly humid microclimate close to the package.
With so many factors to consider, the subject of shelf life is a complex matter when it comes to chilled liquid foods. When deciding on the target shelf life for your products, you need to consider the whole chain from raw material through processing, filling, packaging, storage, and distribution to expected consumer behaviour. One miscalculation, regardless of where in the chain, can significantly impact your final product’s shelf life. So, by working with the same partner for as many steps as possible, getting the most out of your shelf life becomes less complicated.
