Determining the shelf life of processed foods is one of the main problems faced by food companies when launching a product to market, or when modifying some of its ingredients. This problem is especially considerable when it comes to non-perishable products, where food needs many months, or even years, to deteriorate. That is why it is very important to employ methods that correctly estimate the shelf life of products.
We understand shelf life to be the time during which a food maintains characteristics and a level of quality that is suitable for human consumption. In the food industry, the shelf life of a food is the time between the production or packaging of the product and the time when it becomes unacceptable under certain environmental conditions (Ellis, 1994) and when the consumption of said food implies a risk to consumer health.
The Directive 2000/13/EC of the European Parliament and of the Council of 20 March 2000 on the approximation of the laws of the Member States relating to the labelling, presentation and advertising of foods defines shelf life as the time from production to expiration, the end of the life of a food is when it exceeds the levels of microbiological contamination, loses its physical-chemical qualities and changes its organoleptic qualities.
From the point of view of food safety, the shelf life of the food depends on four main factors:formulation, processing, packaging and storage, which we will explain later. In countries such as Spain, each marketing company is responsible for establishing and guaranteeing the shelf life of the food it places on the market given the importance and impact on consumers’ health; making a mistake on the guarantee of the shelf life of the food carries serious consequences such as product recall, consumer complaints, market mistrust and brand reputation problems.
The expiration date and best by dates of foods
Prior to understanding the methods to determine the shelf life of foods, we must take into account two elementary concepts: the expiration date and the best by date.
The expiration date is indicated on perishable products with biological risk(s), meaning its consumption after that date can generate some degree of food poisoning. This date depends largely on factors such as optimal storage conditions, water activity, microbiological criteria or oxidation.
On the other hand, the best by date, which is applicable to more durable and stable products, indicates when a product begins to lose its physical qualities such as colour, smell or taste, but its consumption remains safe and presents no microbiological risk for the consumer. The use of antioxidants can help delay the best by date of a product.
Factors that influence food shelf life
There are several factors involved in the deterioration or loss of the original quality of a food. These factors can be divided into two types: intrinsic (inherent to the nature of the food itself) or extrinsic (external conditions facing food), and are determined by different quality parameters: organoleptic, nutritional, hygienic, physical, chemical or microbiological.
The intrinsic factors that affect shelf life are those that respond to the formulation of the food. In the food industry, it is imperative that the manufacturer has the following knowledge about its products:
- Raw materials
- Composition and formulation of the product (additives used)
- Water activity
- Total acidity and pH value
- Potential Redox
- Available oxygen
Taking all this information into account, the producer can choose the systems that maximise the life of a product according to the needs that it may have. For example, the oxidation of edible oils is a significant problem for the food industry due to the considerable increase in the use of fat and polyunsaturated oils (Frankel, 2010), so it is important to know the nutritional quality and the possible processes that the different raw materials have gone though, and to determine what antioxidants can slow down the oxidation process.
The extrinsic factors that affect the shelf life of the food are those that are present in the process, packaging and storage of the product. Mainly they are:
- Exposure to sunlight
- Damage to packaging
- Distribution and places of sale
During the different manipulation processes of the product, it is necessary to control its interaction with the components of the external system. To control of the process used every detail counts: the light permeability of the packaging, the distribution of humidity and the relative temperature, both in storage and in transportation, are the main external factors to be monitored and optimised.
Methodologies to determine foods’ shelf life
The methods most used today to estimate the shelf life of foods are:
Direct method: These are real-time studies that consist of storing the product under conditions similar to those that it will actually face, to monitor its evolution in regular intervals of time. The main advantage of this method is that it creates a very accurate estimation of the time it takes for a product to deteriorate; however, they are studies that usually take a long time and do not consider the fact that storage conditions of a product are not always stable over time.
Challenge Test: This method consists of experimentally introducing pathogens or microorganisms into the food during the production process, so that the product is exposed to the real conditions it will suffer in real life. The main disadvantage of this type of test is that the effects caused by the studied parameters are the only things analysed, and the fact that the product can be faced with multiple factors at the same time is not addressed. In addition, they are studies that are quite complex and difficult to implement.
Predictive microbiology: This methodology studies the different microbial responses of foods to varying environmental conditions, based on mathematical and statistical models, in order to predict the behaviour of the microorganisms in the product. This type of study, widely used when developing a new product, does consider the possible changing conditions of a product, however, its major limitation is that it implies greater complexity for the manufacturer and that the results correspond to a simulation, which may not be accurate.
Accelerate shelf life tests: In these tests, conditions such as temperature, oxygen pressure or moisture content are modified to accelerate spoilage reactions of a food. These predictions allow one to predict the behaviour of foods in certain conditions and to estimate how they will evolve under certain storage conditions. Accelerated tests allow the inclusion of changing environmental conditions and concentration variations of the ingredients that they are composed of. These studies are very versatile, low cost for the manufacturer and allow for the comparison of different scenarios. Obviously, since it is not an exact representation of reality, there is some margin of error in the obtained results.
Survival method: It is a type of study that is based on the opinion of the consumer about the physical characteristics of the product. It consists in knowing the attitude of people towards the same product with different dates of manufacture, to determine if they would consume it or not. This method seeks to establish a relationship between the shelf life and the perceived quality of the product. Although it is not a method to accurately estimate the shelf life, it is important to do it in a complementary way to establish the best by date of a product.
The interest in preserving food goes very far back in time. Salting, pickling or drying in the sun and air were the first attempts to extend the shelf life of food. Today, thanks to food industrialisation, companies have the responsibility to determine the shelf life of their products and to provide good quality food. The search for this quality leads producers to conduct studies and to learn the factors that cause the deterioration of their products in order to have a sustainable basis for decision making, from the type of packaging, distribution or use of antioxidants to protect food against oxidation.