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Storage Stability: Shelf Life Testing
Storage Stability: Shelf Life Testing E Torrieri, University of Naples Federico II, Portici, Italy ã 2016 Elsevier Ltd. All rights reserved.
Introduction The quality of most foods and beverages decreases from the moment of the packaging until its consumption due to complex reactions based on biological, chemical, or physical mechanisms. There is a finite length of time after which the product becomes unacceptable. This length of time in which a foodstuff material is stored under specified packaging and environmental conditions is referred to as shelf life. A useful definition of shelf life of food has been available in IFST Guidelines: Shelf life is defined as the time during which the food product will
• • •
remain safe; be certain to retain desired sensory, chemical, physical, and microbiological characteristics; comply under the recommended conditions.
Safety and quality are the two main aspects of shelf life of food. While food safety is a legal requirement, the food quality is a choice of the manufacture to respond to consumer requirement. In fact, the words quality, acceptability, and desired characteristics can have several interpretations, depending on the product and on the market. Thus, a proper determination of the shelf life of a product is a critical point for a factory, mainly if one considers that inadequate shelf life will often lead to consumer dissatisfaction and complaints. The critical factors affecting the shelf life of a food are
• • •
product characteristics (formulation and processing), packaging, storage conditions.
All the previously mentioned factors are critical, but their relative importance depends on the specific food. In general, for most perishable foods, such as fruits and vegetables, fresh meat, and fish, factors related to initial quality of the product and to packaging and storage conditions are critical for the shelf life, whereas for thermally stabilized foods, such as some beverages, sauce, or cooked ready meal, process parameters, such as pasteurized temperature–time couple, are critical for the ending of their shelf life. The possible interplay among critical factors must be taken into consideration because they can lead to different storage conditions as a function of the product and package property. Appropriate shelf life testing is normally required to take into account the different scenarios brought about by this interplay. Once the scenarios are defined, an understanding of the key factors affecting the shelf life of a commodity is the starting point to shelf life prediction. This article focuses on the methods used to assess shelf life and in particular will focus on the description of the main steps involving shelf life determination by simulation method, in particular
188
1. identification of the deterioration mechanisms, 2. identification of critical factors for the identified deterioration mechanism, 3. identification of a quality index high related to the deterioration mechanism and its critical level related to the unacceptability of the product, 4. quantification of the evolution of the quality index as function of the time.
Identification of Deterioration Mechanisms Knowledge on deterioration reactions, which affect food quality, is the first step to determine the shelf life of a specify commodity. Fresh and processed foods deteriorate by a variety of mechanisms including oxidation, browning (enzymatic and nonenzymatic), moisture content changes, enzymatic reactions, microbial attack, structural changes, photodegradation, senescence, loss of carbonation, recrystallization of sugars, and caking. The major modes of food degradation are senescence, microbiological decay, chemical deterioration, and physical degradation. The relation between deterioration mechanisms and quality attributes that change during storage is reported in Table 1. Although several deterioration modes may occur simultaneously, it is the most sensitive one that limits shelf life. A good understanding of the interaction between the product, the package, and the environmental system can lead to a right identification of the limiting deterioration mechanism.
Factors Influencing Deterioration Process Many factors can influence the deterioration process and therefore the shelf life of foods. They can be classified as intrinsic factors, related to the final property of the product, and extrinsic factors, related the environment encountered by the product through the food chain (temperature, atmosphere composition, relative humidity (RH), and light). Food deteriorations dependent on environmental factors can be controlled by the package or package technologies. All these factors are important for the stability of the product. In this section, a brief description of the most important activation factors for foods stability in terms of shelf life is given. In particular, among the intrinsic factor, water activity will be examined, while among the extrinsic factors, temperature and gas composition will be considered. It should be emphasized that loss of quality often occurs by a combination of these factors and, when such interactions are indicated or suspected, consideration must be given to mixed effects. For instance, nonenzymatic browning is a function of food composition, moisture content, and temperature.
Encyclopedia of Food and Health
http://dx.doi.org/10.1016/B978-0-12-384947-2.00666-8
Storage Stability: Shelf Life Testing
Table 1
189
Relationship between deterioration mechanisms and quality attributes that change during storage
Deterioration process
Mechanism
Quality attribute changes
Environmental critical factors
Senescence
Catabolism of carbohydrates
Color, flavor, nutrients, texture, tissue damage
Microbiological decay Chemical reaction Physical degradation
Growth of pathogen or spoilage organism
Off-flavor, visual appearance, unsafe food
Enzymatic or nonenzymatic reaction, lipid oxidation Physical crushing, moisture loss/gain, staling
Color, aroma, taste, off-flavor loss of vitamins, degradation of proteins Texture, sedimentation, creaming
Temperature, oxygen, relative humidity Temperature, relative humidity, oxygen Temperature, oxygen, light Temperature, relative humidity
Environmental critical factors are also reported for each deterioration process.
Water Activity Water is the most important diluent of water-soluble food components and plasticizer (softener) of various watermiscible polymeric compounds as well as often the main food components. Chemical reaction, enzymatic changes, and microbial growth may occur readily in foods with high water content, while foods with low water content undergo loss of quality by water absorption. Water has several effects on food stability, palatability, and overall quality, and they are a function of the physicochemical state of water, which is related to water activity. The parameter water activity is defined as the ratio of vapor pressure above a sample, pw, and that of pure water (pwo) at the same temperature. Mathematically, aw ¼
pw pwo
The most common and generally valid relationship used to describe the influence of a constant storage temperature on the reaction rate is the Arrhenius equation: Ea [2] K ¼ A exp RT where A is the so-called pre-exponential factor, Ea is the activation energy, and R and T are the gas constant and absolute temperature, respectively. The value of Ea is a measure of the temperature sensitivity of the reaction. High activation energy implies that the reaction is strongly temperature-dependent, that is, accelerates greatly with increase in temperature. An alternative way of expressing temperature dependence of a reaction is to use the concept of Q10: Q10 ¼
[1]
It describes the degree to which water is free or bound to other components. Water activity depends on the composition, temperature, and physical state of the compounds. When a food is placed in an environment at a constant temperature and RH, it will eventually come to equilibrium with that environment. The corresponding moisture content at steady state is expressed as the equilibrium moisture content. The difference between the RH of the surrounding environment and water activity of the food determines whether a food gains or loses moisture during storage. For the main food product, steady-state relationships between aw and water content at a constant temperature are described by a sigmoid-shaped curve. This curve is known as sorption/desorption isotherm. Such plots are very useful in assessing the stability of foods and in selecting effective packaging.
Temperature Temperature is one of the most important factors affecting the shelf life of foods. Raising the storage temperature will accelerate many aging processes, as well as inactivate enzymatic reaction and/or microbiological spoilage. The relationships among temperature, respiration, transpiration, and other deteriorative changes are well documented. The models used to represent the dependence of reaction rate on temperature include the Arrhenius model, the temperature quotient, and Williams–Landel–Ferry (WLF) model.
KðTþ10Þ KT
[3]
It indicates how fast a reaction will occur if the temperature is raised by 10 C and can be used to predict expected product shelf life. The last relationship generally used to describe the dependence of food stability from temperature derives from the application of polymeric science principle and theory to food science, which has led to the glass transition temperature (tg) being put forward as an alternative, potentially preferable parameter, for defining food stability, where the rate of degradation processes is critically dependent on TTg, particularly at temperature close to Tg. If a change in a quality factor is diffusion-controlled and dependent on viscosity, the observed rate constant may become a function of the true rate constant and diffusivity. In similar studies, the observed rate constant of diffusion-controlled reactions and quality changes in amorphous food matrices have been assumed to be proportional to 1/, which has led to the direct application of the WLF equation. Mathematically, log
ks C1 T Ts ¼ k C2 þ T Ts
[4]
where k is the observed rate constant and ks is the observed rate constant at the reference temperature Ts. C1 and C2 are constants for which values of 14,44 and 51,6 are often used, although they are not always applicable. In general, because it is difficult to measure the mechanical parameter at Tg, it is more accurate to use Tg þ50 C as a reference temperature (in place of Tg) and to determine constant C1 and C2 graphically
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Storage Stability: Shelf Life Testing
from experimental data. Nelson and Labuza had discussed methods of determining WLF coefficients for assessing the applicability of the WLF equation to the temperature dependence of chemical reactions in food.
Gas Atmosphere The composition of the atmosphere that surrounds the product affects its deteriorative process. When the food product is packed, the gas composition of the package head space is a function of the packaging properties (gas permeability) and product properties (respiration rate or chemical reaction kinetics). Oxygen is an active reactant in many chemical reactions (oxidation, enzymatic browning, and physiological metabolism). Its availability has detrimental effects on the nutritive quality of foods. Thus, in many cases, removing oxygen from the surrounding atmosphere of a product leads to positive effects on the shelf life of the product. Exceptions are all the respiring products, which, after harvest, keep on respiring, consuming oxygen and sugar, and producing carbon dioxide, water, and energy. If the product has less oxygen available for the metabolic process, it reduces the respiration velocity to preserve itself. Low oxygen content also suppresses the production of ethylene, therefore delaying the onset of ripening. Moreover, concentrations of oxygen lower than the threshold value below which anaerobic condition establishes can be even detrimental for the shelf life of the product. At the same time, it is reported that high concentrations of CO2 inhibit the respiration rate, and as for oxygen, high levels of CO2 may affect negatively the quality of the product. Carbon dioxide and carbon monoxide improve the stability of the foods. In fact, high levels of carbon dioxide lead to a reduction in microbiological activity, especially mold growth, denaturation of some protein by acidification, inhibition of pectin hydrolysis, and a reduction in the cold damage of the vegetable tissue. However, different commodities have different optimum gas composition requirements for maximum shelf life.
Quality Index and Acceptability Level Once the major deterioration modes are known, and the critical factors for the identified deterioration mechanism have
Table 2
been identified, the following step is to identify quality indexes to quantify the extent of deterioration. Any quality factor should possess three characteristics: measurable, reproducible, and relevant. Moreover, the quality index has to be correlated to the acceptability of the product and must describe well the kinetic of the degradation process. An index could be a sensory, chemical, or physical parameter, depending on the product and on the degradation process. In Table 2, the main quality indexes used for measuring shelf life are reported. As a function of the food product, each quality index has a different relative importance. Thus, for bakery product, the water content and mechanical properties are the most important quality indexes, whereas for fresh red meat, microbial count, color, and odor attributes have the main role in defining product acceptability, and for fresh fish, color, odor attribute, and oxidation index are critical for shelf life evaluation. In order to establish the product shelf life, it is also necessary to define the critical level of each attribute that describes the quality evolution: the critical level must represent the level of the quality under which the product is not anymore acceptable by the consumers. Apart from microbiological quality aspect, for which the legislation provides clear indication (EU regulation (EC) No. 852/2004; 2073/2005), for the other aspects of quality, the critical level must be arbitrarily defined. Nevertheless, considering that the shelf life of food depends on the interaction of the food with the consumer, a possible approach to define the critical level is to perform a consumer acceptability test. Consumer acceptability test gives a direct measure of liking that can be used more directly to estimate the shelf life. The most common procedure is to ask consumer representative of the target population to scale acceptability on a 9-point category scale, anchored from like extremely to dislike extremely acceptability test. For this kind of test, a large number of consumers are required (minimum of 50 but preferably over 100 consumers), and suitable experimental designs should be used, in conjunction with appropriate statistical analysis. An alternative method to take consumer response into account is to estimate the probability of a consumer accepting a product beyond a time t using the survival analysis. In this method, samples with different storage times are presented to consumers. Consumers are asked a question such as ‘Would you normally consume this product? Yes or No?,’ and a survival
Main quality indexes used to measure the principal quality change of food during storage
Deterioration process Senescence Microbiological decay Chemical reaction
Physical degradation
Quality index
Quality changes
Respiration rate, ethylene, water content, sensory attributes; appearance, flavor, taste/texture, electronic noses Microbial count, pH, acidity, metabolite production, electronic noses
Color, flavor, nutrients, texture, tissue damage
Vitamin concentration, acidity, peroxide value (PV), free fatty acid content (FFA), thiobarbituric acid value (TBA), malondialdehyde content (MDA), hexanal, fatty acid composition, volatile compounds, sensory attributes, color Water content, water activity, weight loss, mechanical parameters, sensory attributes, rheological properties, glass transition temperature (Tg)
Off-flavor, visual appearance, unsafe food Color, texture, aroma, taste, off-flavor loss of vitamins, degradation of proteins, loss of essential fatty acids Texture, sedimentation, creaming
Storage Stability: Shelf Life Testing
function is defined as the probability of consumers accepting a product beyond a certain storage time. In general a 50% probability can be fixed as level of acceptability. This method has been applied to determine the shelf life of fresh-cut apple, yogurt, pan bread, UHT milk, minced beef, and ‘Fuji’ apple. Both methods described are useful to establish in specific package and storage condition the acceptability level in terms of storage days. Then, in order to define the critical level of each quality attribute, it is necessary to know the value of the attribute at the specific time (critical storage days) in the specific condition. For example, if, from the survival analysis, results highlight that after 7 days, the probability that a food product packed in a specific package and a fixed environmental condition can be not anymore acceptable by 50% of the consumers, the corresponding critical level of each quality attribute will be the value of the attribute at 7 days in the same storage conditions. Then, that value will be a reference value for whatever storage conditions, that is, the critical acceptability value of quality index of the specific product.
Quantification of the Evolution of the Quality Index: Shelf Life Test The knowledge of the main deterioration processes and of the factors affecting food stability is the starting point to measure the shelf life of a given product. With the expression ‘simulation of the shelf life,’ one refers to a classic approach consisting in representing in laboratory the real storage condition in order to determine at which time the food product loses its commercial quality. This approach is useful when exact knowledge on the deterioration process is not available and the property of a new package is unknown or when their interaction is too complex to be predicted. Moreover, this approach gives the most realistic answer, but data obtained with this method cannot be generalized and are not applicable to different conditions of storage or to different product typologies. The determination of shelf life by simulation of real storage condition is costly and time-consuming. Efficient design of experiments is important. Commonly, a single batch of product (or replicate batches) is stored under controlled condition and sampled at time zero and at regular time intervals on the bases of the expected shelf life. This type of design has the clear advantage that data related to shelf life are generated at intervals and provide a moving picture of deteriorative change taking place. An ideal design for evaluating sensory and instrumental properties at different times of storage requires the availability of a large number of samples to be tested all together. In principle, this can be done in three different ways: A possibility is to use samples from successive production batches so that samples with a given shelf life are available on a wide range of time. This approach may suffer if there is fluctuation in production quality and is convenient only in situations in which production consistency can be assured. A variant of this design consists in the use of a single large batch, which is held in conditions that assure that quality changes are effectively zero, for example, frozen storage. Samples are removed from the frozen chamber at appropriate intervals and stored under the
191
desired conditions. The last possibility is that a large batch of the product is sampled and samples are picked up after appropriate time intervals and kept under nonchanging conditions until the required storage has been reached. The main difficulty of the last two experimental designs is to identify appropriate nonchanging storage conditions, since a few foods can be stored at freezing temperature without changes in some important quality attributes. Once the most appropriate design for shelf life experimental is chosen, the storage conditions need to be chosen carefully, and a good understanding of the storage, handling, and climacteric conditions of the markets in which the product will be sold is essential to the accuracy of shelf life tests. Generally, for chilled foods and frozen foods, temperatures are typically set between 0 and 10 C and around 18 C, respectively. For shelf-stable food, shelf life test temperature must be chosen based on climatic conditions: 38–40 C/80–90 RH for tropical climates and 20–25 C/50–70% RH for temperate conditions. Once the storage condition is defined, the quality over time should be monitored. This is done by measuring attributes correlated to the deterioration processes. Chemical, physical, and microbiological analyses are performed according to the product characteristics and the deterioration processes in act (Table 2). As a function of the expected shelf life, one must decide the time interval of test and the number of samples for each test. Time interval mainly depends on quality degradation kinetics. In the majority of the cases, a constant time interval can be convenient in order to describe the quality changes during time, even if in the same situation, it can be useful to have a high frequency at the end of the shelf life. For example, for microbial growth, the first part of the growth curve is stationary, whereas in the exponential phase, the quality changes are faster. Then, one must decide the duration of the test. Usually, for highly perishable products, shelf life tests last maximum to 15 days, whereas for more stable product, the duration of a test can range from months to years. In this last case, it can be convenient to follow a different shelf life testing procedure based on shelf life prediction.
See also: Emerging Foodborne Enteric Bacterial Pathogens; Food Poisoning: Classification; Food Poisoning: Tracing Origins and Testing; Foodborne Pathogens; Heavy Metal Toxicology; Meat: Eating Quality and Preservation; Mycotoxins: Occurrence and Determination; Preservation of Foods; Spoilage: Bacterial Spoilage; Spoilage: Yeast Spoilage of Food and Beverages.
Further Reading Gambaro A, Fiszman S, Gimenez A, Varala P, and Salvador A (2004) Consumer acceptability compared with sensory and instrumental measures of white pan bread: sensory shelf-life estimation by survival analysis. Journal of Food Science 69(9): S401–S405. Hough G, Langohr K, Go´mez G, and Curia A (2003) Survival analysis applied to sensory shelf life of foods. Journal of Food Science 68(1): 359–362. IFST (1993) Shelf life of foods – guidelines for its determination and prediction. London: IFTS. Kilkast D and Subramaniam P (2011) Food and beverage stability and shelf life. England: Woodhead Publishing.
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Labuza TP (1982) Introduction to open dating. In: Shelf life dating of food. Westport, CT: Food & Nutrition Press. Labuza TB (1984) Application of chemical kinetics to deterioration of foods. Journal of Chemical Education 61(4): 348–357. Lee DS, Yam KL, and Piergiovanni L (2008a) Shelf life of packaged food product. In: Lee DS, Yam KL, and Piergiovanni L (eds.) Food packaging science and technology, pp. 479–540. Boca Raton, FL: CRC Press/Taylor & Francis Group. Lee DS, Yam KL, and Piergiovanni L (2008b) Vacuum/modified atmosphere packaging. In: Lee DS, Yam KL, and Piergiovanni L (eds.) Food packaging science and technology, pp. 397–422. Boca Raton, FL: CRC Press, Taylor & Francis Group. Man CMD and Jones AA (2000) Shelf life evaluation of foods, 2nd ed. Gaithersburg, MD: AN Aspen Publication. Nelson KA and Labuza TP (1994) Water activity and food polymer science: implication of state on Arrhenius and WLF models in predicting shelf life. Journal of Food Engineering 22: 271–289. Robertson GL (1993) Shelf life of food. In: Food packaging. Principles and practice. New York: Marcel Dekker, Inc. Robertson LG (2010) Food packaging and shelf life: a practical guide. Boca Raton, FL: CRC Press/Taylor & Francis Group. Torrieri E, Cavella S, Villani F, and Masi P (2006) Influence of modified atmosphere packaging on the chilled shelf life of gutted farmed bass (Dicentrarchus labrax). Journal of Food Engineering 77: 1078–1086. Torrieri E, Di Monaco R, Cavella S, and Masi P (2008) Fresh-cut annurca apples: acceptability study and shelf life determination. Journal of Sensory Studies 23(3): 195–203.
Torrieri E, Russo F, Di Monaco R, Cavella S, Villani F, and Masi P (2011a) Shelf life prediction of fresh Italian pork sausage modified atmosphere packed. Food Science and Technology International 17(3): 223–232. Torrieri E, Carlino PA, Cavella S, et al. (2011b) Effect of modified atmosphere and active packaging on the shelf-life of fresh bluefin tuna fillets. Journal of Food Engineering 105(3): 429–435. Torrieri E, Pepe O, Ventorino V, Masi P, and Cavella S (2014) Effect of sourdough at different concentrations on quality and shelf life of bread. LWT - Food Science and Technology 56(2): 508–516. Van Boekel MAJS (1996) A critical review: statistical aspects of kinetic modelling for food science problems. Journal of Food Science 61(3): 477–485.
Relevant Websites www.eatbydate.com – Eat By Date: How long does food last? www.efsa.europa.eu – European Food Safety Authority. www.eufic.org – European Food Information Council. https://www.fsai.ie/faq/shelf_life.html – Food Safety Authority of Ireland: Shelf-Life. www.food.gov.uk – Food Standards Agency. www.ift.org – Institute of Food Technologists. https://food.unl.edu – University of Nebraska-Lincoln: Food. www.fda.gov – US Food and Drug Administration.
Introduction The quality of most foods and beverages decreases from the moment of the packaging until its consumption due to complex reactions based on biological, chemical, or physical mechanisms. There is a finite length of time after which the product becomes unacceptable. This length of time in which a foodstuff material is stored under specified packaging and environmental conditions is referred to as shelf life. A useful definition of shelf life of food has been available in IFST Guidelines: Shelf life is defined as the time during which the food product will
• • •
remain safe; be certain to retain desired sensory, chemical, physical, and microbiological characteristics; comply under the recommended conditions.
Safety and quality are the two main aspects of shelf life of food. While food safety is a legal requirement, the food quality is a choice of the manufacture to respond to consumer requirement. In fact, the words quality, acceptability, and desired characteristics can have several interpretations, depending on the product and on the market. Thus, a proper determination of the shelf life of a product is a critical point for a factory, mainly if one considers that inadequate shelf life will often lead to consumer dissatisfaction and complaints. The critical factors affecting the shelf life of a food are
• • •
product characteristics (formulation and processing), packaging, storage conditions.
All the previously mentioned factors are critical, but their relative importance depends on the specific food. In general, for most perishable foods, such as fruits and vegetables, fresh meat, and fish, factors related to initial quality of the product and to packaging and storage conditions are critical for the shelf life, whereas for thermally stabilized foods, such as some beverages, sauce, or cooked ready meal, process parameters, such as pasteurized temperature–time couple, are critical for the ending of their shelf life. The possible interplay among critical factors must be taken into consideration because they can lead to different storage conditions as a function of the product and package property. Appropriate shelf life testing is normally required to take into account the different scenarios brought about by this interplay. Once the scenarios are defined, an understanding of the key factors affecting the shelf life of a commodity is the starting point to shelf life prediction. This article focuses on the methods used to assess shelf life and in particular will focus on the description of the main steps involving shelf life determination by simulation method, in particular
188
1. identification of the deterioration mechanisms, 2. identification of critical factors for the identified deterioration mechanism, 3. identification of a quality index high related to the deterioration mechanism and its critical level related to the unacceptability of the product, 4. quantification of the evolution of the quality index as function of the time.
Identification of Deterioration Mechanisms Knowledge on deterioration reactions, which affect food quality, is the first step to determine the shelf life of a specify commodity. Fresh and processed foods deteriorate by a variety of mechanisms including oxidation, browning (enzymatic and nonenzymatic), moisture content changes, enzymatic reactions, microbial attack, structural changes, photodegradation, senescence, loss of carbonation, recrystallization of sugars, and caking. The major modes of food degradation are senescence, microbiological decay, chemical deterioration, and physical degradation. The relation between deterioration mechanisms and quality attributes that change during storage is reported in Table 1. Although several deterioration modes may occur simultaneously, it is the most sensitive one that limits shelf life. A good understanding of the interaction between the product, the package, and the environmental system can lead to a right identification of the limiting deterioration mechanism.
Factors Influencing Deterioration Process Many factors can influence the deterioration process and therefore the shelf life of foods. They can be classified as intrinsic factors, related to the final property of the product, and extrinsic factors, related the environment encountered by the product through the food chain (temperature, atmosphere composition, relative humidity (RH), and light). Food deteriorations dependent on environmental factors can be controlled by the package or package technologies. All these factors are important for the stability of the product. In this section, a brief description of the most important activation factors for foods stability in terms of shelf life is given. In particular, among the intrinsic factor, water activity will be examined, while among the extrinsic factors, temperature and gas composition will be considered. It should be emphasized that loss of quality often occurs by a combination of these factors and, when such interactions are indicated or suspected, consideration must be given to mixed effects. For instance, nonenzymatic browning is a function of food composition, moisture content, and temperature.
Encyclopedia of Food and Health
http://dx.doi.org/10.1016/B978-0-12-384947-2.00666-8
Storage Stability: Shelf Life Testing
Table 1
189
Relationship between deterioration mechanisms and quality attributes that change during storage
Deterioration process
Mechanism
Quality attribute changes
Environmental critical factors
Senescence
Catabolism of carbohydrates
Color, flavor, nutrients, texture, tissue damage
Microbiological decay Chemical reaction Physical degradation
Growth of pathogen or spoilage organism
Off-flavor, visual appearance, unsafe food
Enzymatic or nonenzymatic reaction, lipid oxidation Physical crushing, moisture loss/gain, staling
Color, aroma, taste, off-flavor loss of vitamins, degradation of proteins Texture, sedimentation, creaming
Temperature, oxygen, relative humidity Temperature, relative humidity, oxygen Temperature, oxygen, light Temperature, relative humidity
Environmental critical factors are also reported for each deterioration process.
Water Activity Water is the most important diluent of water-soluble food components and plasticizer (softener) of various watermiscible polymeric compounds as well as often the main food components. Chemical reaction, enzymatic changes, and microbial growth may occur readily in foods with high water content, while foods with low water content undergo loss of quality by water absorption. Water has several effects on food stability, palatability, and overall quality, and they are a function of the physicochemical state of water, which is related to water activity. The parameter water activity is defined as the ratio of vapor pressure above a sample, pw, and that of pure water (pwo) at the same temperature. Mathematically, aw ¼
pw pwo
The most common and generally valid relationship used to describe the influence of a constant storage temperature on the reaction rate is the Arrhenius equation: Ea [2] K ¼ A exp RT where A is the so-called pre-exponential factor, Ea is the activation energy, and R and T are the gas constant and absolute temperature, respectively. The value of Ea is a measure of the temperature sensitivity of the reaction. High activation energy implies that the reaction is strongly temperature-dependent, that is, accelerates greatly with increase in temperature. An alternative way of expressing temperature dependence of a reaction is to use the concept of Q10: Q10 ¼
[1]
It describes the degree to which water is free or bound to other components. Water activity depends on the composition, temperature, and physical state of the compounds. When a food is placed in an environment at a constant temperature and RH, it will eventually come to equilibrium with that environment. The corresponding moisture content at steady state is expressed as the equilibrium moisture content. The difference between the RH of the surrounding environment and water activity of the food determines whether a food gains or loses moisture during storage. For the main food product, steady-state relationships between aw and water content at a constant temperature are described by a sigmoid-shaped curve. This curve is known as sorption/desorption isotherm. Such plots are very useful in assessing the stability of foods and in selecting effective packaging.
Temperature Temperature is one of the most important factors affecting the shelf life of foods. Raising the storage temperature will accelerate many aging processes, as well as inactivate enzymatic reaction and/or microbiological spoilage. The relationships among temperature, respiration, transpiration, and other deteriorative changes are well documented. The models used to represent the dependence of reaction rate on temperature include the Arrhenius model, the temperature quotient, and Williams–Landel–Ferry (WLF) model.
KðTþ10Þ KT
[3]
It indicates how fast a reaction will occur if the temperature is raised by 10 C and can be used to predict expected product shelf life. The last relationship generally used to describe the dependence of food stability from temperature derives from the application of polymeric science principle and theory to food science, which has led to the glass transition temperature (tg) being put forward as an alternative, potentially preferable parameter, for defining food stability, where the rate of degradation processes is critically dependent on TTg, particularly at temperature close to Tg. If a change in a quality factor is diffusion-controlled and dependent on viscosity, the observed rate constant may become a function of the true rate constant and diffusivity. In similar studies, the observed rate constant of diffusion-controlled reactions and quality changes in amorphous food matrices have been assumed to be proportional to 1/, which has led to the direct application of the WLF equation. Mathematically, log
ks C1 T Ts ¼ k C2 þ T Ts
[4]
where k is the observed rate constant and ks is the observed rate constant at the reference temperature Ts. C1 and C2 are constants for which values of 14,44 and 51,6 are often used, although they are not always applicable. In general, because it is difficult to measure the mechanical parameter at Tg, it is more accurate to use Tg þ50 C as a reference temperature (in place of Tg) and to determine constant C1 and C2 graphically
190
Storage Stability: Shelf Life Testing
from experimental data. Nelson and Labuza had discussed methods of determining WLF coefficients for assessing the applicability of the WLF equation to the temperature dependence of chemical reactions in food.
Gas Atmosphere The composition of the atmosphere that surrounds the product affects its deteriorative process. When the food product is packed, the gas composition of the package head space is a function of the packaging properties (gas permeability) and product properties (respiration rate or chemical reaction kinetics). Oxygen is an active reactant in many chemical reactions (oxidation, enzymatic browning, and physiological metabolism). Its availability has detrimental effects on the nutritive quality of foods. Thus, in many cases, removing oxygen from the surrounding atmosphere of a product leads to positive effects on the shelf life of the product. Exceptions are all the respiring products, which, after harvest, keep on respiring, consuming oxygen and sugar, and producing carbon dioxide, water, and energy. If the product has less oxygen available for the metabolic process, it reduces the respiration velocity to preserve itself. Low oxygen content also suppresses the production of ethylene, therefore delaying the onset of ripening. Moreover, concentrations of oxygen lower than the threshold value below which anaerobic condition establishes can be even detrimental for the shelf life of the product. At the same time, it is reported that high concentrations of CO2 inhibit the respiration rate, and as for oxygen, high levels of CO2 may affect negatively the quality of the product. Carbon dioxide and carbon monoxide improve the stability of the foods. In fact, high levels of carbon dioxide lead to a reduction in microbiological activity, especially mold growth, denaturation of some protein by acidification, inhibition of pectin hydrolysis, and a reduction in the cold damage of the vegetable tissue. However, different commodities have different optimum gas composition requirements for maximum shelf life.
Quality Index and Acceptability Level Once the major deterioration modes are known, and the critical factors for the identified deterioration mechanism have
Table 2
been identified, the following step is to identify quality indexes to quantify the extent of deterioration. Any quality factor should possess three characteristics: measurable, reproducible, and relevant. Moreover, the quality index has to be correlated to the acceptability of the product and must describe well the kinetic of the degradation process. An index could be a sensory, chemical, or physical parameter, depending on the product and on the degradation process. In Table 2, the main quality indexes used for measuring shelf life are reported. As a function of the food product, each quality index has a different relative importance. Thus, for bakery product, the water content and mechanical properties are the most important quality indexes, whereas for fresh red meat, microbial count, color, and odor attributes have the main role in defining product acceptability, and for fresh fish, color, odor attribute, and oxidation index are critical for shelf life evaluation. In order to establish the product shelf life, it is also necessary to define the critical level of each attribute that describes the quality evolution: the critical level must represent the level of the quality under which the product is not anymore acceptable by the consumers. Apart from microbiological quality aspect, for which the legislation provides clear indication (EU regulation (EC) No. 852/2004; 2073/2005), for the other aspects of quality, the critical level must be arbitrarily defined. Nevertheless, considering that the shelf life of food depends on the interaction of the food with the consumer, a possible approach to define the critical level is to perform a consumer acceptability test. Consumer acceptability test gives a direct measure of liking that can be used more directly to estimate the shelf life. The most common procedure is to ask consumer representative of the target population to scale acceptability on a 9-point category scale, anchored from like extremely to dislike extremely acceptability test. For this kind of test, a large number of consumers are required (minimum of 50 but preferably over 100 consumers), and suitable experimental designs should be used, in conjunction with appropriate statistical analysis. An alternative method to take consumer response into account is to estimate the probability of a consumer accepting a product beyond a time t using the survival analysis. In this method, samples with different storage times are presented to consumers. Consumers are asked a question such as ‘Would you normally consume this product? Yes or No?,’ and a survival
Main quality indexes used to measure the principal quality change of food during storage
Deterioration process Senescence Microbiological decay Chemical reaction
Physical degradation
Quality index
Quality changes
Respiration rate, ethylene, water content, sensory attributes; appearance, flavor, taste/texture, electronic noses Microbial count, pH, acidity, metabolite production, electronic noses
Color, flavor, nutrients, texture, tissue damage
Vitamin concentration, acidity, peroxide value (PV), free fatty acid content (FFA), thiobarbituric acid value (TBA), malondialdehyde content (MDA), hexanal, fatty acid composition, volatile compounds, sensory attributes, color Water content, water activity, weight loss, mechanical parameters, sensory attributes, rheological properties, glass transition temperature (Tg)
Off-flavor, visual appearance, unsafe food Color, texture, aroma, taste, off-flavor loss of vitamins, degradation of proteins, loss of essential fatty acids Texture, sedimentation, creaming
Storage Stability: Shelf Life Testing
function is defined as the probability of consumers accepting a product beyond a certain storage time. In general a 50% probability can be fixed as level of acceptability. This method has been applied to determine the shelf life of fresh-cut apple, yogurt, pan bread, UHT milk, minced beef, and ‘Fuji’ apple. Both methods described are useful to establish in specific package and storage condition the acceptability level in terms of storage days. Then, in order to define the critical level of each quality attribute, it is necessary to know the value of the attribute at the specific time (critical storage days) in the specific condition. For example, if, from the survival analysis, results highlight that after 7 days, the probability that a food product packed in a specific package and a fixed environmental condition can be not anymore acceptable by 50% of the consumers, the corresponding critical level of each quality attribute will be the value of the attribute at 7 days in the same storage conditions. Then, that value will be a reference value for whatever storage conditions, that is, the critical acceptability value of quality index of the specific product.
Quantification of the Evolution of the Quality Index: Shelf Life Test The knowledge of the main deterioration processes and of the factors affecting food stability is the starting point to measure the shelf life of a given product. With the expression ‘simulation of the shelf life,’ one refers to a classic approach consisting in representing in laboratory the real storage condition in order to determine at which time the food product loses its commercial quality. This approach is useful when exact knowledge on the deterioration process is not available and the property of a new package is unknown or when their interaction is too complex to be predicted. Moreover, this approach gives the most realistic answer, but data obtained with this method cannot be generalized and are not applicable to different conditions of storage or to different product typologies. The determination of shelf life by simulation of real storage condition is costly and time-consuming. Efficient design of experiments is important. Commonly, a single batch of product (or replicate batches) is stored under controlled condition and sampled at time zero and at regular time intervals on the bases of the expected shelf life. This type of design has the clear advantage that data related to shelf life are generated at intervals and provide a moving picture of deteriorative change taking place. An ideal design for evaluating sensory and instrumental properties at different times of storage requires the availability of a large number of samples to be tested all together. In principle, this can be done in three different ways: A possibility is to use samples from successive production batches so that samples with a given shelf life are available on a wide range of time. This approach may suffer if there is fluctuation in production quality and is convenient only in situations in which production consistency can be assured. A variant of this design consists in the use of a single large batch, which is held in conditions that assure that quality changes are effectively zero, for example, frozen storage. Samples are removed from the frozen chamber at appropriate intervals and stored under the
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desired conditions. The last possibility is that a large batch of the product is sampled and samples are picked up after appropriate time intervals and kept under nonchanging conditions until the required storage has been reached. The main difficulty of the last two experimental designs is to identify appropriate nonchanging storage conditions, since a few foods can be stored at freezing temperature without changes in some important quality attributes. Once the most appropriate design for shelf life experimental is chosen, the storage conditions need to be chosen carefully, and a good understanding of the storage, handling, and climacteric conditions of the markets in which the product will be sold is essential to the accuracy of shelf life tests. Generally, for chilled foods and frozen foods, temperatures are typically set between 0 and 10 C and around 18 C, respectively. For shelf-stable food, shelf life test temperature must be chosen based on climatic conditions: 38–40 C/80–90 RH for tropical climates and 20–25 C/50–70% RH for temperate conditions. Once the storage condition is defined, the quality over time should be monitored. This is done by measuring attributes correlated to the deterioration processes. Chemical, physical, and microbiological analyses are performed according to the product characteristics and the deterioration processes in act (Table 2). As a function of the expected shelf life, one must decide the time interval of test and the number of samples for each test. Time interval mainly depends on quality degradation kinetics. In the majority of the cases, a constant time interval can be convenient in order to describe the quality changes during time, even if in the same situation, it can be useful to have a high frequency at the end of the shelf life. For example, for microbial growth, the first part of the growth curve is stationary, whereas in the exponential phase, the quality changes are faster. Then, one must decide the duration of the test. Usually, for highly perishable products, shelf life tests last maximum to 15 days, whereas for more stable product, the duration of a test can range from months to years. In this last case, it can be convenient to follow a different shelf life testing procedure based on shelf life prediction.
See also: Emerging Foodborne Enteric Bacterial Pathogens; Food Poisoning: Classification; Food Poisoning: Tracing Origins and Testing; Foodborne Pathogens; Heavy Metal Toxicology; Meat: Eating Quality and Preservation; Mycotoxins: Occurrence and Determination; Preservation of Foods; Spoilage: Bacterial Spoilage; Spoilage: Yeast Spoilage of Food and Beverages.
Further Reading Gambaro A, Fiszman S, Gimenez A, Varala P, and Salvador A (2004) Consumer acceptability compared with sensory and instrumental measures of white pan bread: sensory shelf-life estimation by survival analysis. Journal of Food Science 69(9): S401–S405. Hough G, Langohr K, Go´mez G, and Curia A (2003) Survival analysis applied to sensory shelf life of foods. Journal of Food Science 68(1): 359–362. IFST (1993) Shelf life of foods – guidelines for its determination and prediction. London: IFTS. Kilkast D and Subramaniam P (2011) Food and beverage stability and shelf life. England: Woodhead Publishing.
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Storage Stability: Shelf Life Testing
Labuza TP (1982) Introduction to open dating. In: Shelf life dating of food. Westport, CT: Food & Nutrition Press. Labuza TB (1984) Application of chemical kinetics to deterioration of foods. Journal of Chemical Education 61(4): 348–357. Lee DS, Yam KL, and Piergiovanni L (2008a) Shelf life of packaged food product. In: Lee DS, Yam KL, and Piergiovanni L (eds.) Food packaging science and technology, pp. 479–540. Boca Raton, FL: CRC Press/Taylor & Francis Group. Lee DS, Yam KL, and Piergiovanni L (2008b) Vacuum/modified atmosphere packaging. In: Lee DS, Yam KL, and Piergiovanni L (eds.) Food packaging science and technology, pp. 397–422. Boca Raton, FL: CRC Press, Taylor & Francis Group. Man CMD and Jones AA (2000) Shelf life evaluation of foods, 2nd ed. Gaithersburg, MD: AN Aspen Publication. Nelson KA and Labuza TP (1994) Water activity and food polymer science: implication of state on Arrhenius and WLF models in predicting shelf life. Journal of Food Engineering 22: 271–289. Robertson GL (1993) Shelf life of food. In: Food packaging. Principles and practice. New York: Marcel Dekker, Inc. Robertson LG (2010) Food packaging and shelf life: a practical guide. Boca Raton, FL: CRC Press/Taylor & Francis Group. Torrieri E, Cavella S, Villani F, and Masi P (2006) Influence of modified atmosphere packaging on the chilled shelf life of gutted farmed bass (Dicentrarchus labrax). Journal of Food Engineering 77: 1078–1086. Torrieri E, Di Monaco R, Cavella S, and Masi P (2008) Fresh-cut annurca apples: acceptability study and shelf life determination. Journal of Sensory Studies 23(3): 195–203.
Torrieri E, Russo F, Di Monaco R, Cavella S, Villani F, and Masi P (2011a) Shelf life prediction of fresh Italian pork sausage modified atmosphere packed. Food Science and Technology International 17(3): 223–232. Torrieri E, Carlino PA, Cavella S, et al. (2011b) Effect of modified atmosphere and active packaging on the shelf-life of fresh bluefin tuna fillets. Journal of Food Engineering 105(3): 429–435. Torrieri E, Pepe O, Ventorino V, Masi P, and Cavella S (2014) Effect of sourdough at different concentrations on quality and shelf life of bread. LWT - Food Science and Technology 56(2): 508–516. Van Boekel MAJS (1996) A critical review: statistical aspects of kinetic modelling for food science problems. Journal of Food Science 61(3): 477–485.
Relevant Websites www.eatbydate.com – Eat By Date: How long does food last? www.efsa.europa.eu – European Food Safety Authority. www.eufic.org – European Food Information Council. https://www.fsai.ie/faq/shelf_life.html – Food Safety Authority of Ireland: Shelf-Life. www.food.gov.uk – Food Standards Agency. www.ift.org – Institute of Food Technologists. https://food.unl.edu – University of Nebraska-Lincoln: Food. www.fda.gov – US Food and Drug Administration.