Predicting the impact of food processing on food constituents

Journal of Food Engineering 56 (2003) 113–117 www.elsevier.com/locate/jfoodeng

Predicting the impact of food processing on food constituents Daryl Lund

*

North Central Regional Association, University of Wisconsin-Madison, Madison, WI 53706, USA

Abstract With the advent of improved analytical techniques, statistical experiment design, computers and knowledge of food constituents, there have been significant advances in our knowledge of the impact of processing on food constituents. Unfortunately some of this information has not contributed significantly to our ability to predict the influence of processing or resulted in refined predictive equations. This paper reviews our current ability to predict the influence of processing on food constituents. Traditional and potential processes are examined and research needs are identified so that food quality quantification and prediction advances. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Modeling; Food processing; Food reaction kinetics; Food engineering

1. Introduction Since the discovery by Pasteur that microorganisms are responsible for food spoilage, scientists have been developing mathematical models to predict the effect of processing and storage on food constituents. Some of these models have been based on thermodynamic principles developed around the end of the twentieth century, while others were strictly empirical. Today models derived from both ends of the spectrum are in common use by industry and regulatory agencies. Generally these models do not provide insight into the actual mechanisms of either microbial inactivation or chemical change. In this paper, the current status of our ability to predict the impact of food processing on food constituents will be reviewed.

2. Framing the issue This Sixth Annual Nutrition Symposium organized by the Federal Research Center for Nutrition Karlsruhe was also organized to honor Prof. Walter Spiess, who has spent his career at the Center. Prof. Spiess and his research group have been very productive in analyzing the effects of thermal processing (Mayer-Miebach, Zanoni, & Spiess, 1997), freezing (Min, Wolf, Morton, & Spiess, 1994) and drying (Adam, *

Tel.: +1-608-262-2349; fax: +1-608-265-6434. E-mail address: [email protected] (D. Lund).

Muehlbauer, Esper, Wolf, & Spiess, 2000a) on several food constituents e.g. n-3 fatty acids (Koller, Wolf, & Spiess, 1997), colorants (Tevini, Spiess, Tevini, & Wolf, 2001), proteins (Mohamed, Wolf, & Spiess, 2000) and food quality factors (Adam, Muehlbauer, Esper, Wolf, & Spiess, 2000b) in food spices (e.g. paprika and marjoram), fish and vegetables (onions and potatoes). Based on excellent studies such as these in which the conditions are very clearly described and the origin of the products are known and characterized, it is possible to utilize the data to validated models to predict effect of processing on food constituents. An example can be found in the paper by Spiess, Boehme, and Wolf (1997) in which they model quality changes in food subjected to deep freezing and chilled storage. There are several papers in this symposium which directly address the effect of processing on food constituents. Some of the most relevant are: DNA changes during processing of food by Hammes, effects of fermentation on nutrients and phytochemicals by Biacs, impact of processing on functional properties of proteins by Meuser, impact of non-thermal processing in plant metabolites by Knorr, probabilistic models of food microbial safety and nutritional quality by Peleg, and an integral approach to deep fat frying by Saguy. In BruinÕs paper, he further helped to frame the subject by pointing out that the most important food constituents are those contributing to flavor, taste, color, structure and nutritional quality all of which are subjected to processes that may be classified as separating, structuring, converting or stabilizing. Although each of these

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processes are useful in transforming raw animal or plant material into consumer ready products, arguably the most important processes are those which stabilize the product preventing or diminishing further change. Without these processes, we would not be able to store food from time of plenty to time of need nor would be able to transport the food over long periods of time. For these stabilization processes, the basis of the process is usually destruction of microbial vegetative cells or spores or inactivation of enzymes. In most preservation processes, a decrease in microbial and enzymatic activities are observed. When microbial activity is the basis of a conventional or aseptic thermal process, the desired extent of inactivation is achieved when all parts of the product is exposed to a time-temperature treatment equivalent to the F-value of the process, established by the regulatory authority. The F-value is the time in minutes equivalent to F minutes at 121.1 °C. A similar expression can be developed for quality factors in foods including color, texture, flavor, and nutrients. The factor is sometimes referred to as the cook value (Cvalue) and is the equivalent time in minutes at 100 °C. Obviously when preservation of a food constituent is paramount, then the process is designed to achieve the required F-value while minimizing the C-value. Another important factor is to consider the importance of the nutrient in the diet of real people. Although we have based intake of nutrients on the ‘‘average’’ individual, none of us are truly average. Rather we belong to a subpopulation. These special subpopulations must be taken into account when any food item is assessed for nutrient availability. Subpopulations like pre-term infants, children, pregnant women, elderly, immune compromised, athletes and those with chronic disease have special needs, and maintaining an especially high level of particular nutrients in their diets may be more important than maintaining it in the diet of the normal, average population. The dietary needs of these special subpopulations have resulted in further specialization of food production, processing and marketing through the introduction of functional foods. In addition to considering that the food is going to be ingested by real people, the actual level of the nutrient in the food should be evaluated relative to the normal contribution of the nutrient from that particular food. Bender (1987) illustrated this point by analyzing a complete meal for its vitamin content. The meal originally contained 16.5 mg of Vitamin A. Approximately 50% was lost during canning and the other 50% disappeared during storage for 18 months. However, the original meal only contained 2% of the recommended dietary allowance (RDA) for Vitamin A. Thus the loss, although high in percentage, was inconsequential to the individual. Similarly, the same meal contained 9 mg of Vitamin B1 (thiamin). 75% was lost after 18 months storage, but the meal contained ten times the RDA for

Table 1 Errors in calculated rate constants k caused by analytical errors (according to Benson (1960)) Percent change in reactant species monitored Analytical precision (%)

1

Error in k (%)
0.1 14
0.5 70
1.0 >100
2.0 >100

5

10

20

30

40

50

2.8 14 28 56

1.4 7 14 38

0.7 3.5 7 15

0.5 2.5 5 10

0.4 2 4 8

0.3 1.5 3 6

Vitamin B1. Consequently the meal still provided adequate ingestion of B1 even though the loss was very high. Finally, there is a big difference between ‘‘kinetic mechanism’’ and ‘‘kinetic model’’. A kinetic mechanism actually describes the reactants, products and all intermediary products. Its description may or may not provide quantitative data on rates of reaction or influence of environmental factors although one can usually predict or infer those effects. A kinetic model, on the other hand, provides quantitative prediction of the extent of reaction under well-defined conditions (the boundary conditions and change of conditions with time). A kinetic model does not necessarily provide insight into the molecular events that occur leading to the reaction products and may only describe the final concentration relative to the initial concentration. Data on the mechanism of a reaction in food are very hard to obtain because of the complex chemical milieu of food. On the other hand, data on the kinetics of a reaction are easier to obtain since one only needs to have a method of measuring the reactants or products accurately. Consequently we have a fairly significant amount of data on which to develop kinetic models. Furthermore, advances in statistical experimental design have resulted in models on the effect of processing on food constituents. These models, it must be emphasized, do not necessarily relate to the molecular mechanism. Benson (1960) critically examined the relationship between extent of reaction (up to one half life), analytical accuracy with which a reactant could be measured, and error in the calculated rate constant. Table 1 shows that within the accuracy of determining the concentration of most food constituents (including nutrients), the reaction must proceed through at least one half life in order to provide a kinetic parameter that is within 5% of the true value.

3. Effect of processing on food constituents Table 2 summarizes the environmental factors that affect changes in food constituents. Most of the methods for extending the shelf-life of food rely on one or more

D. Lund / Journal of Food Engineering 56 (2003) 113–117 Table 2 Environmental factors affecting food constituents Physical

Tissue

Temperature Pressure Oxidation/reduction potential pH Enzymes Metals Leaching Light Processing/packaging chemicals Water activity

Maturity Cultivar State of the tissue Composition

Table 3 Processes for extending shelf-life of foods Traditional

Recent

Thermal processing Freezing Cooling Drying Fermentation Packaging

High hydrostatic pressure Ultrasonic Pulsed electric field Oscillating magnetic fields High intensity light Irradiation

physical factors. For example, conventional thermal processing relies on the effect of temperature on microorganisms and food constituents followed by appropriate packaging and storage conditions. Electromagnetic radiation effects are evident in preservation processes such as irradiation, ultraviolet light and high-intensity light. The combination of two or more physical factors leading to preserved food has been referred to as the ‘‘hurdle effect’’ and has been widely promoted and described by Leistner and Gorris (1995). Obviously when these physical principles are applied to the preservation of food, the laws of thermodynamics must be obeyed. LeChatelierÕs Principle applies to all physical processes and holds that when a system at equilibrium is disturbed the system responds in a way that tends to minimize the disturbance. Similarly the law of mass action applies. If a system reacts to produce a gas, then increasing gas pressure will reduce the rate of reaction. In our exuberance to proclaim our discoveries, sometimes the very obvious results in a publication. The most important traditional methods for extending the shelf-life of food are given in Table 3 along with the processes receiving the most recent attention. Rather than cover all the methods (since the physical laws of nature do apply), I will illustrate the quantitative models available on a selected few methods. Thermal processing, high pressure and pulsed electric field will be briefly reviewed.

4. Thermal processing Predicting the effect of thermal processing on selected food constituents has been well practiced since the early

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Table 4 Relative rate constants and activation energy for physical and chemical processes important in food science (according to Thijssen and Kerkhof (1977)) Chemical or physical process Physical properties Physical process rates Enzyme-catalyzed reactions Chemical reactions Destruction of microorganisms

D121 (min)

widely varying 0.1–400 1010 –10

Ea (kJ/ mol) 2–40 17–60 17–60 6–210 200–700

1900s. Generally the reactions are characterized as first order processes with a temperature dependence described by the Arrhenius equation. The basis of the process, usually microbial inactivation, is also generally described by first order kinetics and an empirical temperature dependence called the Thermal Death Time model. Although these models form the basis of the regulatory agency evaluation of the adequacy of thermal processes, recently several investigators have questioned the application of the first order model (Peleg, Nussinovitch, & Horowitz, 2000). It would appear that, right or wrong, the empirical model using first order kinetics has resulted in adequate thermal processes for protection of the consuming public. Design of thermal processes for maximizing nutrient retention has been practiced for several decades. Thijssen and Kerkhof (1977) summarized relative rate constants and activation energy for physical and chemical processes important in food processing (Table 4). Several observations are readily apparent by examination of this table. For example, if a process is dependent on a physical property (like viscosity), then increasing temperature will increase the rate generally but the process is not heavily temperature dependent. The same is true for processes dependent on physical rate processes (such as mass transfer coefficients). Another very significant observation from Table 3 is that destruction of microorganisms (or their spores) is much more temperature dependent than chemical reactions or enzyme-catalyzed reactions. Therefore, a higher temperature-shorter time process will favor retention of nutrients and quality factor in foods, constituents that are destroyed through conventional chemical reactions. This observation has led to the development of aseptic processing of food for those foods that can be pumped.

5. High pressure One of the more recently applied processes for preservation of food is high pressure. Commercial systems are now in use and available. Several mechanisms have been suggested for the inactivating effect of high pressure including denaturation of proteins and changes in

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cell wall permeability. Much of the literature reports little or insignificant changes in the quality factors (color and flavor especially) which would lead one to surmise that there is probably little effect on nutrients as well (Barbosa-Canovas, Pothakamury, Palou, & Swanson, 1997a). The effect of pressure on reaction rate can be described by an equation analogous to the Arrhenius equation.   d ln k RT ¼ DV þ ð1Þ dP T DV þ is the activation volume   d ln k ¼ DEþ RT dT P

ð2Þ

DEþ is the activation energy. This equation has been generally satisfactory for predictive use.

6. Pulsed electric field The effect of pulsed electric fields on microorganisms has been studied for at least three decades. Currently, commercialization of the process is under development. For it to be accepted by regulatory agencies, reproducibility and reliability for inactivation of microorganisms or their spores will be essential. The process results in inactivation presumably through changes in cell wall permeability (Barbosa-Canovas, Pothakamury, Palou, & Swanson, 1997b). Hulshegger, Potel, and Niemann (1983) described the effect of pulsed electric fields on microorganisms through an equation similar to the Arrhenius equation. n t ¼ exp½ðE  Ec =KÞ ð3Þ n0 tc E ¼ applied electric field, c ¼ critical or reference value of t and E and K ¼ kinetic parameters. More recently, Peleg (1995) suggested the following equation to describe the effect of number of pulses and their duration on inactivation of microorganisms. n 1 ¼ n0 1 þ expf½E  EðnÞ=KðnÞg

ð4Þ

EðnÞ, KðnÞ function of number of pulses (n) and/or treatment time   D ðE  ER Þ ð5Þ log ¼ DR ZðEÞ ZðEÞ is the increase in E to produce a 10 fold change in D. Very little has been published on the effect of pulsed electric fields on other food constituents (BarbosaCanovas et al., 1997b). However, given the lack of effect

thus far reported it is likely that the process is very mild, and food constituent retention is expected to be very high.

7. Conclusions Although there are many studies reported in the literature on the effects of processing on food constituents, many of them cannot be used to develop predictive equations. Frequently the history of the food is not given, and extrinsic factors that affect rates of reactions are not recorded. When investigators design experiments to study the effect of processing on food constituents, they should consider the basic tenants of good laboratory practice to obtain quantitative data that will be useful in developing predictive equations. Although many nutritionists have pointed out that the relevance of the particular constituent in a food product is dependent on the contribution of that source to the recommended daily allowance frequently studies are done and results reported that have no relevance to the modern diet. A good experimenter will take relevance in the diet into consideration when initiating a study. Although the effects of temperature and pressure on rates of reaction are well studied, there are still conflicts with interpretation of the data. Furthermore, the effects of more recent preservation/stabilization techniques on chemical degradation and loss of biological activity are described primarily for their effect on microorganisms. In most cases, little is known about their effects on rates of chemical reaction. It appears that free radical reactions are frequently involved in degradation reactions for many food constituents. Effects of pulsed electric fields, oscillating magnetic fields, and high intensity light need further elucidation. Finally, real advances in our knowledge about changes in food constituents are increasing rapidly primarily because of advances in the basic sciences. With the elucidation of the human genome and detection and measurement of reaction products, we can expect rapid improvement in our ability to predict the impact of processing on food constituents. Application of advances in basic sciences to further our understanding in the applied sciences is the responsibility of all scientists and engineers. Clearly this is the case with food science and engineering.

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