Department of Food Engineering

Dr. Mehmet Özkan

Department of Food Engineering

Ankara University

Phone: 203 3300/3621

e-mail: ,

Office Hours: 09:00–11:00 (Thursday)

Teaching Assistant Fatmagül Hamzaoğlu

Department of Food Engineering

Ankara University

Phone: 203 3300

e-mail:

FDE206 REACTION KINETICS

Ø  REACTIONS AFFECTING THE QUALITY OF FOODS DURING PROCESSING

Ø  GRAPHICAL PRESENTATION OF EXPERIMENTAL DATA

Construction of arithmetic, semi-logarithmic and logarithmic graphics, calculation of linear regression for arithmetic, semi-logarithmic and logarithmic equations

Ø  REACTION RATES

Reaction rates, Order of reactions in foods (zero-, first- and second-orders)

Ø  THEORIES ABOUT REACTION RATES

Collusion theory, Transition state theory

Ø  FACTORS AFFECTING REACTION RATES IN FOODS

Chemical nature of the reactants, Ability of the reactants to contact with each other, Concentration of the reactants, Temperature, Catalysts, Water activity

Ø  CALCULATION OF KINETIC PARAMETERS FOR REACTIONS IN FOODS

¨  Calculation of rate constants for:

ü  Zero-order reactions (non-enzymatic browning)

ü  First-order reactions (microbial growth/death, pigment degradation –particularly anthocyanins and chlorophylls–, vitamin degradation –particularly ascorbic acid and thiamin)

ü  Second-order reactions (ascorbic acid degradation in baby foods and packaged foods stored in low O2 concentration)

¨  calculation of half-lives for first- and second-order reactions

ü  first-order reactions (microbial growth/death, pigment degradation, vitamin degradation)

ü  second-order reactions (ascorbic acid degradation in packaged foods stored in low O2 concentration)

Ø  EFFECT OF TEMPERATURE ON REACTION RATES IN FOODS

¨  Arrhenius equation

¨  Determination of activation energy (Ea) and Q10 values

¨  Examples of Ea and Q10 calculations in foods (non-enzymatic browning, ascorbic acid degradation, pigment degradation, microbial growth/death, desulfurization of dried fruits)

Suggested reading (primary)

1)  Toledo RT. 1994. Fundamentals of Food Process Engineering. 2nd ed., Chapman & Hall, New York, NY.

Chapter 1: Review of mathematical principles and applications in food processing, pp. 1–50.

Chapter 8: Theory of chemical reactions in foods, pp. 302–314.

2)  Özkan M, Cemeroğlu B, Toklucu AK. 2010. Gıda Mühendisliğinde Reaksiyon Kinetiği. 174 s, Gıda Teknolojisi Derneği Yayınları No: 42, Bizim Grup Basımevi, Ankara.

Suggested reading (secondary)

1)  Earle R, Earle M. 2003. Fundamentals of Food Reaction Technology. 2nd ed., Leatherhead Food International, Leatherhead, United Kingdom.

2)  Brady JE, Russell JW, Holum JR. 2000. Kinetics: The study of rates of reaction. In Chemistry Matter and its Changes, pp. 573–623, 3rd ed., John Wiley & Sons, Inc., New York, NY.

1. REACTIONS AFFECTING THE QUALITY OF FOODS DURING PROCESSING

Food processing includes all the stages between the agricultural and meat and marine production and its final eating of consumers. Basically, it includes everything from the transport and storage of fresh meat, fish, fruits and vegetables to the production of final consumer product. Food processing makes the food products more attractive, more satisfying, safer, and easier to eat and preserves the foods from deterioration. It includes building of desirable constituents (formation of lactic acid in pickle fermentation) and removing the undesirable ones (oxygen causing oxidation of ascorbic acid and lipids), encouraging enzymes to develop desirable flavors and inhibiting the enzymes causing undesirable changes (the enzyme polyphenol oxidase in fruits), growing microorganisms to create desirable flavor and destroying microorganisms to prevent harm to the consumer.

It is very important to identify the desirable product qualities and the undesirable and even unsafe product qualities.

Agricultural,

meat and marine products → Processing change → Food Product

1.1.  Changes during food processing

The primary purpose of food processing is to preserve the foods for a long time. For example, heat processing (pasteurization and sterilization) is applied to foods to prevent the undesirable biochemical and microbiological changes in foods by inactivating the undesirable enzymes and microorganisms that cause such changes. On the other hand, during heat processing, the degradation of food constituents can occur depending on the temperature and time applied.

Other than heat processed foods (such as canned foods), the foods frozen are also exposed to high temperatures by blanching and frying. The food constituents can also be degraded even at lower temperatures, although the degradation rate will be much lower compared to heat processing temperatures. Therefore, the food constituents are degraded during food processing and storage, depending on the degree and the length of the exposure of temperature and time, respectively.

Processing causes changes in the food material, some of these changes are listed in Table 1.1 and Table 1.2.

Table 1.1 Changes in food materials during processing

Chemical / : / Hydrolysis, oxidation, polymerization, denaturation, browning
Physical / : / Gelation, hardening, softening, color loss/gain
Biological / : / Growth and death of microorganisms, glycolysis, physiological changes during ripening
Nutritional / : / Proteins changes, loss of vitamins, amino acid loss, destruction of anti-nutritional substances
Sensory / : / Aroma and flavor loss, aroma and flavor changes, texture changes, color bleaching and darkening

Reference: Earle and Earle (2003).

Table 1.2 Some of the chemical and biochemical deterioration reactions in foods and the environmental factors affecting these reactions

Reaction / Typical sensorial
changes / Typical nutritional
Constituent loss / Environmental factor affecting these reactions
▲  Non-enzymatic browning / ·  Loss of dissolution of proteins
·  Browning
·  Stale and cooked type of off-flavor / ·  Loss of ascorbic acid
·  Loss of some essential amino acids / ·  Temperature and water activity
·  Temperature, water activity and composition of gaseous in packaging
▲  Lipid oxidation / ·  Rise in fat viscosity
·  Rancid off-flavor
·  Browning / ·  Loss of A and E vitamins
·  Loss of essential fatty acids / ·  Temperature and water activity
▲  Forming of cross-links in polymers such as proteins and polysaccharides / ·  Hardening in texture
·  Loss in water holding capacity / ·  Loss of protein digestion / ·  Temperature, water activity and composition of gaseous in packaging
▲  Enzymatic changes in fruits and vegetables stored at refrigerated temperatures / Off-flavor formation / ·  Loss of vitamins / ·  Temperature, water activity and composition of gaseous in packaging
▲  Oxidation of meat pigments / Color deterioration in meats / – / –

Reference: Karel (1984).

Changes given in Table 1.1 and 1.2 can be measured, so their progress during processing can be followed and studied by the food technologists. The progress of processing can be measured in many ways, such as chemical analysis, physical measurements, counts of microorganisms, and color, texture and flavor assessments by sensory panel. Changes can often be described by the changes in chemical composition, but sometimes sensory, physical and microbiological measurements are used to quantify the changes.

These measurements reveal continuing change with time during the process as well as during the storage of foods. During storage, the samples of food stored at certain temperature are periodically taken, and analyzed for the targeted quality criteria and the changes in these criteria were determined over a period of storage period.

As seen in Table 1.2, the most important environmental factor affecting the deterioration of foods is the temperature of processing and the storage. Other than temperature; water activity, composition of gaseous in packaging are the other factors affecting the deterioration of foods.

The quantitative data obtained from the laboratorial measurements can be fitted to mathematical equations and to models. Once the models are sufficiently established, they can be used to predict changes in processing between and sometimes beyond the original processing purposes as well as during prolong storage. The models can be employed industrially to guide the processing, to control the processing and to design new processing and equipment. Important processing variables include temperature, time, moisture level and pH.

During the processing and storage of foods, the primary purpose is to preserve the food constituents and to minimize the changes in the quality of foods. The end point of food (shelf-life) is very important and is defined as the time that a food lasts as an acceptable and safe product in distribution, storage, in marketing and in the home. The most important criterion in defining the shelf-life of food is to know the primary quality factor of a food in question.

There are some examples given below on how to determine the primary quality criteria in defining the shelf-life of foods. The quality attributes can be classified as critical, important and unimportant.

Example 1.1: Identify the critical attributes of orange juice.

Ø  Aroma and flavor: After the extraction of orange juice, aroma and flavor diminish with time. This depends on especially temperature and oxygen level. The changes in aroma and flavor can be monitored by chromatography, and quality thresholds can be determined by sensory panel. One of the potential contributors to off-flavor is the oil passing from peel into juice during the extraction.

Ø  Pathogenic microorganisms: More critically, pathogenic microorganisms can contaminate the juice. Legal regulation requires that the packed orange juice must be subjected to a process sufficient to inactivate the pathogens in the finished product by a factor of 5 log cycles. This imposes a requirement of processor to identify the most resistance pathogen in the product, and to know the reaction order. To obtain this reduction, either heat processing (pasteurization) or non-thermal processing such as high pressure and irradiation should be applied.

Ø  Inactivation of pectin methyl esterase

Ø  Browning

Ø  Ascorbic acid

Ø  Carotenoids

For the packaged orange juice, the quality criteria is the destruction of pathogens, followed by aroma and flavor, pectin methyl esterase, browning, ascorbic acid and carotenoids.

In studying the shelf life of a food, the following information should be known:

Ø  product attributes significant to acceptable quality,

Ø  acceptable levels of these critical product attributes,

Ø  changes of product attributes with time,

Ø  reactions causing these changes

Ø  rates of these reactions

Ø  effects of storage variables on rates of reactions

It is important in studying shelf-life to control the storage variables, such as time and temperature that affect the rate of deterioration. Storage variables are shown in Table 1.3.

Table 1.3 Storage variables affecting the shelf life of foods

Food materials / Environment / Packaging
Microbial quality: / Temperature / Permeability to:
Pathogens / Water activity / Oxygen
Spoilage bacteria / Gas composition (atmos): / Water vapor
Yeasts / Oxygen / Carbon dioxide
Moles / Carbon dioxide / Ethylene
Composition: / Inert gases / Odors
moisture / Ethylene / Solvents, oils
Acidity/pH / Light / Light transmittance
Salt / Microorganisms / Packaging migration
Preservatives / Pests / Product/packaging interaction
Contaminants

Reference: Earle and Earle 2003.

Example 1.2: Shelf life of frozen foods

In studying the frozen storage of fruits, vegetables, meats and fish, some chemical and sensory attributes were measured. Samples at cold storage at a constant temperature were compared with samples held at a much lower temperature considered low enough for no change in the quality to occur. Sensory difference and statistical analysis were used to compare the test and control samples. Shelf life of foods is considered to be expired when a difference between the quality of the test and control samples was statistically detected by a trained sensory panel.

Shelf-life of various foods at various frozen temperatures is given in Table 1.4. As seen in Table 1.4, as the storage temperature was lowered, shelf-life of foods increased substantially.

Table 1.4 Practical storage lives (shelf-life) of frozen foods

Practical storage life (months)
Foods / –12°C / –18°C / –24°C
Peaches, apricots, cherries / 4 / 18 / >24
Green beans / 4 / 15 / >24
Corn / 4 / 15 / >24
Peas / 6 / 24 / >24
Carrots / 10 / 18 / >24
Beef carcasses / 8 / 15 / 24
Beef minced / 6 / 10 / 15
Fish lean / 4 / 9 / >12
Fish fatty, glazed / 3 / 5 / >9

The best way to follow and interpret the various reactions occurring in foods is to use “reaction kinetics.” “Reaction kinetics” or “chemical kinetics” is the branch of chemistry, which deals with the reaction's mechanism as well as the rates of chemical reactions. Reaction kinetics investigates how different experimental conditions can influence the rate (speed) of a chemical reaction and the construction of mathematical models that can describe the characteristics of a chemical reaction. In food applications, reaction kinetics is used in the following of the microbial and biochemical reactions as well as the chemical reactions that affect the quality adversely, and in calculating the rate of these reactions.

In investigating the chemical reactions in terms of rate determination, the net reaction is important and intermediate products are not taken into consideration. As known, most chemical reactions occur stepwise, which means chemical reactions take more than one elementary step to complete. During these steps, the intermediate products are formed. An intermediate is a compound that is produced from the preceding reactants and is then reacted with another reactant to form a final product. The lifetime of the intermediates usually do not last very long because they are usually catalyzed to make the next product of the reaction sequence. Since they have short lifetime, they do not remain in the product mixture.

Most of the time, intermediate products formed are not very important because these products are immediately consumed and cannot be measured analytically. However, in some cases, the formation of intermediate products can be very important, especially in foods. These intermediate products give important clues for the formation of the final products as well as the clues for predicting the conditions (temperature, pH etc.) at which the food products are exposed. For example, the formation of hydroxymethyl furfural (HMF) as a result of the non-enzymatic reactions between proteins and carbohydrates indicates that the browning reactions undergo and the conditions are suitable for the formation of brown pigments (For example, foods are exposed to high processing temperature or storage temperature or exposed to high temperatures for prolonged time).

In food science, reaction kinetic calculations are used not only for the degradation or formation of food constituents but also used for the inactivation of microorganisms and enzymes by heat or chemicals.

In practice, it is important to know the rate of chemical reactions. For example, by knowing the reaction rate as well as the conditions affecting the rate of given reaction, the producer of chemical compounds can increase the yield and reduce the processing cost. The producers of chemicals aim to produce the high amount of products from the reactants. On the contrary, food processors aim to preserve the quality, in essence aiming the preserving the reactants. For food processors, reaction kinetics is the tool to preserve the quality parameters of food products. For example, sour cherry juice producers aim to preserve the characteristic red-violet color of sour cherries, resulting from the anthocyanin pigments. If the degradation rate of anthocyanins as well as the factors affecting this degradation is known, both processing and storage conditions (temperature and time) are optimized and by doing so, the color of sour cherry juice can be preserved.