How Enzymes Are Used in Food Processing

 

ACCM Concepts

This Exemplar will include the following concepts from the ACS Examinations Institute General Chemistry ACCM:

VII. A. 2. Rate is generally defined as the change in concentration of a reactant or product as a function of time.

VII. B. Empirically derived rate laws summarize the dependence of reaction rates on concentrations of reactants and temperature.

VII. B. 1. The “order” of a reaction is derived from the exponent on the concentration term of a given reactant in the rate law.

VII. E. 1. A catalyst is defined as an agent that increases the rate of the reaction while not being consumed by the reaction.

VII. E. 2. A catalyst increases the rate of the reaction by providing a new reaction pathway with a lower activation energy.

Introduction

If you have ever looked around at the various labels on packaged foods and drinks, you might have noticed “shelf life” or “sell by” labels somewhere on the container. These labels tell you the amount of time the food can be stored before it becomes unsuitable for consumption. How do food processing companies determine these values?

 

        

Figure 1. “Shelf life” and “sell by” dates as shown on this milk carton and chip bag are determined by analyzing the rates of chemical reactions occurring in the food to maximize the amount of time the food lasts while still being consumable.

 

Chemical Kinetics in Food

Chemical kinetics plays a significant role in the amount of time that food can last and stay fresh. Several different chemical reactions take place during and after the preparation of foods. After all, foods are compositions of several different molecules and compounds that all interact and react with each other in the food. These reactions could be intrinsically driven through thermodynamics or caused by microorganisms in the food. Some of these reactions are beneficial and necessary for the food, producing the molecules and compounds that give the food its taste, nutrition, appearance, and texture.¹ Other reactions reduce the longevity of the food through degradation, causing the food to become rotten or spoiled.¹ To prevent foods from spoiling too soon, food processors turn to chemical kinetics.

In chemistry, kinetics deals with the rates at which chemical reactions are occurring.⁴ When foods are processed and packaged to be sold, food processing engineers and companies must consider the rates at which certain reactions occur in the foods. To efficiently preserve foods, one must take into consideration the best conditions to decrease the rates at which detrimental reactions occur (i.e. reactions that lead to spoilage) while also increasing the rates at which the beneficial reactions occur in order to maintain the flavor, freshness, and nutrition of the food.² Without this application of chemical kinetics, very useful products such as MREs (Meal, Ready to Eat), a type of ration primarily used by the military, would not exist, and your ice cream would not last nearly as long in the freezer!

 

The Basics of Kinetics

The study of kinetics primarily involves reaction rates and reaction mechanisms, which are the steps that are involved in a reaction to lead it to completion. A reaction rate is equal to the change in the concentration of the reactants or products in a reaction over time. In general chemistry, we assume that the reverse reaction in reversible reactions is insignificant and negligible, meaning that the rate generally depends on the change of reactant concentration over time.³ Reaction rates can be expressed through equations called rate laws, which generally take the form:

Rate = k[A]^m[B]ⁿ

Rate laws can only be determined experimentally as the values of each variable are unique for each reaction. Here is what each variable in the expression represents:

• k = rate constant: a constant that describes the relationship between the reaction rate and the concentrations of the species present in the reaction

• [A] and [B] = the concentrations of the species that were present in the reaction

• m and n = the reaction order relative to that concentration/species

The reaction order describes the dependence of the reaction rate on the concentration of that species. The main types of reaction orders in general chemistry are zero order, first order, and second order. In a zero-order reaction, the rate is not dependent on the concentration and only on the rate constant. In a first-order reaction, the rate is directly proportional to the concentration of the present species. In a second-order reaction, the rate is exponentially proportional to the concentration and quadruples as the concentration doubles. 

Example 1

In a certain food, an amino acid (AG) is added to sugar, glucose (C₆H₁₂O₆),  and forms glycosylamine and water as given in the reaction below:

C₆H₁₂O₆ + AG —> glycosylamine + H₂O

The following kinetic data was determined experimentally:

Experiment	[C6H12O6]0 (mol/L)	[AG]0 (mol/L)	Initial Rate (mol * L^-1s^-1)
1	0.0570	0.114	6.56e-2
2	0.114	0.114	2.62e-1
3	0.114	0.0570	1.31e-1

 

Determine the overall rate law for this reaction.

Solution

In order to determine the overall rate law, one must first determine the order of reaction with respect to each species in the reactants. First find the order of the reaction with respect to glucose (C₆H₁₂O₆).

Using the data in the table, write out the rate law for the experiments in which the concentration of C₆H₁₂O₆ changes but the concentration of AG does not. This allows you to see how a change in the concentration of C₆H₁₂O₆ affects the reaction rate.

Rate law for experiment 1:

rate₁ = k[C₆H₁₂O₆]^m[AG]ⁿ

Rate law for experiment 2: 

rate₂ = k[C₆H₁₂O₆]^m[AG]ⁿ

Now that the rate laws are written, divide the second rate law by the first rate law in order to determine the value of the variables. Since you are determining the order of reaction with respect to [C₆H₁₂O₆], you will solve for the variable m. Plug in the values for the rates and concentrations and simplify in order to solve for m.

\(\dfrac{rate_2}{rate_1} = \dfrac{k[C_6H_{12}O_6]^m[AG]^n}{k[C_6H_{12}O_6]^m[AG]^n}\)

\(\dfrac{2.62e-1mol/L*s}{6.56e-2mol/L*s} = \dfrac{k[0.114M]^m[0.114M]^n}{k[0.114M]^m[0.114M]^n}\) 

\(\dfrac{2.62e-1mol/L*s}{6.56e-2mol/L*s} = \dfrac{[0.114M]^m}{[0.0570M]^m}\) 

4 = 2^m

m = 2

Since m=2, the reaction order with respect to glucose is second order.

Now, let's find the reaction order with respect to [AG]. Repeat the same process as previously stated above but with the experiments in which the concentrations of AG stay the same and the concentrations of glucose change.

Rate law for experiment 2:

rate₂ = k[C₆H₁₂O₆]^m[AG]ⁿ

Rate law for experiment 3: 

rate₃ = k[C₆H₁₂O₆]^m[AG]ⁿ

Now that the rate laws are written, divide the third rate law by the second rate law in order to determine the value of the variables. Since you are determining the order of reaction with respect to [AG], you will solve for the variable n. Plug in the values for the rates and concentrations and simplify in order to solve for n.

\(\dfrac{rate_3}{rate_2} = \dfrac{k[C_6H_{12}O_6]^m[AG]^n}{k[C_6H_{12}O_6]^m[AG]^n}\)

\(\dfrac{1.31e-1mol/L*s}{2.62e-1mol/L*s} = \dfrac{k[0.114M]^m[0.0570M]^n}{k[0.114M]^m[0.114M]^n}\) 

\(\dfrac{1.31e-1mol/L*s}{2.62e-1mol/L*s} = \dfrac{[0.0570M]^n}{[0.114M]^n}\) 

0.5 = 0.5ⁿ

n = 1

Since n=1, the reaction order with respect to AG is first order.

Now that both m and n have been determined, the values can be substituted into the overall rate law. 

Overall rate law for the reaction:

rate = k[C₆H₁₂O₆]²[AG]

Other factors can also affect the rates of reaction. An increase in temperature causes an increase in the reaction rate because the molecules in the reactants will have a higher average kinetic energy, meaning that more molecules have sufficient energy to collide and react, which causes the rate to go faster. The introduction of a catalyst also increases the rate of reaction by lowering the activation energy of the reaction. A catalyst is a species that is introduced into a chemical reaction to provide another reaction path with a lower activation energy so that more molecules of reactants are able to react, and catalysts are also not used up in the reaction, so after being introduced into the reaction, the catalyst will still remain at the end of a reaction 

 

Figure 2. This graph shows the effects of catalysts on the activation energy and reaction mechanism of a reaction. The catalyst creates a different reaction pathway in which the activation energy is lower, therefore allowing more molecules to react and increasing the rate of reaction.

 

The Role of Enzymes in Food Preservation

In the food processing industry, enzymes are used as preservatives by functioning as catalysts for certain biochemical reactions in foods. Because only specific substrates can bind to the active sites of certain enzymes, enzymes can be highly selective and therefore can control the reactions and start and stop them whenever necessary. For food processing, enzymes are most commonly sourced from microorganisms such as bacteria due in part to the cheaper production costs and greater availability.

Enzymes function as catalysts by reducing the activation energy of the reaction, which allows the reaction to occur sooner with less required energy. This results in increased reaction rates for certain reactions. For food preservation, the enzyme activity results in the increase of the rates of beneficial reactions, and the reactions that create the food components take place in the least amount of time possible due to the lessened activation energy created by the enzyme catalyst.¹ Enzyme and catalytic activity can be affected by any changes in pH, temperature, and concentration, so those factors also must be taken into account in order to create the most optimal conditions to allow for the quickest reaction time and processing efficiency.⁴ 

Example 2

Many reactions within the food processing industry use catalysts in order to help the reaction. 
  
Overall Reaction 2A+E+G —> H

Step 1   A+B—> C 
Step 2   C+E—> F
Step 3.  F+G+A —>B+H

Identify the catalyst in the reaction. 

Solution

The catalyst is B.

References

(1) Cavalierei, R. P.; Reyes De Corcoran, J. I. Kinetics of Chemical Reactions in Foods. Food Engr. 2011, 1, 1-8

(2) Juárez-Enríquez E; Levario-Gómez A; Ochoa-Reyes E; etc. Significance of enzyme kinetics in food processing and production. In Value-Addition in Food Products and Processing Through Enzyme Technology; Kuddus, M., Aguilar C. N., Eds; Academic Press: 2022; pp 467-482

(3) Zhang, H. Kinetics. https://www.learnchem.net/tutorials/kinetics.shtml (accessed Dec 7, 2022)

(4) Zumdahl, S. S.; DeCoste, D. J. Chemical Principles; Cengage Learning, 2017.

Image Sources:

Figure 1:

https://www.pbs.org/wgbh/nova/articl...iration-dates/

https://www.reddit.com/r/funny/comme...to_the_minute/

Figure 2: https://saylordotorg.github.io/text_...catalysis.html