Ch. 7 - The Control of Microbial Growth

Tortora - Microbiology: An Introduction 14th Edition

Tortora14th EditionMicrobiology: An IntroductionISBN: 9780138200398Not the one you use?Change textbook

Tortora 14th Edition

Ch. 7 - The Control of Microbial Growth

Problem 7

Chapter 7, Problem 7

How do salts and sugars preserve foods? Why are these considered physical rather than chemical methods of microbial control? Name one food that is preserved with sugar and one preserved with salt. How do you account for the occasional growth of Penicillium mold in jelly, which is 50% sucrose?

Verified step by step guidance

Understand that salts and sugars preserve foods primarily by creating a hypertonic environment, which causes water to move out of microbial cells by osmosis, leading to dehydration and inhibition of microbial growth.

Recognize that this preservation method is considered physical rather than chemical because it relies on altering the physical environment (water availability) rather than chemically altering or killing the microbes.

Identify examples of foods preserved by sugar, such as jelly or jam, and foods preserved by salt, such as salted fish or cured meats.

Explain that despite high sugar concentration (e.g., 50% sucrose in jelly), some molds like Penicillium can occasionally grow because they are osmotolerant and can survive in low water activity environments.

Summarize that the effectiveness of salts and sugars depends on their ability to reduce water activity, which is a physical parameter, and that microbial resistance varies among different species.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Osmotic Pressure and Microbial Preservation

Salts and sugars preserve food by creating a hypertonic environment that draws water out of microbial cells through osmosis, leading to dehydration and inhibition of microbial growth. This reduction in available water prevents spoilage and extends shelf life without directly killing microbes.

Physical vs. Chemical Methods of Microbial Control

Physical methods inhibit microbial growth by altering environmental conditions, such as water availability, temperature, or radiation, without chemically altering microbial structures. In contrast, chemical methods involve substances that react with microbial components to kill or inhibit them. Salting and sugaring are physical because they rely on osmotic effects rather than chemical reactions.

Microbial Growth in High Sugar or Salt Environments

Despite high sugar or salt concentrations, some microbes like Penicillium molds can occasionally grow due to their ability to tolerate lower water activity or form resistant spores. In jelly with 50% sucrose, Penicillium can survive by exploiting microenvironments or slight variations in sugar concentration, leading to spoilage.

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