Deconstructing Yogurt: A Different Method for Understanding Fermentation in an Undergraduate Microbiology Laboratory Setting

INTRODUCTION

Humans have been producing and consuming yogurt for several millennia as fermentation has historically been the only reliable method for preserving milk besides drying it. Fermented milk products can be found in almost every culture and have even been observed in Egyptian hieroglyphs (1). The earliest recorded references to the health benefits of consuming yogurt date back to Indian scripts from 6000 BC (2). Today, yogurt remains one of the most popular fermented dairy products worldwide, making up a 70-billion-dollar industry. Thus, yogurt making is often taught in microbiology laboratory courses as a way to introduce students to the basics of microbiology and its application in food preservation. Although students can produce yogurt, due to safety concerns, they are often not allowed to taste the yogurt they produce, which can lead to student dissatisfaction and lack of engagement with the laboratory procedure. Therefore, we designed a laboratory exercise in which students deconstruct commercially available yogurt to identify the two main bacteria present, Streptococcus thermophilus and Lactobacillus bulgaricus. Federal regulations require that anything sold as “yogurt” contain only these two species of bacteria. The variety of textures and flavors that is achieved between the different commercially available yogurts is due to strain differences of these two bacterial species as well as differences in the way the yogurt is processed. This hands-on procedure introduces students to the basic concepts of food fermentation and allows them to practice many of the basic microbiology techniques they have been learning, while mitigating the safety concerns around food preparation in a laboratory setting.

LEARNING OBJECTIVES

Due to the large nature of the microbiology classes at our institution (6 sections, ~24 students per section), finding an alternative space in which students could prepare yogurt had become untenable. Therefore, in order to still allow students to interact with microbes involved in the fermentation process, we created a laboratory exercise in which students were tasked with identifying the bacteria in commercially available, store-bought, yogurt. Prior to the beginning of these activities, students had received a lecture on the role of microorganisms in the fermentation of human foods, which included how yogurt and cheese were produced. At the end of these activities, which are designed to occur over the course of four days/lab periods, students will be able to 1) Gram stain and identify the bacterial species used in yogurt production, 2) collect and analyze results from MRS agar and phenol red broths to identify the bacterial species used in yogurt production, 3) provide the characteristics of bacterial species used in yogurt production, and 4) explain the role of bacteria and their metabolism in making yogurt.

PROCEDURE

Students worked in pairs following the protocol provided in Appendix 1. A visual representation of the four days of tasks can be found in Figure 1. On the first day, a single container of CHOBANI Greek unflavored yogurt was provided to the class. On day 1, students were tasked with Gram staining the yogurt and streaking the yogurt on an MRS plate (Hardy Diagnostics). This agar was developed as an alternative non-selective medium for the cultivation of fastidious Lactobacillus (3). On day 2, students had to identify the two different types of bacteria on their MRS plates based on the provided colony morphology and choose one of each type to streak for isolation of colonies on new MRS plates. As the two bacterial species are not present in equal densities, the isolation of colonies is a challenging but attainable task for students. On day 3, students Gram stained each isolated bacterial type to confirm isolation and inoculated sucrose and glucose phenol red broths with Durham tubes. On day 4, students recorded the results of their phenol red broths and confirmed the identification of their two species of bacteria. During the lab, students record their results on the provided worksheet (Appendix 2). This worksheet also includes a number of questions for students to answer regarding the procedures and reagents used as well as the overall fermentation process that occurs in yogurt.

FIGURE 1.

Overview of student work during laboratory activities

All work for these activities should be performed in a biosafety level 1 (BSL1) space and assumes students have had basic safety training in performing laboratory experiments with bacteria. Instructors should refer to the ASM BSL1 guidelines for laboratory space requirements, personal protection requirements, and standard laboratory practices. In particular, instructors should make sure that students have been instructed on safe use of Bunsen burners, the safe handling of bacteria, and the proper methods for disposing of biohazardous waste.

CONCLUSION

Students were assessed for their participation in the overall lab procedure, completion of the worksheet, and their laboratory skills (Gram staining, microscopy, aseptic technique, streaking for isolation, inoculating broths, and ability to identify their unknown bacteria). Although we did not include this in our assessments, student participation could be assessed during discussions in lab and/or the lecture portion of class. In general, the provided materials are meant as a guide, and we hope instructors will edit them to meet the needs of their classrooms. This laboratory procedure highlights the following ASM curriculum guidelines (4): “Humans utilize and harness microorganisms and their products” and “Microorganisms and their environment interact with and modify each other.” Further, this laboratory procedure touches on almost all the core laboratory skills recommended by the ASM guidelines and can serve as an excellent end-of-year assessment of student learning (4).