Fun Microbiology: Making Plain & Frozen Yogurt

Abstract

Yogurt is one of the oldest fermented milk products known. It is believed to have originated approximately 4,000 years ago in the Middle East (the name comes from Turkish) and spread with the migration of herdsmen in search of water and pastures to stake a living (Vedamuthu 1993). Although yogurt has a long history, its popularity and growth in sales in the United States has occurred primarily over the last 20 years. This could be attributed (but not limited) to the following developments: use of sweeteners to mute tart acid flavor of plain yogurt, introduction of fruits and nuts, inclusion of low-fat and non-fat yogurts containing non-nutritive sweeteners with advertising aimed at calorie-conscious persons, introduction of frozen yogurt as a low-calorie dessert, and accumulation of scientific data indicating health benefits of yogurt consumption (Halpem et al. 1991; Isolauri et al. 1991; Pochart et al. 1989; Vedamuthu 1993). Yogurt is a solid, custard-like milk product fermented with a mixture of the bacteria Streptococcus salaivarius subsp. thermophilus (coccus shaped) and Lactobacillus delbrueckii subsp. bulgaricus (rod shaped)(Atlas 1997; Jay 1996). The pairing of coccus and rod is important in yogurt production because there is an associative beneficial relationship formed between these two bacteria, which in biology is referred to as symbiosis. Modern cultured yogurt production usually begins with concentrating milk by 1/4 volume in a vacuum pan by heating at 90? C (1940 F) for a few minutes. The milk is fortified at 380