1. Introduction





1.0 Introduction

If one wishes to enjoy a warm cup of coffee, with the constraint of being lactose intolerant, one simply adds in a dose of soy milk as an alternative to ordinary milk.  However, those who do indulge in coffee with soy milk encounter a literal bittersweet challenge at the bottom of their mugs - curdled soy milk.  

According to recent studies done by the U.S. National Library of Medicine® in 2010, approximately 65% of the human population has a reduced ability to digest lactose after infancy. Lactose intolerance in adulthood is most predominant in people of East Asian descent, affecting more than 90% of adults in such communities.

Curdling is affected by two main factors - temperature and acidity. (The Kitchn, 2011)  It happens because of the way the proteins in the soy milk react to the acidity of the coffee. The protein in the milk is normally suspended in a colloidal solution, meaning that the small protein molecules float freely and independently. These floating molecules refract light and is the reason why milk is white in colour. Under normal circumstances, these protein molecules repel each other, allowing them to float about without clumping together. But when the pH of their solution changes - due to the coffee’s acidity - they coagulate and attract one another and form clumps, causing the milk to curdle.

Another factor that could affect the amount of curdling is the sequence of pouring coffee into soy milk and vice versa. Research has shown that the method of pouring the coffee gradually into the soy milk will reduce the curdling as it gives the soy milk time to adjust to the coffee’s high temperature as compared to pouring the soy milk into the coffee. (The Kitchn, 2011)

568denaturation.gifThe factor that we are experimenting on, however, is the temperature. The temperature affects the rate of curdling as it changes the protein structure of the soy milk, otherwise called as the denaturation of proteins. Heat affects the protein structure as it disrupts hydrogen bonds and nonpolar hydrophobic interactions due to the increase in kinetic energy. The protein molecules vibrate rapidly, hence the bonds are disrupted. In order for the soy milk to curdle, the protein molecules are denatured due to the temperature of the coffee and coagulates due to the coffee’s acidity. 


Hence, the protein molecules clump together and form curdles. (Charles E. Ophardt, 2003)

Our hypothesis is that the higher the temperature of the coffee, the more the soy milk will curdle as the high temperature will allow the soy milk to curdle at a faster rate, hence there will be more curdling in a fixed amount of time as compared to other temperatures.


1.1 Research Questions 


  1. How does the temperature of the coffee affects the amount of curdle?
  2. How does the sequence (coffee into soy milk vice versa) affect the amount of curdle?

     1.2 Hypothesis

The higher the temperature of the coffee, the more the soy milk will curdle.


1.3 Independent, Dependent and Controlled Variables

Our independent variable is the temperature of the coffee, and we will be using five different temperatures, 10°C, 20°C, 30°C, 40°C, and 50°C. We will also be reversing the process. First, we will be pouring the soy milk into the coffee before reversing the process and pouring the coffee into soy milk.

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