Friday, June 24, 2005

Calibrating a rice cooker

Table of contents:
  • Introduction
  • The rice cooking process
  • The model
  • Experimentally determining the parameters
  • Summary
  • Cite this article

  • Cooking rice can prove difficult for inexperienced persons (like me). I find it a tricky business fiddling with the water levels to make rice of suitable wetness. Making edible rice is dead easy. Making reasonably decent rice is not hard. Making good rice is doable, once in a while. However, making perfect rice in a consistent fashion requires experience and a certain touch, none of which I happen to possess.

    There must be a better way than this fiddly, inconsistent, unreliable and occasionally emotional way of learning to make rice by trail and error. It gets emotional when one is hungry and the rice turns out to be hard, translucent bits of starch granules fused together into little shrapnel fragments.

    There are various guidelines for cooking rice. Some recommend that the depth of water should be double (or triple or whatever) that of the rice grains, while others recommend that the water level above the level of rice should be 1cm (or 2 cm or whatever). Of course, these guidelines are only valid within a small range (a range that the person recommending the guideline is familiar with). If you are cooking 1cm deep of rice, both guides would recommend 2cm of water. What if you want to cook for more guests, and need 6cm of rice? Do you add 12cm of water or 7cm of water?

    Presented below is a method to calibrate a rice cooker. It involves modelling the rice cooking process with some valid simplifications, and the data collection will be a straightforward process

    The rice cooking process:

    Cooking of rice involves dry, uncooked rice grains absorbing some water while being heated at about 100 degrees C. Traditionally, rice is steamed in a bowl filled with uncooked rice and an amount of water. Below this bowl of rice will be some boiling water. Steam from the boiling water then heats the rice to 100 degrees C. We should note that there are two seperate bodies of water- one to be boiled to produce steam, and another that would be absorbed into the rice.

    Modern rice cookers do away with this business of separating the boiling water and absorbed water. As such, one would need to add a certain amount of excess water into the rice cooker to allow for some boiling. Otherwise, if one sticks to the usual proportions used in rice steaming, the electric rice cooker will inevitably produce overly dry rice, and possibly tears from a certain hungry person.

    The model:

    Simplifying assumptions:
  • Assume that the heating process from ambient (room) temperature to boiling temperature is reasonably fast, and no significant amount of cooking occurs during the heating up phase.
  • Assume that the rice cooker will boil water for a fixed length of time. This is analogous to someone steaming a bowl of rice for a fixed length of time, irrespective of the number of servings that is being cooked.
  • Assume that the water boils at a constant temperature.
  • Assume that heat supply to the water and rice is at a constant rate.
  • As a consequence of these simplifying assumptions, the amount of water that is boiled to steam is constant, irrespective of the quantity of rice being cooked.

    It is perfectly reasonable to assume that the amount of water absorbed into the rice grains is proportional to the amount of rice. The amount of water absorbed by 2 cups of dry rice is precisely double the amount absorbed by 1 cup of dry rice.

    The amount of water that needs to go into the electric rice cooker is thus the amount that will be absorbed by the rice, plus the amount that will be boiled to steam during the cooking process.

    In general, the amount of water needed can be expressed as a linear equation:

    (Amount of water needed) = (Amount of water absorbed per unit dry rice) x (amount of dry rice) + (amount of water boiled)

    These can be expressed in any unit you desire. For example, 3/4 cup of water for every cup of dry rice, and 1 inch deep of extra water to boil. Or 56 kiddie pools of water for every kiloton (million kilograms) of dry rice, and 300 gallons to boil.

    Experimentally, there are 2 variables that need to be determined- the quantity of water boiled, and amount of water absorbed for each unit of dry rice.

    At this point, it is noteworthy to mention that the amount of water per unit of rice is only dependent on the kind of rice used, and the water boiled is only dependent on the model of rice cooker employed. When you buy a different variety or rice, the only variable that needs to be tuned is the water per unit rice. Likewise, changing rice cookers will mean that the amount of water boiled changes.

    Experimentally determining the 2 variables:

    We will first find out how much water is boiled by the rice cooker as it is the easier variable to determine.

    Fill your rice cooker with a precisely known quantity of water. “Cook” this vat of water as you would cook rice. When the cooking is complete and after everything has cooled to a safe temperature, measure the quantity of remaining water, precisely. Obviously, the quantity of water boiled is the difference between the start and end quantities. For convenience’s sake, we will call this quantity of water B (for boiled).

    Cook some rice. Precisely measure the volume of dry rice (245ml, for example, or 1.05 cups). We will refer to this volume of dry rice as R (for rice). Add water to the rice, measuring it as you add. We will refer to this quantity of water as W (for water added).

    As of now, our equation is
    W = kR + B, where k is the amount of water absorbed for each cup/ml/gallon/ton of rice

    If you had been a good boy/girl and had measured all previous quantities (B, R and W) to a reasonable accuracy, it would be an easy matter finding the value of k for this particular batch of rice. k = (W-B)/R

    Was the rice good? If you want drier rice, reduce k. For wetter rice, increase k. In the process of optimising your value of k for this particular variety of rice, it is best to keep a record of your past values of k and some remarks like “too damn wet”, “bone dry” or “slightly wet”. With the aid of your history of attempted k values and corresponding results, it will be easy to approach the ideal quantity of water. And then you would have perfect rice, every time.

    In summary:

    The rice cooker boils a fixed amount of water. For the rice to be good, it must absorb a certain quantity of water. Water added into the cooker must be the sum of these 2 quantities of water.

    First determine the amount of water boiled. Then find the quantity of water absorbed that will result in good rice.

    Cite this article as:
    Tan Yee Wei(2005), "Calibrating a rice cooker", from "Snippets of This and That"

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