ice, science of its use. For an understanding of how to most efficiently use ice to control temperature and dilution in beverages, and of how clear ice is produced both at home and in ice machines, we will look at the science of freezing water and melting ice. For simplicity’s sake, we will assume the typical conditions of freezer-temperature ice submerged in room-temperature beverages, rather than all possible situations.
The cooling of beverages with ice is due mostly to its melting, which results in the dilution of the beverage with water. Ice melts thus: first the solid ice will warm up to its melting temperature of 0° C, then change its state by melting from solid into liquid, and then the liquid water, which is still at 0° C, can warm further. Only one-half calorie of energy is required to warm 1 g of ice by 1° C, and one full calorie is needed to warm 1 g of water by 1° C, but the change of state requires that a relatively huge 80 calories be expended before 1g of ice at 0° C becomes 1 g of water at 0° C. So from a typical freezer temperature of −18° C, to warm up 1 g of ice to its melting temperature requires only 9 calories of energy but another 80 calories to melt it.
This energy needed to melt the ice is taken from its surrounding environment: the beverage in which the ice is placed. As the beverage loses energy, its temperature decreases. A little of that energy stolen from the beverage goes toward warming up ice, and a lot toward melting it, so we can simply round up to say that the chilling of an iced beverage is the result of ice melting, and also that the chilling of an iced beverage results in dilution. For iced drinks, chilling and dilution are critically linked.
Rate of Chilling and Dilution
Ice melts at its surface, so the greater the surface area of ice in a beverage, the more melting that can take place at a given time. For a given total volume (or weight) of ice, smaller pieces will have a greater combined surface area than a single large piece. For example, a single, large cube of ice 10 cm square has a volume of 1,000 cm3 and a surface area of 600 cm2, while that same cube split into eight 5-cm cubes will have the same combined volume of 1,000 cm3 but double the surface area at 1,200 cm2. Thus the smaller group of cubes with their larger total surface area can chill a beverage at a faster rate than a single large cube.
However if both the larger and smaller cubes melt completely into a beverage, they will chill the drink to the same final temperature (and dilution), since the same amount of energy is required to melt the same volume of ice. In other words, the volume (or weight) of ice we add to a drink determines how much it can chill, while the surface area of the ice determines how quickly it can chill. In the real world we use more ice than is necessary to chill a drink, and it will reach an equilibrium temperature with no significant additional chilling or dilution from ice. But unless we are holding the beverage in a perfectly insulated container, additional dilution will occur over time in response to heat gain from ambient temperature.
If we want to chill (and therefore dilute) a drink with ice at the slowest rate possible, as may be desired for whisky on the rocks, we can use one large ice sphere, because spheres have the lowest surface-area-to-volume ratio of any shape. Conversely, small chips of ice will chill a drink faster than big cubes.
To further increase the rate of chilling, we stir or shake drinks, moving more of the warm beverage over the surfaces of the ice. This sweeps away the just-melted surface water on the ice so that the warmer beverage can continue to be in contact with the ice’s surface (again, most of the cooling of the drink comes from ice melting). Shaking with ice hastens this process and also increases the ice’s surface area by shattering it into smaller pieces, thus speeding chilling even more. Shaking cocktails reduces their temperature faster than stirring (assuming roughly the same size ice), but it also adds texture via air bubbles.
Wet, glossy-appearing ice (such as ice kept out in an ice bowl) has newly melted water clinging to its surfaces. When we add wet ice to a drink, we are adding both ice that is very effective at cooling and water that is not. The water will still dilute the drink without the payoff of chilling it much, so if we want to chill drinks most effectively, we should use non-wet ice. Ice can be strained or toweled off to remove some of its surface water.
Freezing and Clear Ice
Ice is unusual in that it floats on its liquid form, water, rather than sinks. When water freezes, its molecules arrange into a hexagonal lattice pattern repeated in all directions to form an ice crystal. The spacious arrangement of atoms in the crystal results in ice being less dense than its liquid water form.
Pure water (pure H2O) chilled to 0° C does not spontaneously crystallize into ice; it needs a nucleus to grow from. Pure water can become supercooled: the condition of remaining in the liquid state below its freezing point. Supercooled pure water will eventually self-nucleate at cold enough temperatures, but in any real-world situation water is not 100 percent H2O, and a foreign particle like a speck of dust or a grain of salt in it will act as a starting point for crystallization at just below water’s freezing temperature.
In a natural setting like a lake, ice crystals grow in a thin layer across the surface and then thicken downward. Ice thickens at a slower and slower rate over time, as the energy from the environment needed to freeze the water must conduct through the existing ice at the surface first.
When ice crystals grow slowly, they reject incorporating impurities including trapped air, minerals, and anything else not pure H2O, which don’t fit into their neat crystal lattice. The pure ice is perfectly clear, and impurities are pushed away from the growing crystal. In a typical freezer, the cold environment surrounds an ice cube tray on all sides, so ice forms not only on the surface but also on the inner walls and thickens in toward the middle. The first part to freeze is typically clear, and the last part to freeze is where the impurities and air are concentrated, forming a cloudy core. (This is easier to see by freezing water in a single container like a plastic cup instead of a tray of cubes.) Additionally, because water expands as it becomes a solid and the last part to freeze is in the center of a cube, the ice will often shatter or crack when fully frozen, increasing perceived cloudiness.
Water can also form cloudy ice when it freezes quickly, as multiple fast-growing crystals surround and trap impurities and air, rather than pushing them away.
Clear ice can be produced at home or in machines by encouraging the water to freeze in one direction only (sometimes called “directional freezing”), rather than forming from the outside in toward the center. A simple way to accomplish this is by filling an insulated container with no top (such as a hard-sided beer cooler with the top off) with water and leaving this in a freezer. The insulation on the sides and bottom of the container prevents or at least slows ice forming on those sides, encouraging the ice to thicken only from the top toward the bottom.
The final part of the block to freeze, the bottom section, will be cloudy, much like the core of a standard ice cube. To harvest only clear ice, the slab of ice can be removed from the insulated container before it has fully frozen to the bottom, leaving behind water. Alternately, if left to freeze all the way, the cloudy bottom of the fully frozen block can be cut off to retain only the clear section. Most recently available clear-ice-producing trays for home use employ directional freezing with insulated containers and clear/cloudy section separators.
Various techniques have been reported to increase ice clarity, whether in a typical ice cube tray or within the directional freezing system. Most are attempts at slowing the rate of freezing, minimizing minerals and other impurities, and/or minimizing gases in the water. Boiling water before freezing it, a much-promoted (but usually ineffective) technique, is meant to capitalize on reducing gases in water.
Many ice sculpture block machines also freeze water in one direction, but from the bottom toward the top. A cold plate is located on the bottom of a large, water-filled container, and a water pump closer to the surface circulates water to prevent ice from freezing over at the top. The block forms from the bottom up, and then the remaining surface water (which would have been cloudy ice if allowed to freeze) is discarded. Other clear-ice-producing machines, typically those that make smaller shapes, pump or spray water over a cold plate and continually discard the runoff, so that only clear ice forms. See Clinebell ice machine.
How Ice Is Used in Modern Mixology
While ice cube machines and the shape and sizes of ice they produce vary in different parts of the world, very few models produce large, clear ice cubes bigger than 32 mm (1.25″) square as of this writing. In recent years larger sizes of clear ice have been desired by bartenders in order to serve cocktails and spirits on a single cube or sphere. Clear ice is largely an aesthetic choice over cloudy ice, but it should also contain fewer minerals/impurities and melt at a slower rate. As cloudy ice’s bubble pockets melt, more surface area is exposed and the melting rate can increase; additionally, cloudy ice tends to crack more readily due to the way it was formed.
This model of cutting sculpture ice blocks into glass-sized smaller cubes was replicated around the United States and in other countries. Blocks are typically cut using power tools (inducing electric chainsaws and band saws) and hand tools to reduce them to desired shapes and sizes. Common sizes of ice produced in this method include large cubes and single tall “spears” to fit Collins glasses, though everything from large spheres to ice punchbowls are cut down from these large blocks. See ice carving.
Beverages can be chilled by methods other than adding ice to liquid—whether the purpose of this is to specifically avoid dilution or not—with varying degrees of effectiveness. Liquid ingredients, bar tools, and glassware can be chilled in refrigerators or freezers, or they can be chilled with ice that is not incorporated into the drink. Frozen objects such as stones and metal and plastic cubes can be added to the beverage in the glass. In specialty freezers, ice can be kept at extremely cold temperatures and used to minimize dilution (more cooling power comes from ice warming up to its melting point). Dry ice (solid carbon dioxide) and liquid nitrogen are sometimes used to chill glassware or are added to drinks, though these require special safety training and handling.
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By: Camper English