Monday, February 3, 2014

Tools and Techniques of Making Ice Cream - Part Three

My post called, “The quest for a truly low carb ice cream” and parts one and two of this series cover mostly all of the basics of making homemade ice cream. Part three is a deep dive into ice cream science. A lot of it is going to seem redundant since I’m going to cover many of the same topics, but in greater detail. If you’re only interested in making (sugar free) homemade ice cream and you’ve read the earlier posts, you can skip this one if you like. Part four will be a short post detailing my Ice Cream Freezing Point Depression Calculator spreadsheet, so it will be written with the assumption that the reader is familiar with these topics. However, there’s nothing stopping anyone from just plugging in numbers to get the recommended thresholds and ignore all the theory behind it. (You can be a great driver even if you don’t know the internal workings of a car.) The amount of information on ice cream science is staggering. What I’m writing here is truly only the tip of the iceberg.

The are five basic ingredients to ice cream: milk product, sweetener, emulsifier, stabilizer, and air. Entire books have been written to describe the effect each of these components have on the finished product. I will attempt to convey the most important points of each. I will be including information about sugar and other high carbohydrate sweeteners, but I will not delve into the art and science of non-fat ice cream methods. I have too little interest and time to write about it. Besides, there is an abundance of non-fat and low-fat frozen dairy available in the USA. There should be no motivation to recreate it in your home unless you’re a masochist. My focus will be on the non-industrial home ice cream maker. I won’t discuss issues such as calculating overrun and fat content to meet legal requirements. I don’t care about saving pennies by eliminating high quality ingredients, like heavy cream, with inferior products.

If you want to do your own research, you should start with these three books:
Douglas Goff is a professor at the University of Guelph in Canada. He maintains a website there about ice cream: https://www.uoguelph.ca/foodscience/ice-cream

I had the pleasure to speak to Steve Young by phone. I can say without a doubt that he has forgotten more about food science than I will learn in my lifetime. Tharp and Young hold a three day course on ice cream every December in Las Vegas. Their website is http://www.onicecream.com/.

Milk


Before we can understand ice cream, it’s important to understand milk. “Milk is comprised of water, milk fat, and milk solids-not-fat (MSNF). The latter two components comprise the total milk solids. MSNF are the solids of skim milk and include lactose, proteins, minerals (ash), water-soluble vitamins, and enzymes plus some minor constituent.”1 The sugar found in milk is called lactose. It’s a disaccharide made up of glucose and galactose. (Sucrose a.k.a. table sugar is also a disaccharide but consisting of glucose and fructose.) “The nonvolatile salts of milk are the minerals that are found in the ash that remains after heating milk at a high temperature... The mineral content of milk ranges from 0.65 to 0.75%, while the average content of minerals in the salt form is 0.9%.”2

Milk contains fat, which “contributes significantly to the rich, full, and creamy flavor and to the smooth texture of ice cream. Part of the flavor contribution comes from the shortchain, volatile fatty acids that are part of the triglycerides of milk fat, particularly butyric acid.”3 Different kinds of milk based dairy products contain varying amounts of fat. Whole milk is approximately 3.25% fat and 5% lactose. As the percentage of fat increases, the percentages of lactose, protein, and ash decrease.

Typical Composition for Fluid Milk Cream Products (%)

Cream Product Water Fat Protein Lactose Ash
Half-and-Half 80.2 11.5 3.1 4.5 0.7
Light Cream 74.0 18.3 2.9 4.2 0.6
Light Whipping Cream 62.9 30.5 2.5 3.6 0.5
Heavy Cream 57.3 36.8 2.2 3.2 0.5
Plastic Cream 18.2 80.0 0.7 1.0 0.1

Source: Chandan, Ramesh C. Dairy-based ingredients. St. Paul, Minn., USA: Egan Press, 1997.

The primary “Sweet Cream Base” documented in the “Ben & Jerry’s Homemade Ice Cream & Dessert Book” uses a 2:1 ratio of heavy cream to milk.

Nondairy Fats


Fats from vegetable oils are sometimes used (usually outside of North America) to enhance flavors at a lower cost than milk fat. “Five factors of great interest in selection of fat source are the rate at which the fat crystallizes (which determines, in part, the aging time); the crystal structure of the fat; the temperature-dependent melting profile of the fat, especially at refrigerator and freezer temperatures (both factors determine how well the fat aggregates on freezing); the content of high-melting triglycerides, which can produce a waxy, greasy mouthfeel; and the flavor and purity of the oil. High molecular weight triglycerides are generally flavorless themselves, but often carry other flavor components. Likewise, they can play a role in the release of flavoring agents during consumption.”4

Milk Solids-Not-Fat/Protein Ingredients


The addition of MSNF components to ice cream mixes are sometimes used to enhance flavor, texture, mouthfeel, etc. Some of these include concentrated (condensed or evaporated) skim milk, dried (skim and whole) milk, whey protein, and nondairy protein ingredients.

Sweeteners


The number of options available to professional ice cream makers is enormous. Cane and beet sugars, corn sweeteners, maple sugar, honey, invert sugar, fructose, molasses, malt syrup, brown sugar, and lactose are some of the possibilities.5 Ben and Jerry have this to say about sweeteners:6
Ice cream requires some sort of sweetener whether it be sugar, honey, or maple syrup. We always use pure cane sugar, because our experience tells us that it’s not possible to make a better ice cream with any other sweetener.

If you prefer the taste of honey, you can substitute it for sugar on a one-to-one basis. Some people feel that honey is sweeter than sugar, others claim the opposite. The literature on the subject is conflicting. The darker the honey, the stronger the flavoring, so keep in mind that honey-sweetened ice cream will always have a slightly detectable honey taste.

When we first opened our store in the gas station, we used honey in several “health food” flavors, but after the ice cream hardened, we found that the texture was crumbly and the honey flavor overpowering. Since then, we’ve abandoned honey in favor of pure cane sugar.

Maple syrup lends a slight maple flavor to the ice cream. If this is what you want, use Grade A light amber maple syrup and substitute it at a ratio of 1 cup maple syrup for each ¾ cup sugar. Because of maple syrup’s high water content, it will dilute the concentration of butterfat in the ice cream mix and affect the final richness and texture of the finished product. Therefore, always be sure to use it in a mix with the highest butterfat content.

You may even consider increasing the proportion of butterfat in the mix by reducing the amount of milk and adding more cream. When we make Honey Apple Raisin Walnut Ice Cream or Maple Walnut Ice Cream, we always start with our richest cream base.

Corn syrup adds both sweetness and body to frozen desserts. It works well with fruit sorbets, but we don’t use it in our ice cream. Many commercial ice creams use corn syrup because it’s a cheap source of sweetness and body. We never recommend it for making ice cream.
Most of the ice cream industry traditionally uses a “combination of sucrose (10–12%) and corn sweeteners derived from hydrolysis of corn starch (corn syrup solids, CSS, usually 3–5%).”7 Ben and Jerry go on to say:
As for artificial sweeteners, we would never use them and don’t recommend them for making homemade ice cream or sorbets.
Well, considering this book was written in the late 80’s, I can understand that sentiment. Artificial sweeteners have come a long way since then.

Sugar Alcohols


People who have been doing low carb for a while will recognize polyols such as sorbitol, mannitol, xylitol, erythritol, lactitol, maltitol, isomalt, etc. These sugar alcohols each have different sweetness intensities and flavor characteristics. Many have a laxative effect and/or may cause gas and bloating depending on the amount ingested.

Name Sweetness relative to sucrose Food energy
(kcal/g) 		Glycemic Index Laxation Threshold
Erythritol 60% - 80% 0.213 ~ 0 0.66 - 0.80 g/kg*
Glycerol 60% 4.3 5 ???
HSH 25% - 50% 3.0 35 ~ 100 g/day
Isomalt 45% - 65% 2.0 2 50 g/day
Lactitol 30% - 40% 2.0 3 20-50 g/day
Maltitol 75% 2.1 35 100 g/day
Mannitol 50% - 70% 1.6 2 20 g/day
Sorbitol 50% - 70% 2.6 4 50 g/day
Xylitol 100% 2.4 12 50 g/day
Compare with:
Sucrose 		100% 4.0 65 N/A

* The laxative threshold of erythritol was estimated as 0.80 g/kg body weight for females and 0.66 g/kg body weight for males

Sources:

Nonnutritive Sweeteners


Non-nutritive sweeteners offer no nutritional benefits such as vitamins and minerals and they are low or have no calories. The FDA has given the label “Generally Recognized as Safe” (GRAS), to aspartame (NutraSweet and Equal), acesulfame potassium (Sweet One), neotame, saccharin (Sweet ‘N Low), and sucralose (Splenda). Stevia doesn’t have a GRAS distinction, but that doesn’t mean it’s dangerous. It just means there isn’t enough evidence yet either way.8

The sweetness of these products compared to sucrose are generally anywhere from 100-600 times greater. The caloric content and glycemic response can for all intents and purposes be considered zero because so little is needed. Truvia is over 99% erythritol, but is over 200% sweeter than sucrose due to the less than 1% stevia component.

Stabilizers


Stabilizers are added to increase mix viscosity, retard or reduce ice and lactose crystal growth during storage, provide uniformity to the product and resistance to melting, and produce smoothness in texture during consumption.9

Gelatin was one of the first stabilizer used in mass produced and home recipes. “However, the ice cream industry has made many improvements in the stabilization and emulsification of ice cream and other frozen desserts, and a range of polysaccharide gums has been available since the 1950s or 1960s, which, for both functional and economical reasons, has largely displaced the use of gelatin.”10 Most mass produced ice cream products today use various gums such as guar, locust bean, and cellulose. Many people who make ice cream at home use xanthan gum, but it’s not generally used by professionals due to its pseudoplastic characteristics.11

Carrageenan is used often in professionally manufactured ice creams. However, due to potential health concerns, I cannot recommend using it.


Emulsifiers


“An emulsifier is a substance that produces a stable suspension of two liquids that do not mix naturally, typically oil and water. In ice cream mixes, there is sufficient protein present to adequately emulsify the mix, so emulsifiers are not needed for fat emulsification in the classic sense. Their mechanism of action in promoting fat destabilization can be summarized as follows: they lower the fat/water interfacial tension in the mix, resulting in protein displacement from the fat globule surface, which, in turn, reduces the stability of the fat globule allowing partial coalescence during the whipping and freezing process. This leads to the formation of a structure of the fat in the frozen product that contributes greatly to texture and meltdown properties.12 The extent of protein displacement from the membrane, and hence the extent of dryness achieved, is a function of the emulsifier type and concentration.”13

“Most homemade ice cream has eggs or egg yolks that act as an emulsifying agent suspending the butterfat particles. (Many commercial ice creams use polysorbate 80.) Eggs also add texture to ice cream and improve its whipping ability. All in all, they help make a richer, creamier ice cream that holds up better in storage.”14 “Egg whites provide proteins with good water-holding properties, but they are not surface active, so the yolk is the functional part of the egg for emulsifying.”15

Air


The reason ice cream mixes need to be churned is to add an essential ingredient to the finished product: air. “All ice cream has some amount of air. If ice cream had no air at all, it would be a solid frozen block and totally unpalatable”16

The amount of air churned into ice cream is called the overrun. Some ice cream makers for home users have different dashers (paddles) for different levels of overrun. Gelato has little to no overrun and thus needs to be served at much higher temperatures. Gelato recipes usually include more egg yolks, more milk and less cream. It actually has less fat than regular ice cream, but gelato’s low overrun makes for an extremely dense, rich and creamy treat.

Flavorings


The most popular choices are vanilla, chocolate, and strawberry, but the combinations of natural and artificial flavors is limited only by the imagination. Continuous flavoring of an ice cream mix must be added prior to churning. Particulates (fruit, nuts, candy pieces, ripple sauces) should be added at the very end of the churning process. Adding a flavoring with a high moisture or water content will dilute the mix and affect the body and texture of the finished ice cream. When making fruit-flavored ice creams, for example, always starting with a high-butterfat mix.17 Ben and Jerry recommend the following:18
For certain fruits, it helps quite a bit if you can prepare them in advance, sometimes just a few hours beforehand, sometimes as much as a day before.

 Usually, we cut up fresh fruit, add sugar, and chill the mixture in the refrigerator in a covered bowl. Every half hour or so, we toss the fruit. When the fruit and sugar mingle, they combine and bring out the best in each other. The sugar lowers the freezing point of the fruit and prevents it from becoming too icy when added to the ice cream. The sugar also extracts the fruit’s natural juices and helps flavor the ice cream during the freezing process.
Ben and Jerry provide this chart of the more popular fruit flavorings and their recommended proportion of added sugar:19

Fruit Proportion of Fruit to Sugar
Bananas no added sugar
Apples 7:1
Strawberries 4:1
Peaches 4:1
Plums 4:1
Pineapple 4:1
Apricots 3:1
Blackberries 3:1
Cherries 3:1
Raspberries 2:1

Salt is sometimes added as a flavor enhancer. It also plays a role in the freezing point depression of the mix. The higher the fat of the milk product used, the lower salt content will be. Alcohol based flavorings also depress the freezing point. Care should be exercised when adding these kinds of flavorings.

Freezing Point


Sugars and milk salts are primarily responsible for the softness and scoopability of ice cream. Surprisingly, fat, proteins, large molecular weight carbohydrates, stabilizers, and emulsifiers do not contribute significantly to freezing point depression.

“The freezing point of ice cream is dependent on the concentration of the soluble constituents and varies with the composition. An average mix containing 12% fat, 11% MSNF, 15% sugar, 0.3% stabilizer, and 61.7% water has a freezing point of approximately -2.5°C (27.5 °F). The initial freezing point of ice cream mix is highly dependent on the sweetener content and MSNF of the mix, specifically the lactose and mineral salts content.”20

Freezing Point Depression Curve
Figure 1: Typical freezing curve for ice cream mixes of different composition showing percentage of water frozen at various temperatures21

In order to determine the total Freezing Point Depression (FPD) of a mix, the contribution of each ingredient must be calculated. A freezing curve based on cane sugar was documented by Alan Leighton in 1926. This has resulted in a freezing point depression table created from these experiments:

Table 6.1 Freezing point depression (°C) below 0°C of sucrose solutions (g/100 g water)22

g Sucrose / 100 g water FPD (°C) g Sucrose / 100 g water FPD (°C) g Sucrose / 100 g water FPD (°C)
3 0.18 63 4.10 123 9.19
6 0.35 66 4.33 126 9.45
9 0.53 69 4.54 129 9.71
12 0.72 72 4.77 132 9.96
15 0.90 75 5.00 135 10.22
18 1.10 78 5.26 138 10.47
21 1.29 81 5.53 141 10.72
24 1.47 84 5.77 144 10.97
27 1.67 87 5.99 147 11.19
30 1.86 90 6.23 150 11.41
33 2.03 93 6.50 153 11.63
36 2.21 96 6.80 156 11.88
39 2.40 99 7.04 159 12.14
42 2.60 102 7.32 162 12.40
45 2.78 105 7.56 165 12.67
48 2.99 108 7.80 168 12.88
51 3.20 111 8.04 171 13.08
54 3.42 114 8.33 174 13.28
57 3.63 117 8.62 177 13.48
60 3.85 120 8.92 180 13.68

Data were extrapolated from Leighton (1927), which were originally derived from Pickering (1891 as cited by Leighton)

From “Ice Cream” by Goff and Hartel:23
To calculate the freezing point of a given mix, the first step is to determine the equivalent content of sucrose in the mix, based on all the mono- and disaccharides that are present. This is referred to as the sucrose equivalence (SE) in g/100 g of mix.

SE = (MSNF × 0.545) + (WS × 0.765) + S + (10DE CSS × 0.2) + (36DE CSS × 0.6) + (42DE CSS × 0.8) + (62DE CSS × 1.2) + (HFCS × 1.8) + (F × 1.9)

Where:

MSNF = milk solids-not-fat, 0.545 is the percentage of lactose typical of MSNF.
WS = whey solids (from dry or condensed whey), 0.765 is the percentage of lactose typically found in whey solids.
S = sucrose or other disaccharides such as maltose or lactose (if added directly as a separate ingredient, otherwise it is accounted for in the MSNF or whey solids calculations) or disaccharide alcohols such as maltitol or lactitol.
DE = dextrose equivalence of the CSS, the various factors convert the carbohydrates in the starch hydrolysate to sucrose equivalents.
HFCS = high fructose corn syrup, which is mostly monosaccharide, hence the factor of 1.8 to convert it to equivalent sucrose content.
F = pure fructose or other pure monosaccharides such as dextrose or monosaccharide 6-carbon alcohols such as sorbitol, the factor of 1.9 to convert it to equivalent sucrose content based on molecular weight ratios; all in g/100 g mix (or %).

If blended protein, lactose, and mineral ingredients are used as a source of MSNF, the lactose and salts in those ingredients should be included directly in the calculation rather than using the factors for MSNF or WP. Simply ensure that all lactose and salts are accounted for and none are double-counted. If xylitol (5-carbon sugar alcohol, molecular weight 152), erythritol (4-carbon sugar alcohol, molecular weight 122), or other such low molecular weight sweeteners are included in the formulation, the molecular weight of sucrose (342) divided by their molecular weight could be used as the appropriate factor

The equivalent concentration of sucrose in water (g/100 g water) is then determined by dividing the SE by the water content.

g sucrose/100 g water = SE × 100 / W

where: W is the water content (100 - total solids, %).

To obtain the freezing point depression associated with this concentration of SE in water, FPDSE, Table 6.1 is used.

The contribution to freezing point depression from salts in MSNF and WS is found using the following equation:

FPDSA = (MSNF + WS) × 2.37 / W

FPDT = FPDSE + FPDSA
The formula for sucrose equivalence includes a lot of sweeteners and other MSNF uncommon to home ice cream making. High fructose corn syrup, pure fructose, whey solids, and maltodextrins are used by the ice cream industry to custom craft ice cream recipes at minimal cost. (Whey protein is an additive that may be desired for nutritional reasons. However, I don’t trust that the multiplier in the original formula will work for over the counter whey proteins. It’s impossible for me to predict all of the additives in the whey protein mix that can affect the freezing point.) The sucrose equivalence formula can be simplified and expanded to include other items that will be needed. Soluble ingredient data needed to calculate FPD for my low carb recipes was erythritol, salt, vanilla extract, alcohol, and glycerol. Take the molar mass of sucrose and divide it by the molar mass of the ingredient and that becomes a new multiplier that can be inserted in the SE formula.

Ingredient Sucrose Equivalence
Erythritol 2.8
Salt 5.9
Vanilla Extract 2.5
Glycerin (Glycerol) 3.7
Alcohol (Ethanol) 7.4
Sucrose 1.0

The SE formula can be rewritten as follows:

SE = (MSNF × 0.545) + S + (E × 2.8) + (NaCl × 5.9) + (VE × 2.5) + (G × 3.7) + (A × 7.4)

Calculating the FPD for your recipe is extremely important if the ingredients in a known good recipe that effect FPD are to be significantly altered. The most obvious example would be replacing sugar with sugar alcohols and non-nutritive sweeteners. A desirable FPD is paramount to modifying sucrose based recipes. “Whelan et al. (2008) examined a number of polyol sweeteners in low glycemic index formulations. Once the freezing curves were matched, other physicochemical properties also were found to match.”24

Let’s start with the first Ben and Jerry’s Sweet Cream Base recipe:
  • 2 large eggs
  • 2 cups heavy cream
  • 1 cup milk
  • ¾ cup sugar
The variables needed for the FPD formulas can be obtained by getting the data for each ingredient from http://nutritiondata.self.com/. The website doesn’t provide the MSNF value, but it can be calculated:

MSNF = Total Mass - Fat - Water

Disclaimer: My formula for MSNF from nutritional data is based on my own observations. Other documented estimates of MSNF vary and there’s no explicit calculation method that I’ve found so far. The formula I provided yields a result close to other published results, but I cannot vouch for its accuracy. Such is the risk of taking advice from an autodidact.

The two cups of heavy cream has a total weight of 476g including 176g of fat and 274g of water. Therefore, its MSNF is 26g or 5.46%. The same can be calculated for the milk: MSNF = 244g - 8g - 215g = 21g or 8.61%. This agrees (more or less) with the documented percentages for heavy cream and whole milk from other sources.

We need to know the total weight of the mix to calculate the sucrose equivalence since the numbers need to be based on 100g. The sugar weighs 150g and the 2 eggs are 100g. Altogether, the mix is 970g. The percent sucrose is 150g / 970g = 15.46%. MSNF contribution of the heavy cream is 26g / 970g = 2.68% and from milk is 21g / 970g = 2.16%.

Sucrose equivalence is easy in this example since we only have to worry about the sugar and MSNF:

SE = (MSNFHeavy Cream × 0.545) + (MSNFWhole Milk × 0.545) + S

SE = (2.68 × 0.545) + (2.16 × 0.545) + 15.46

SE = 18.10

Basically, all we did was estimate the lactose from the MSNF. I’d feel more comfortable calculating the lactose directly than use a MSNF fudge factor, but that data isn’t available from the http://nutritiondata.self.com/ website. The recipe is very simple so we didn’t need to calculate the sucrose equivalence for anything other than the milk and cream. Notice that I didn’t consider the effect of the eggs. I don’t know the fudge factors to convert the minimal salt and sugar content of eggs to sucrose equivalents, so we’ll have to ignore them. (I suspect the yolks have no effect since they’re primarily fat. The egg whites are mostly protein, so they may have some very small effect.)

Next, we have to find “g sucrose/100 g water.” First we need to know how much water all of the ingredients (including the eggs) yield and calculate the percentage of water for the mix. All of the individual ingredient water weight can be obtained from the http://nutritiondata.self.com/ website. The total water content of the mix is 564.8g and dividing that by the total weight of 970g yields 58.23%.

g sucrose/100 g water = SE × 100 / W

g sucrose/100 g water = 18.10 × 100 / 58.23

g sucrose/100 g water = 31.08

We need to look up the 31.08 value on the FPD table to get the FPDSE value. There’s an entry for 30 and 33, but nothing specifically for 31.08 so we have to interpolate. The interpolated value is 1.92.

Finding FPDSA is straightforward for this example.

FPDSA = MSNF × 2.37 / W

FPDSA = (26 + 21) / 970 × 2.37 / 58.23

FPDSA = 0.20
And so...

FPDT = FPDSE + FPDSA

FPDT = 1.92 + 0.20 = 2.12

We now know that the Ben and Jerry’s Sweet Cream Base has a FPD of 2.12 or -2.12 °C. See how easy that was? No! It wasn’t easy at all. This is why I created a spreadsheet to do the calculations automatically based on the recipe inputs. This example was as simple as it gets. Throw in the complexity of artificial sweeteners and other components and you’ll be pulling your hair out.

Summary


I’ve presented what I believe are some of the most important scientific aspects of making ice cream at home. You should now be able to take the recipes from Ben and Jerry (or any other source), add stabilizers, and swap out sugar for other sweeteners, if desired. While you may understand the importance of FPD and know the method for calculating it, automating the process will be the next logical step. Part four will discuss my ice cream freezing point depression calculator spreadsheet and how to use it. I recently discovered and fixed some errors in the calculations, so check back periodically for updates.

References

  1. Goff, H. Douglas, and Richard W. Hartel. Ice cream. 7th ed. New York: Springer, 2013. p 45.
  2. p 50-51.
  3. p 52.
  4. p 54.
  5. p 66.
  6. Cohen, Ben, Jerry Greenfield, Nancy J. Stevens, and Lyn Severance. “Ice Cream Theory.” In Ben & Jerry’s Homemade Ice Cream & Dessert Book. New York: Workman Pub., 1987. p 20.
  7. Goff, H. Douglas, and Richard W. Hartel. p 67.
  8. American Heart Association. “Non-Nutritive Sweeteners (Artificial Sweeteners).” Getting Healthy. http://www.heart.org/HEARTORG/GettingHealthy/NutritionCenter/Artificial-Sweeteners_UCM_305880_Article.jsp (accessed February 1, 2014).
  9. Goff, H. Douglas, and Richard W. Hartel. p 75-76.
  10. p 76.
  11. p 80.
  12. Goff HD, Jordan WK (1989) Action of Emulsifers in Promoting Fat Destabilization During the Manufacture of Ice Cream. Journal of Dairy Science 72:18–29
  13. p 82.
  14. Cohen, Ben, Jerry Greenfield, Nancy J. Stevens, and Lyn Severance. p 20.
  15. Goff, H. Douglas, and Richard W. Hartel. p 83.
  16. Cohen, Ben, Jerry Greenfield, Nancy J. Stevens, and Lyn Severance. p 21.
  17. p 22.
  18. p 24.
  19. “Fruit Flavors.” p. 50.
  20. Goff, H. Douglas, and Richard W. Hartel. p 147.
  21. p 148.
  22. p 182.
  23. p 181-183.
  24. p 441.
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