Simple Sorbet Science

By Michael Laiskonis – Culinary Director, Boiron Americas

Sponsored by 

Few things are better than a smooth, clean, and full-flavored sorbet showcasing the peak flavor, aroma, and color of fruit. On its surface, sorbet is quite simple. Perfecting frozen desserts, however, requires familiarity with the complex interplay of ingredients, some important math, and attention to technical best practices throughout the process. I’ve spent years gaining deeper knowledge of sorbet and ice cream technology, but for those just getting started, I like to highlight three key aspects of understanding: formulation and ingredient composition, the role of sweeteners, and the nature of ice. With these fundamentals, pastry chefs can begin to refine their recipes, troubleshoot problems, and, ultimately, create their own recipes.

Formulation: Know Your ingredients

Like so many pastry preparations based on proportion and ratio, the formulas needed to create a perfectly textured, flavorful frozen dessert are quite narrow in scope, and standards remain largely unchanged since early technological advancements more than a century ago. There may be no single ‘ideal’ sorbet formula, but we can assemble bespoke recipes, much like an algebraic equation, based on the flavors at hand and our desired result. The key to success is knowing which components are fairly static and which are variable. And then we need to know the composition of our ingredients and how they supply those basic components such as water and solids, namely sugars.

In very simple terms, we can think of a sorbet (for which there is no legal definition) as a frozen mixture containing roughly 30% solids — with sugars providing the bulk of that mass — along with stabilizers, flavorings, and non-soluble solids. The balance of the formula is water, roughly 70%, and the nature of the solids ultimately controls how that water behaves in the finished sorbet.

We can further refine a typical sorbet formula within the following ranges:

• Fruit or other bulk ingredients: 25-70%
• Sucrose: 10-15%
• Glucose Powder: 4-6%
• Dextrose Powder: 0-5%
• Invert Sugar:  0-4%
• Nonfat Milk Solids: 0-2%
• Stabilizers: 0.2- 0.5%

Total water: 67-70%
Total Solids: 30-33%*

*Some exceptions made for non-soluble solids, fats, alcohols, types of stabilizer, etc.

If we want to build any frozen dessert formula, it’s crucial to know the composition of our individual ingredients. Of course, this basic information is important to pastry chefs no matter the preparation at hand. With knowledge of an ingredient’s composition, structure, and function comes true power to the cook. Rather than thinking of, say, an apple, as simply an ‘apple’, one must strip it down to its base components of water, fiber, sugar, pectin and acid in order to understand how it will behave in an application. For the purpose of creating a balanced sorbet recipe, a fruit’s water and solids content is key.

For example, among the range of Les vergers Boiron 100% purées, such technical aspects are available to guide us. Mango will have a consistent Brix or sugar content of 19% (with a range of +/-2%) and a slightly higher corresponding total solids content of 19.5% by weight, while Purple Fig provides a sugar content of 20% and total solids reaching 24%. Useful for building the recipe to follow, the PGI Corsican Clementine purée has a solids content of 11.2% and 11% sugar. The relative intensity of a fruit’s flavor will also determine how much of that fruit is used; subtle White Peach may make up 60% or more of the total recipe, but bolder Bergamot or Kalamansi may only comprise 30% of the total recipe. The varying proportions of water and solid in each of these fruits will of course determine the added water and sugars needed to achieve a balanced recipe.

The properties of sweeteners and stabilizers will be covered in further detail below, but from a composition perspective, we need to account for the water content of ‘dry’ ingredients as well – invert sugar (roughly 20%), glucose powder (5%), or dextrose (7%). Such information can set us on the path of formulating new sorbets and ice creams, or it can be helpful in reverse-engineering existing recipes to see where its components may fall along the formulation spectrum.

Soluble Solids: Sweetness and Freeze Point Depression

Non-soluble solids in sorbet are usually minimal and, thus, their effects are of little concern to us. However, the soluble solids – sugars – are of great importance in terms of tempering overall sweetness and dialing in texture and firmness. Water containing dissolved solids, such as salt and sugar, is affected by colligative properties. These solutes will raise the boiling point of water on the high end of the temperature range (as in a boiled sugar syrup), and at the low end, they lower the freezing point of water. It is this very property of freeze-point depression that makes sorbet and ice cream possible at all so that at serving temperatures below water’s freezing point it is soft enough to scoop and consume.

Different solutes – for the sake of our discussion, sweeteners – will lower the freezing point of water to varying degrees. The measurement that we use to correlate freeze-point depression is a sweetener’s molecular weight – the lower the molecular weight, the greater the effect of freeze-point depression. Sucrose, for example, has a molecular weight of ‘342’, with dextrose coming in at ‘180’ and an average powdered glucose at ‘472’. With this we can say that a solution of dextrose will lower the freezing point of water to nearly twice that of sucrose, while a glucose solution will raise the freezing point relative to sucrose. What does this mean for us? Simply put, we can use multiple sweeteners to modify the freezing point – the relative firmness or softness – of sorbet or ice cream.

Different sweeteners also have a different ‘sweetening power,’ which also allows us the ability to fine tune the perceptible sweetness, all while maintaining a fairly constant percentage of total solids. As a standard sucrose is given a sweetening power of ‘100’, with invert sugar at about ‘125’, and glucose somewhere in the range of ‘50’ (glucose can offer a range of properties based on how it is processed – reflected in its DE, or level of dextrose equivalence). Thus, for the sake of comparison, replacing some of the sucrose in a formula with invert sugar will simultaneously give us more sweetness, while also lowering its freezing point. It is even more useful that added glucose will offer less sweetness while also raising the freezing point, giving us firmer textures at higher temperatures. It is also helpful to know that lactose in dry milk powders, with a relatively low perceptible sweetness (its score is less than ‘20’), will lower the freezing point to the same degree as sucrose.

The molecular weight of sodium chloride – salt – at ‘54’ will lower the freezing point of water over six times that of sucrose (which is why we put salt on icy roads and not sugar!). And ethanol – pure alcohol – will lower the freezing point by a factor of seven times that of sucrose, with its molecular weight of ‘46’. Formulating ice creams with alcohol can often be frustrating; two general rules of thumb to consider are the need for a reduction in dissolved solids (down to 23-25%) and the addition of a maximum of about 7% pure alcohol. Formulations must also be adjusted when adding ingredients like chocolate and fruit – which might bring sweeteners, water, or fats of their own. With more ingredients, the math becomes more complex.

Controlling Water: The Amount and Size of Ice Crystals

Sorbet, by nature, is made up of a lot of ice. Improper formulation or handling can result in the negative attributes of too much ice, or in too large a crystal size. Two important concepts to remember:

The amount and type of solutes (sugars) in the unfrozen water phase of sorbet determines the volume of ice crystals that form…

and

…the rate and speed of freezing the base mix determines crystal size. The lower the temperatures, the faster the base freezes to produce the smallest possible ice crystal. These ice crystals will always be at their greatest number and at their smallest size the moment they are extracted from the batch freezer and they can never get smaller.

In other words, whereas the type of sweeteners we choose will determine the overall sweetness of the ice cream, the sum total of those sweeteners and their effect on the freezing point determine how much of the water will turn to ice.  It is also interesting to consider the idea of freeze concentration: as a solution freezes, only pure water crystallizes into ice, which means the concentration of solutes in the remaining unfrozen water increases, which also means that the freezing point of that unfrozen water continues to drop as more water turns into ice. Thus, even at a temperature of about 3˚F/-16° C – below the typical serving temperature of sorbet and ice cream – only about 72% of the total water in a base mix is frozen as ice. The rest remains unfrozen as a very concentrated sugar solution.

For optimum mouthfeel, our goal is to ensure the ice crystals are as small as possible. It’s all about speed and temperature. A high-end batch freezer that can process sorbet in a few minutes will make ‘better’ sorbet, with a smoother texture. We determine ‘doneness’ or ideal extraction of sorbet and ice cream by temperature, usually a minimum of -5˚°C or 23°F; at this point only half the water in the mix has frozen. We then transfer the sorbet to a blast freezer to ‘harden’ fully, and we freeze the remaining water.

From here, it makes sense that as the sorbet is exposed to increasingly higher temperatures, some of that frozen water will melt, forming increasingly larger crystals if and when the temperature drops again. This is usually referred to as thermal shock. Accretion, the fusing of large ice crystals over time, is related, but slightly different. For example, the ‘shelf-life’ – texturally speaking – of sorbet and ice cream stored below -20°C or -4°F may extend beyond two weeks but increase the storage temperature to -15°C or 5°F and that shelf-life dramatically drops to as short as two days.

Stabilizers collectively refer to a category of additives – most often polysaccharide hydrocolloids – that act upon the water phase of a sorbet base. While the most common components in sorbet stabilizers are the thickeners locust bean and guar gums, we may see a range of other ingredients in recipes and stabilizer blends, such as pectin, alginate, carrageenan or xanthan. Gelatin is particularly well suited for the task, but has fallen out of favor in order to accommodate dietary restrictions. Though each of these ingredients offer unique properties, they all work to bind water and add body.

Stabilizers are responsible for adding viscosity to the unfrozen portion of the water, contributing to overall mouth feel and enhancing the ability of the base mix to hold air during the freezing process. Binding water stabilizes it, so that it cannot migrate within the frozen product. Without the stabilizers, the sorbet would become coarse and icy very quickly due to the migration of this free water and the growth of existing ice crystals. Stabilizers improve melting qualities and help to prevent thermal shock. Stabilizers can be easily misused; in excess, they contribute off-favors and gummy textures. The typical dosage of a stabilizer blend rarely exceeds 0.2 to 0.4% by weight of the total base.

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The study of frozen desserts and the ingredients that go into them can be a lifelong pursuit, but these basic principles can be put to immediate use toward the goal of enhancing frozen desserts in our own kitchens. For more recipe inspiration and technical information, visit: https://www.my-vb.com/.

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PGI Corsican Clementine & Roasted Red Pepper Sorbet

By Michael Laiskonis
Culinary Director, Boiron Americas

The flavor of this sorbet evokes the sunny Mediterranean, pairing smoky and savory roasted pepper with sweet citrus and a subtle note of coriander seed – perfect for a transitional palate cleanser or pre-dessert.

Yield: approximately 12 pre-desserts

PGI Corsican Clementine & Roasted Red Pepper Sorbet

• 100 g granulated sugar, divided
• 3 g sorbet stabilizer
• 125 g water
• 70 g glucose powder
• 50 g invert sugar
• 1 g coriander seed, ground
• 100 g roasted red pepper, peeled, seeded, and puréed
• 400 g Boiron PGI Corsican Clementine purée

1. Combine 20 g of the sugar with the sorbet stabilizer; reserve.

2. In a saucepan, heat the water to 120°F (50°C). Whisk in the stabilizer mixture, the remaining 80 g sugar, the glucose powder, invert sugar and coriander seed.  Bring just to a boil and remove from heat. Chill and allow the syrup to mature at least 4 hours.

3.  Combine the purées and syrup and process in batch freezer; extract the mix at 23°F (-5˚C). Alternatively, transfer to PacoJet canisters and freeze; process as needed. Continue to harden the sorbet at 0°F (-18˚C) as necessary.

Neutral Caramel

• 100 g fondant syrup
• 100 g glucose syrup
• 100 g isomalt

1.  Combine the fondant, glucose, and isomalt in a saucepan and bring to a boil. Cook the mixture to 325°F (162°C).

2.  Remove from the heat and pour onto a silicone mat and allow to cool completely. Transfer to a food processor and grind to a fine consistency.

3. Sift over a desired stencil onto a silicone mat. Remove the stencil and cover with a second mat. Bake at 300°F (150°C) for 90 seconds. Remove from the oven and allow to cool.

4. Store in an airtight container.

PGI Corsican Clementine Coulis

• 200 g Boiron PGI Corsican Clementine purée
• 25 g granulated sugar
• 0.5 g xanthan gum

1. Combine all ingredients and blend well.

Assembly

• Extra virgin olive oil
• Piment d’Espelette
• Maldon salt
• Herbs or flowers, as desired

1. Place a scoop or quenelle of the sorbet into a chilled dish and garnish with the clementine coulis, neutral caramel, piment d’Espelette, salt, olive oil and herbs.

(This article appeared in the Winter 2024 issue of Pastry Arts Magazine)