Part IV — Management and Quality
Diagnosing Defects
I difetti del gelato
A gelato that has gone wrong always carries a reason. Too hard, gritty, watery — each of these is the result of something that happened in the formulation or in the process. This chapter gathers the craft of working back from the symptom to the cause. Every chapter so far becomes one toolbox here.
A defect points at its cause
Judging the quality of gelato is harder than it looks. A clean line between pass and fail is difficult to draw, and judgment carries some subjectivity. Even so, a plain defect (difetto) — grittiness, hardness, wateriness — is a firm handhold. A defect is a symptom, and a symptom always points at a cause.
Diagnosis is the work of reasoning back from the symptom to the cause. It calls on every chapter so far — on what each ingredient brings in (Part III), and on how the components behave (Part I). It calls too on what happens in the steps (Part II). Knowing these is what lets the question be read: why did this symptom appear? This chapter is that knowledge, rebuilt into the shape of a diagnosis.
The order of diagnosis — temperature, formulation, process
Hunt for the cause at random and the trail is lost. There is an order to the sorting.
Suspect the temperature first. With the same formulation, a few degrees off in the case moves the hardness a great deal (Chapter 7). Then the formulation — corpo, the amount and the composition of the solids (Chapters 2 to 5). Last the process — struttura, the conduct of pasteurization, freezing, and hardening (Chapters 6 to 8). The distinction introduced in Chapter 1 works here as the two pillars of diagnosis. The nearer the cause stands to the front, the easier it is to fix, and the faster the effect.
From symptom to cause — a table to read at a glance
The principal defects, set out as symptom, chief cause, and countermeasure. Every cause answers to a chain of cause and effect treated in an earlier chapter.
| Symptom | Chief cause | Countermeasure and reference |
|---|---|---|
| Hard, heavy | Formulation: too little total solids; too little sugar (too little PAC) Process: storage temperature too low; aging too short; too little air taken in; stabilizers used wrongly | Read the product temperature first. Then, for a crema, see whether the total solids have fallen under 32 % or the sugars under 16 %, and replace part of the sucrose with a sugar of higher PAC (Chapters 2, 5, 7) |
| Soft, watery | Too much PAC; too little solids and fat | Review the split of the sugars, and the solids and fat (Chapters 2, 3, 5) |
| Sandy (gritty) | There are two kinds. Lactose crystals: too much MSNF; too much milk powder (half of it is lactose); milk powder blended with whey powder; a formulation using sucrose alone in excess; condensed milk that has already begun to crystallize Ice crystals: too little MSNF — the water goes unheld and freezes Process: swings in the storage temperature | Take 12 % of MSNF as the guide, and confirm the ceiling calculated for that formulation. Check that the milk powder is not blended with whey powder. Replace part of the sucrose with another sugar. Warm old condensed milk to dissolve the crystals before use. Where the MSNF is short instead, raise it (Chapters 3, 5, 7) |
| Coarse (ice particles felt) | Freezing too slow; hardening too slow; swings in temperature; the scraper blade no longer shaving | Freeze and harden fast, and hold recrystallization off. Check as well that the blade is well sharpened, undeformed, and close against the wall of the cylinder (Chapters 1, 7, 8) |
| Buttery | Formulation: too much fat; too little MSNF Process: homogenization or aging too short | Review the balance of the fat and of the milk solids both, and check the homogenization and the aging (Chapters 3, 6) |
| Gummy | Too much stabilizer; the wrong kind of stabilizer | Reduce the amount. If that does not settle it, change the product itself (Chapter 4) |
| Spongy | A poor balance; too much yolk; too much stabilizer; too much air taken in | Review the balance, the stabilizer, and the overrun (Chapters 4, 5) |
| White spots on the surface (sorbetto) | Formulation: supersaturation of sugar (fruit formulations run higher in sugar than milk ones); too much sucrose Process: storage temperature not low enough; swings in the storage temperature | Check the cabinet temperature first, and its steadiness. Then keep the surface from drying, and replace part of the sucrose with glucose syrup or another sugar (Chapters 2, 7, 9) |
The correspondence of symptom, cause, and countermeasure rests on the chains of cause and effect established in Chapters 1 to 9 of this book. It is an original arrangement; the table in the technical literature is not reproduced. The row for white spots is corrected against a second source, which names the trigger as evaporation from the surface rather than the formulation.
One thing is worth holding in mind when the table is used. The same symptom has a cause on the formulation side and a cause on the process side, both.
The table in the literature sets these two in separate sections. The first is the formulation — causes tied to the composition of the materials. The second is the process — faults in the conduct of homogenization, aging, mantecazione, hardening, and storage. And the second section carries a note: a defect arising from the process can be amplified by a poor formulation. The reverse holds as well. A defect arising from the formulation can come off lightly where the process is sound.
So the road back from symptom to cause always runs in two lanes. The distinction set up in Chapter 1 between corpo, the formulation, and struttura, the process, becomes a tool at the moment of diagnosis. Only the row for "hard, heavy" is written out in the two lanes above. It is the symptom on which the pair shows most plainly.
Note that in this division, the temperature of storage and serving counts on the process side. The opening of this chapter cut temperature out and put it in front. Among the process causes, it is the one confirmed with the least effort.
And in practice, the side to look at first is the process. The reason is simple: it is quicker to get to a fix. Suspect the formulation and a trial batch must be made again, while reading the thermometer in the cabinet is finished today.
Reading a few defects through
The reasoning behind the table, confirmed on representative cases.
Sandiness comes mostly from an excess crystallization of lactose. Raise the MSNF and the lactose rises with it, past what will dissolve, and grows into coarse crystals (Chapter 3). And a storage temperature that moves up and down accelerates that growth. So sandiness is born on two sides at once. One side is the formulation, meaning the amount of lactose. The other is the process and the storage, meaning the steadiness of the temperature. The diagnosis has to look at both.
Buttery texture arises less from having too much fat than from having handled the fat badly. Where homogenization is insufficient and the fat globules stay large, the fat slips its control and gathers into lumps. The same happens where aging is short and the fat freezes still liquid (Chapters 3, 6). More often than not, "review the homogenization and the aging" comes before "reduce the fat".
A coarse texture is the result of ice crystals grown large. The causes are three: freezing too slow, hardening too slow, and swings of temperature during storage. Each of them means the critical temperature zone was crossed slowly (Chapters 1, 7). Let the process be careless and this defect appears, however correct the formulation.
White spots on the surface are easily misdiagnosed. Conclude that there is too much sugar and set about cutting the total, and the aim is usually off. The trigger lies not in the formulation but in water having escaped from the surface. Where an exposed face loses water to evaporation, the sugar concentrates just there, and sucrose comes out of solution as crystals in time. Sucrose is very slow to crystallize, so the defect itself is rare. Turn that around, and it says something: when it does appear, the surface has been dry for that long. What to suspect first is the covering and the storage, not the recipe.
Approfondimento — deep diveHow ice grows in storage — Ostwald ripening and heat shock↓ Go deeper
Defects on the tongue — taste and aroma, not texture
Everything the table has handled so far is a defect of texture. Hard, gritty, gummy — faults of structure, known to the hand and the mouth. But defects appear on one more layer: taste and aroma.
This layer has a character of its own. A defect of texture has its cause in the formulation or the process — in what the maker did. A defect of taste or aroma, though, has often begun before the material entered the workshop. Rancid oil, old milk powder, cream on the turn. However correct the making, what came in bad comes out in the product.
The literature lists more than ten defects of taste and aroma. Five are taken here — those whose cause this book can explain with the chains of cause and effect it has already treated. The rest lie in territory where this book holds no mechanism yet — the material of the utensils, the dosing of flavorings. So they are named and no more: metallic, salty, weak in taste, too strong in taste, tallowy.
| Symptom | Chief cause | Countermeasure and reference |
|---|---|---|
| Sour | The mix cooled outside the pasteurizer, at room temperature; aging run too long; the material already on the turn | Finish the cooling inside the pasteurizer. Take a single night as the default for aging, and do not carry it over to a second (Chapter 6) |
| Cooked | Overheating, above all in a formulation carrying egg yolk taken too far up the scale | Hold 85 °C as the mark even for high pasteurization, and do not pass it. With yolk in, the proteins begin to cluster at 83–85 °C (Chapters 6, 10) |
| Of milk powder | Too much skim milk powder; the powder itself degraded | Confirm the ceiling on MSNF by calculation. The same cause produces sandiness (lactose crystals), so read this with the table above (Chapters 3, 5) |
| Pasty, like glue | Too much stabilizer | Reduce the amount. It is the same cause as gummy texture, come out in the mouth in another form (Chapter 4) |
| Rancid (the taste of old oil) | Oxidation of fat. Materials held too long in store; nut paste left with its oil separated and gone old; aging overrun; storage too long | Turn over the fat-rich materials faster. Knead a separated nut paste back together and use it up. Put a limit on frozen storage as well (Chapters 6, 10) |
The correspondence of symptom, cause, and countermeasure rests on the chains of cause and effect established in Chapters 1 to 10 of this book. Of the twelve kinds the literature lists, five were chosen and rebuilt — those whose mechanism this book can explain. This is not an exhaustive list. The table in the literature is not reproduced.
Set the five side by side and something comes into view. The taste of milk powder and sandiness share one cause — too much milk powder. Pastiness and gummy texture share one cause — too much stabilizer. An excess, that is, appears on the texture and on the taste at once, wearing a different face on each. Set out to fix one alone and the aim is off. When the tongue says "pasty", that is a diagnosis about the texture as well.
The artisan's view
The best countermeasure is the record
The strongest weapon for a fast and accurate diagnosis is the record. Keep the formulation, the pasteurization temperature, the aging time, and the storage temperature day by day (Chapter 12). Then, when a defect appears, what was different this time can be found at once. A workshop without records begins the hunt for the cause of the same defect from scratch every time.
Diagnosing defects is the craft that bridges the knowledge of Parts I to III to the trouble on the floor. With this, the round of designing gelato, making it, guarding it, and repairing it is complete. The remaining two chapters of Part IV take up what it needs to go out into the world as a product. Regulation and labeling, and cost and price.