Ice cream with 30% overrun, sandy texture and shrinkage
A 500 L batch of ice cream mix contains 10.5% milk fat, 10.8% SNF, 14% sugar and 0.35% stabiliser–emulsifier. It is pasteurised, homogenised at 65°C, aged for 4 hours at 4°C, and frozen on a continuous freezer with a draw temperature of −5°C. The freezer run produces 650 L of ice cream. After three weeks in a storage freezer that cycles between −18°C and −12°C, packs show shrinkage, a sandy mouthfeel and visible ice crystals. The plant target is 70% overrun. The production manager blames the stabiliser and wants to increase it to 0.6%; the freezer operator says the mix is fine. You have the batch sheet and the finished-product complaints.
1. Calculate the actual overrun and the mix required to make 1,000 L at the target overrun. What does the gap mean commercially?
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Actual overrun: overrun (%) = (volume of ice cream − volume of mix)/volume of mix × 100 = (650 − 500)/500 × 100 = 30%.
The target is 70%. At 70% overrun, 500 L of mix should yield 500 × 1.70 = 850 L of ice cream. The run is 200 L short, so the freezer is incorporating far too little air.
Mix needed for 1,000 L at 70% overrun: mix volume = 1,000/1.70 = 588 L. The plant would need about 588 L of mix, not the 500 L it might otherwise assume.
Commercially, low overrun means more mix solids per litre and a denser, more expensive product. It also reduces the insulating air phase, so the product is more prone to heat shock, shrinkage and a cold, heavy eating quality. The 0.6% stabiliser proposal would not correct a 40-percentage-point overrun deficit; the freezer and mix processing must be checked first.
2. Distinguish the causes of the sandy mouthfeel and the shrinkage. Why is increasing stabiliser to 0.6% unlikely to solve the problem?
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Sandy mouthfeel is usually lactose crystallisation, not simply large ice crystals. Lactose is less soluble at low temperature; with 10.8% SNF, lactose is present at roughly 5.5–5.8% in the mix. Temperature cycling between −18°C and −12°C causes freeze-concentration and repeated lactose crystallisation, and crystals above about 15 µm are perceived as sandy. Large ice crystals from heat shock add a separate coarse sensation.
Shrinkage is mainly heat-shock damage: ice melts during temperature rises, water refreezes into larger crystals, air cells collapse, and the product contracts and pulls away from the pack. Low overrun (30%) makes the product denser and less able to resist this damage. A very short 4-hour ageing period may also leave the fat and stabiliser incompletely hydrated, weakening the structure.
Why 0.6% stabiliser is not the answer: 0.35% is already within the usual range. Raising it to 0.6% can make the product gummy, slow-melting and unpleasant without preventing lactose crystallisation. The dominant faults are air incorporation, total solids/water content and storage temperature cycling. Stabiliser adjustment should be a small refinement after those are corrected, not the main fix.
3. Give a prioritised corrective plan covering formulation, freezing, hardening and storage.
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1. Fix the freezer and overrun control. Check the air supply, dasher, mix feed pump, draw temperature and back-pressure. Measure overrun by weight for every run and aim for 70% ± 5%. The draw temperature of −5°C is reasonable, but it cannot compensate for poor air incorporation.
2. Improve the mix and ageing. The current total solids are about 35.7%, low for a 10.5% fat ice cream; aim for about 38–40% total solids. Increase milk solids carefully, but avoid simply raising SNF, which adds lactose. Replace part of the sucrose with corn syrup solids or invert sugar to control lactose crystallisation and improve body. Keep stabiliser–emulsifier near 0.35–0.4%, not 0.6%, and age the mix for 12–24 hours at 2–4°C for proper hydration and fat crystallisation.
3. Harden rapidly and store steadily. Harden at about −30 to −35°C with good air circulation, then store at −18°C or below without cycling. The measured swing to −12°C is enough to cause heat shock, shrinkage and sandiness. Check door seals, defrost cycles and stock rotation.
4. Verify the result. Re-test overrun, sensory texture and crystal size after one and three weeks. If sandiness persists after stable storage and reduced lactose load, review the SNF source and consider lactose-hydrolysed solids or a different sweetener blend.