Two powders on the same quotation sheet, both labelled “trehalose”, both white and crystalline, and a 10% difference in how much sugar you actually get per kilogram. That is the practical problem behind the anhydrous versus dihydrate decision. This article gives the arithmetic, the certificate traps, the process consequences and a decision table so that a buyer can compare offers without chemistry support in the room. The reference case is a food-grade anhydrous crystal, assay ≥98.0%, loss on drying ≤1.5% (CAS 99-20-7), compared with the dihydrate (CAS 6138-23-4).
1. The one number that decides everything
Anhydrous trehalose is C12H22O11, molecular weight 342.30. The dihydrate is C12H26O13, molecular weight 378.33, because two water molecules are part of the crystal lattice, not loosely adsorbed on the surface. Dividing the two gives 0.9048: one kilogram of perfectly pure dihydrate contains 0.905 kg of trehalose and 0.095 kg of water that you have paid for and will not use.
| Quantity | Anhydrous (99-20-7) | Dihydrate (6138-23-4) |
|---|---|---|
| Molecular weight | 342.30 | 378.33 |
| Water of crystallisation | 0 | 9.52% theoretical |
| Trehalose per 1,000 kg purchased | 1,000 kg at 100% purity; about 985 kg at LOD 1.5% | 904.8 kg at 100% purity; about 885 kg at LOD 9.5% |
| Water paid for per tonne | up to 15 kg (residual only) | about 95 kg |
| Maximum as-is assay a laboratory can report | 100.0% | 90.48% |
| Dry-basis assay at typical moisture | 99.5% (LOD 1.5%) | 99.4% (LOD 9.5%) |
Note the last line: on a dry basis the two crystals are equally pure. The difference is entirely water. This is why a supplier who quotes purity figures can make either form look identical, and why the purchase specification has to name the hydrate form and the assay basis in the same sentence.
2. Reading a certificate of analysis: four checks
2.1 Hydrate-versus-moisture consistency
Loss on drying for the dihydrate sits around 9.0 – 9.6%; for the anhydrous form it is at or below 1.5%. Any certificate showing an as-is assay of 98% together with roughly 9.5% moisture is internally impossible – pure dihydrate cannot exceed 90.48% as-is. Either the assay is dry-basis and unlabelled, or the numbers belong to a different batch. Both cases require a written clarification before a purchase order.
2.2 Assay basis conversion
Dry basis = as-is / (1 – LOD). A material at 98.0% as-is with 1.5% moisture is 99.5% dry basis. A competitive-looking offer of “98% purity” that turns out to be dry basis at 1.5% moisture is only 96.5% as-is – a shortfall of 1.5 percentage points, or about 15 kg of sugar in a tonne.
2.3 Colour and turbidity
Chromaticity ≤0.100 and turbidity ≤0.05 are the markers of a carbon-treated and ion-exchanged liquor. These limits matter far more in clear beverages and in diagnostic reagents than in bakery, where a slightly darker powder performs identically. Paying for colour control you cannot use is a common and avoidable cost.
2.4 Particle size and bulk density
Mesh cut affects dissolution time and dusting, and bulk density affects how much fits in a jumbo bag or a container. Two offers at the same price per kilogram can differ by 8 – 12% in container payload once bulk density is taken into account.
| COA line | Anhydrous expectation | Dihydrate expectation | Failure mode to watch |
|---|---|---|---|
| Loss on drying | ≤1.5% | 9.0 – 9.6% | dihydrate sold as anhydrous after partial drying, giving a mixed and partially amorphous solid |
| Assay | ≥98.0% as-is | 85 – 90.5% as-is, or 98% dry basis | dry-basis figure presented as as-is |
| Ignition residue | ≤0.05% | ≤0.05% | inadequate demineralisation, visible as pH drift in solution |
| Chromaticity / turbidity | ≤0.100 / ≤0.05 | ≤0.100 / ≤0.05 | colour masking by high solids in the test solution |
3. Which form for which application
| Application | Recommended form | Reason |
|---|---|---|
| Dry blends, tablet premixes, powder sachets | anhydrous | no water of crystallisation to migrate; better free-flow in low-humidity blending rooms |
| Freeze-dried biologics | either, but anhydrous is easier to standardise | the sugar is dissolved before use; the hydrate water contributes to the initial solids and must be accounted for in the formulation mass balance |
| Frozen dough and bakery | either; dihydrate is the cost-effective choice | added to water as a solution, so crystallisation water is irrelevant to the dough |
| Beverage syrups and liquid concentrates | either; dihydrate is usually cheaper per unit of sugar | dissolved immediately; hydrate water simply reduces the water addition |
| Diagnostic reagents and standards | anhydrous where a defined absorbance baseline matters | reduced risk of residual moisture variation between lots |
| Long storage in humid climates | anhydrous | lower hygroscopicity in the crystalline state; less caking risk |
The general rule: if the material is dissolved before use, choose on delivered cost per kilogram of trehalose, which usually favours the dihydrate. If the material is blended dry, tabletted, packaged as a powder or used as a reference standard, choose the anhydrous crystal and absorb the premium. Deciding the other way round is where most cost surprises come from.
4. What changes at the plant when you switch forms
4.1 Formulation mass balance
Replacing 100 kg of dihydrate with 100 kg of anhydrous material adds about 9.5 kg of sugar to the batch. In a bakery or beverage system this shifts sweetness, water activity and solids. The correct substitution is 90.5 kg of anhydrous for every 100 kg of dihydrate, with the balance of the water added in the process – not a one-for-one swap.
4.2 Dissolution and dusting
Anhydrous crystals dissolve slightly faster per unit of surface area but generate more airborne dust in handling. In plants with dust-explosion classification, the mesh cut and the use of a closed transfer system matter more than the hydrate form.
4.3 Cost per unit of sugar rather than per kilogram
The comparison that survives procurement review is cost per kilogram of trehalose delivered, computed as quoted price divided by (assay x (1 – LOD)). Using the same specifications as above: anhydrous at 98.0% as-is with 1.5% LOD delivers 0.965 kg of sugar-equivalent per kilogram, while dihydrate at 99.0% dry basis with 9.5% LOD delivers 0.896 kg. The ratio – 1.077 – is the price premium the anhydrous form is allowed to carry before it stops being the cheaper option.
4.4 Storage and re-test
Both forms should be stored below 25 °C and below 65% relative humidity. The anhydrous crystal has the better shelf profile in humid climates; the dihydrate is less prone to dusting but cakes if the crystal surface partially dehydrates in a dry, hot warehouse, which produces a hard crust on the outside of the granule. In either case, opened bags should be consumed within the campaign, not carried into the next quarter.
5. Commercial and regulatory notes
Both forms are the same substance for food-labelling purposes in most markets: trehalose is declared as trehalose, not as a hydrate, and the water of crystallisation is not declared separately. The practical regulatory difference appears in two places. First, excipient monographs are usually written for the dihydrate, so a pharmaceutical customer specifying a monograph grade is implicitly asking for the hydrate and needs a written variation if an anhydrous excipient is wanted. Second, import documentation and duty classification normally follow the CAS number on the invoice, which means the stated CAS must match the form actually shipped – a mismatch that gets discovered at customs far more often than in the QA lab.
| Commercial question | Anhydrous | Dihydrate |
|---|---|---|
| Typical price position per kg | higher | lower |
| Delivered sugar per container | higher (no inert water) | about 10% lower |
| Available mesh cuts | 60 – 200 mesh | often coarser, 40 – 100 mesh |
| Lead time for custom cut | 4 – 6 weeks | 3 – 5 weeks |
| Documentation needed | CAS 99-20-7 on COA, SDS and invoice | CAS 6138-23-4 on COA, SDS and invoice |
6. FAQ
Can I dry the dihydrate to convert it to the anhydrous form?
Not reliably. Heating the dihydrate drives off water but also collapses part of the crystal lattice, producing a mixed amorphous/crystalline solid with unpredictable flow and dissolution behaviour. The anhydrous form should be bought as such, made by high-temperature crystallisation.
Which form does a “crystalline trehalose, assay ≥98.0%, LOD ≤1.5%” specification describe?
An anhydrous crystal. The LOD limit alone rules out the dihydrate, and the as-is assay of 98% is arithmetically impossible for a pure dihydrate, whose ceiling is 90.48%.
Does the hydrate form affect microbiological stability?
Indirectly. Water of crystallisation is not available for microbial growth, but it does raise the water activity of a dry blend slightly during storage and it contributes to the total water in a formulation mass balance. In practice the microbiological limits on the certificate (≤100 CFU/g total plate count, pathogens absent) are what the buyer should police.
Is one form more suitable for organic or clean-label products?
The hydrate form has no bearing on certification; both are the same substance. What matters is the documentation package – identity preservation, allergen and non-GMO statements – and that is issued per order.
Conclusion
Choose the dihydrate when the sugar is going into water and cost per kilogram of sugar is the deciding metric. Choose the anhydrous crystal when the powder is used as a powder, when dusting and flow matter, or when a reference standard demands lot-to-lot consistency in residual moisture. In both cases, specify the CAS number, the hydrate form and the assay basis, and compute cost per kilogram of delivered trehalose before signing. For the full technical background see the trehalose main article, and for the specification of our own material see the trehalose product page. For biological stabilisation, continue with trehalose in freeze-drying.