SOP: Determination of Grain Size of Sugar Crystals

GIFSQ — Global Institute for Food Safety & Quality

Principle

Sugar crystals are graded by sieve analysis: a weighed sample is shaken on a nest of sieves and the fractions retained on each sieve are weighed; the weight of each fraction in grams is the percentage of sugar retained on that sieve. The cumulative percentages are compared with the printed grade requirements (Table 8-5); if the figures do not comply, the sugar is considered below ISI Grade. The size distribution may also be expressed by the M.A./C.V. notation: the weighted frequency distribution of granular sugar plotted against particle size conforms to the Gaussian normal distribution; the Mean Aperture (M.A.) is the aperture of the sieve that would retain 50% by weight of the sample, and the Coefficient of Variation (C.V.) is the standard deviation of the distribution expressed as a percentage of the Mean Aperture.

Apparatus required

• Nest of sieves (IS and Tyler series) with lid and receiver
• Mechanical sieve shaker
• Rotary (mechanical) sample divider
• Weighing dishes

Procedure — Sieve analysis and grading

Ensure the sample is truly representative: draw a sufficient number of samples to represent the whole consignment, using a rotary divider to get the required weight (or use the whole sample)

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Place 100 g of sugar on the top sieve of a nest of sieves, with the lid on the top sieve and the receiver at the bottom

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Shake for 5 min on a mechanical sieve shaker

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Brush the fractions of sugar from each sieve separately into weighing dishes and weigh accurately

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The weight of each fraction in grams is the percentage of sugar retained on that sieve; calculate the cumulative percentage and compare with Table 8-5 — if the figures do not comply, the sugar is considered below ISI Grade

Table 8-4 — Grain size requirement for crystals:

Note: the printed table is garbled in the extraction; the printed row A additionally shows “2.0” (Tyler mm), and row B’s retained-IS value is illegible (“LiR/S”), rendered here as 1.68, the aperture of a Tyler No. 10 sieve.

Table 8-5 — Requirements for grain-size group of crystal sugar:

Note: the printed last entry reads “oe)”; rendered as 99, consistent with the pattern of every other group.

Procedure — M.A./C.V. method

Ensure the sample is truly representative, subdividing in the divider so that the whole of the final subdivision is used for the sieve analysis

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Select the sieves so that between 10 and 20% is retained on the top sieve and between 10 and 20% passes through the bottom sieve (a 50-g sample shaken on 8-inch sieves for 15 min is satisfactory in most cases; the required weight depends on the sieve diameter and the time of sieving)

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Place the lid on the top sieve and the receiver at the bottom sieve; shake for 15 min in a sieve shaker

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Note the weight of each fraction; plot the cumulative percentage retained on the sieve on the x-axis and the aperture size on the y-axis on arithmetic probability paper; draw the best straight line through the points

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From the graph read: (i) the aperture size corresponding to 50% retention, which gives the M.A.; (ii) the apertures corresponding to 16% and 84% retention

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For a sample having normal distribution, the difference between the apertures at the 16% line and the 84% line equals twice the standard deviation; calculate the C.V. from the values so found

Calculation

C.V. = [(A − B) / (2 × C)] × 100

Where,

• A, B, C = apertures corresponding to 16%, 84% and 50% (mean aperture) retention respectively, as read from the graph
• C.V. = coefficient of variation
• M.A./C.V. = the size distribution expressed in terms of the two parameters

Note: the printed expression (“V.= EiMeee < 100 / ae CX 2”) is garbled in the extraction; the formula above is derived from the definitional relationships the passage states in words (standard deviation = half the difference between the 84% and 16% apertures; C.V. = standard deviation as a percentage of the mean aperture).

Precautions

• Sampling has great significance in grain-size analysis — draw a sufficient number of samples to represent the whole consignment.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Specification for Sugar used in Food Preservation Industry IS: 1679-1960, Indian Standards Institution, New Delhi — cited in the book.