AOAC Methods for Review in Codex STAN 234_11-2018

AOAC Official Methods Listed in CXS 234 for Milk and Milk Products

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( c ) Testing milk test portions .—To test bottle from ( a ) containing tempered milk, add appropriate amount of H 2 SO 4 [as determined in ( b )] by delivering acid in one delivery that washes all traces of milk into bulb and cleanly layers acid under milk. Immediately shake bottle by hand rotation as in ( b ), until all traces of curd disappear. Place bottle in Babcock bottle shaker set at medium speed. Continue to add acid to all test portions, and then shake full set 1 additional min. Temperature of milk plus acid in first bottle should not be less than 50.5°C at time bottles are transferred to centrifuge. Place bottles in heated centrifuge, counter-balance, and centrifuge 5 min after proper speed is reached. Add distilled water at 50 ° –52°C until bulb of bottle is filled. Centrifuge 2 min. Add distilled water at 50 ° –52°C until top of fat column approaches 7% mark on bottle neck. Centrifuge 1 min longer at ca 51°C. Transfer bottle to water bath maintained at 48 ° ± 1°C, immerse bottle to level slightly above top of fat column, and leave in water bath until column is at equilibrium and lower fat surface assumes final convex form ( ³ 5 min). Remove 1 bottle from bath, wipe dry, and with aid of reading light and magnification, C ( k ), use dividers or calipers to quickly measure fat column (before it begins to cool and contract). Place caliper points in vertical line on neck of bottle, with 1 point at lowest surface of lower meniscus and other point at top of upper meniscus. Without changing distance between 2 points on calipers, move calipers down bottle neck until lower point rests in etched horizontal graduation mark at 0%. Place upper point of calipers against bottle graduation and read test in % to nearest 0.05%. Repeat for each bottle. Fat column, at time of measurement, should be translucent, golden-yellow or amber, and free of visible suspended particles. Reject all tests in which fat column is milky or shows presence of curd or charred matter, or in which meniscus is indistinct or distorted; repeat test, adjusting volume of H 2 SO 4 added, to obtain proper color and milk-acid reaction temperature. References: JAOAC 8 , 4(1924); 8 , 471(1925); 56 , 1401(1973); E. Alternative Apparatus (First Action 1986; Final Action 1991) ( a ) Gercock-style milkfat test bottle .—(Available from Forcoven Products, Inc., PO Box 1556, Humble, TX 77338, USA.) 5%, 162 ± 3 mm high. Bottom of bottle is flat, and axis of flat graduated tube is vertical when bottle stands on level surface. Quantity of milk charge is 18 g; 18 g is acid-etched and pigmented in clear space above 5% graduated line. Specifications: ( 1 ) Bulb .—Capacity of bulb to junction with neck must be ³ 45mL. Shape of bulb is cylindrical, 36 ± 0.5mmod. Wall thickness 1.4 ± 0.2 mm. Word “Sealed”, manufacturer’s number or name, and matted marking spot must be legibly and permanently marked on surface. ( 2 ) Side filling tube .—8 mm od, 0.1 mm wall thickness. To enter bulb below junction of bulb and neck of flat graduated tube along curved portion of bulb and with minimum angle extend to center of bulb and maintain vertical axis for 40 mm to height of 3 ± 2 mm above bottom of bottle. ( 3 ) Neck .—Graduated portion of flat tube is uniform cross section ³ 4 mm beyond extremes of graduated scale. Length of graduated portion of flat neck ³ 75 mm. Graduated at whole %, 0.5, 0.1, and 0.05%. Graduations are acid-etched, with black pigment annealed to graduation. Graduation widths ³ 0.2 mm. Each whole % 58 , 949(1975); 71 , 898(1988). J. AOAC Int. 80, 845(1997).

with full centrifuge should be checked periodically with tachometer. “Diameter of wheel” means distance between inside bottoms of opposite cups measured through center of rotation of centrifuge wheel while cups are horizontally extended. Relative centrifugal force is approximately 164 ´ g . ( e ) Dividers or calipers .—For measuring fat column. ( f ) Water bath for test bottles .—Provided with thermometer and device to maintain temperature of fat column at 48 ° ± 1°C. ( g ) Water bath for tempering milk samples prior to weighing. —Provided with thermometer and device to maintain temperature of milk at 38 ° ± 1°C. ( h ) Supp l y o f ho t wa t e r add e d a f t e r 1 s t and 2nd centrifugations .—Provided with thermometer, device to maintain temperature of distilled H 2 O at 50 ° –52°C, and device to deliver H 2 O into Babcock bottles. ( i ) Bottle shaker .—Variable speed and matched to maximum capacity of centrifuge. ( j ) Digital thermometer for measurement of milk-acid reaction temperature. —Capable of reading temperature to the nearest degree in range of 100 ° –120°C. Use acid-resistant probe, with small diameter ( £ 0.5 mm) to ensure rapid response time. Length of temperature probe should be such that its tip is ca 1 cm above bottom of bottle, when fully inserted. ( k ) Reading light with magnifying glass .—As background when measuring fat columns. Light should be diffused (soft white color) and provide illumination from angles above and below level of fat column. Magnification device must be used to aid reading. ( l ) Tachometer .—For determining rotation speed of centrifuge (capable of measuring between 600–1000 rpm). D. Determination ( a ) Test sample preparation and temperature adjustment. —Place test sample in water bath at 38 ° ± 1°C. Level of H 2 O should be at or above milk level. Mix test sample 10 ´ by inversion. If fat line remains on inside surface of container, run hot tap water (ca 50 ° –60°C) over outside surface for 15–20 s. Mix test sample thoroughly by inversion and pipet immediately. Do not allow test samples to remain in water bath more than 15 min after reaching 38°C. With pipet, C ( b ), transfer 17.6 ± 0.05 mL prepared test portion at 38°C to milk-test bottle. Also, transfer 17.6 ± 0.05 mLmilk at 38°C to another milk test bottle for measurement of milk-acid reaction temperature. Blow out milk in pipet tip ca 30 s after free outflow ceases. Adjust temperature of milk in bottle to 20 ° –22°C. Adjust temperature of H 2 SO 4 to 20 ° –22°C. ( b ) Measurement of milk-acid reaction temperature and determination of amount of H 2 SO 4 to use .—Before testing group of test portions, determine correct amount of acid to be used in analysis by measuring milk-acid reaction temperature. Add 17.5 mL H 2 SO 4 to bottle from ( a ) containing tempered milk test portion prepared for measurement of milk-acid reaction temperature. Add acid in one delivery that washes all traces of milk into bulb and cleanly layers acid under milk. Fully insert digital thermometer probe down bottle neck and immediately shake by hand rotation until all traces of curd disappear. Peak reaction temperature should be 108 ° ± 2°C. Adjust amount of H 2 SO 4 added, until reaction temperature is within this range and fat column is translucent golden-yellow to amber. Amount of acid requiredmay vary between technicians and between batches of acid.

ã 2005 AOAC INTERNATIONAL

10/9/2018