Howard Mold Counting Chamber

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The Counting Chamber is of all glass construction. In the center of the slide is a rectangle 15 x 20 mm, which is flanked by shoulders on each side 0.1mm higher. The cover glass is supported on these shoulders and leaves a depth of 0.1mm between the underside of the cover glass and the rectangle. The rectangle and the cover glass have optically plane surfaces. To facilitate calibration of the microscope, the rectangle has two engraved parallel lines spaced 1.382mm apart.

An accessory eyepiece micrometer is available which is ruled into squares, each of which is equal to 1/6 of the diameter of the eyepiece diaphragm opening. For mold counting, the microscope must give an area of 1.5 sq. mm.(circle 1.382 mm in diameter) at magnification of 90-125.

Instructions for Use:


In making mold counts of tomato products, use juice and catsup as it comes from the container. If necessary add clean, mold free, water-soluble gum to assist in making more uniform mounts; in case of puree and paste mix H2O to make total tomato solids of diluted product 8.37-9.37%.

Clean the Howard cell so that the Newton rings are produced between slide and cover glass. Remove cover glass and place small drop of well mixed sample upon central rectangle; using knife blade or scalpel, spread drop evenly over rectangle, and cover with glass so as to give an even spread. Use only sufficient sample to bring material to edge of rectangle. (It is of utmost importance that the drop be taken from a thoroughly mixed sample and spread evenly over rectangle. Otherwise, when cover slip is put in place, insoluble material, and consequently molds, may be more abundant at the center of mount). Discard any mount showing uneven distribution or absence of Newton rings, or liquid that has been drawn across moat and cover glass.

Place slide under microscope and examine with such adjustment that each field of view covers 1.5sq. mm. which is a circle with a diameter of 1.382 mm.. This adjustment can be facilitated by use of the two parallel lines engraved at the side of the rectangle which are spaced 1.382 mm apart. When the instrument is properly adjusted, quantity of liquid examined per field is 0.15 cu. mm.

From each of two or more mounts examine at least 25 fields taken in such manner as to be representative of all sections of mount. Observe each field, noting presence of absence of mold filaments and recording results as positive or negative, as case may be. (No field should be considered positive unless aggregate length of not more than 3 of filaments present exceeds 1/6 of diameter of field which is equal to the distance between any to lines of the Howard eyepiece micrometer). Calculate proportions of positive fields from results of examination of all observed fields and report the percentage of fields containing mold filaments. In case greater accuracy is desired more fields should be counted.


Drain contents of can 2 min. on No. 2 sieve. For containers less than 3 lbs. net weight, use sieve 8" in diameter; for containers of 3 lbs. or more net weight, use 12" sieve. Examine drained tomatoes and record number and size of any rotten portions present. Run drained tomatoes through laboratory cyclone with screen openings ca 0.027" in diameter, or brush through No. 30 sieve with stiff-bristle brush. Make mold counts on both drained juice and pulped tomatoes as directed in 42.57.




(a) Tomato soup with or without cream. -Place can in hot H2O and heat until contents are thoroughly warmed; then open. Transfer 10 ml of thoroughly mixed soup to 50 ml centrifuge tube and add 3 ml of KOH soln. (1X1). Stir until starch in soup has been dissolved and tissues cleared. Add sufficient H2O to fill tube, and centrifuge. (Time required to centrifuge sample varies greatly. With centrifuge arm length of 5¼" and at speed of ca 1600rpm, ca 20 minutes is required for average sample. In heavy soups the gelatinizing of so much starch sometimes interferes with proper settling out of solids during centrifuging. If liquid remains cloudy it may be necessary to discard sample and start again with only 5 ml of soup and proceed with the usual 3 ml of KOH). When supernatant liquid is clear, pour it off. If it is not entirely clear, check supernatant liquid for mold before discarding. Add enough H2O to residue in tube to bring to original volume of soup, mix, and count mold as directed in 42.57.

(b) Pork and beans, spaghetti with sauce, spaghetti with meatballs or meat, ravioli, chili con carne, and tamales. - Place unopened can in hot H2O and heat until contents are thoroughly warmed. Open can and transfer contents onto No. 6 sieve. Drain until major portion of liquid part has passed through. (With some products sauce runs through at once, but in case of some beans and spaghetti 10 or more minutes may be required). Mix sauce thoroughly, place 10 ml in centrifuge tube, and proceed as directed in (a). Use care in products containing meat so as not to confuse mold filaments and muscle fibers that bear a superficial resemblance to each other; muscle fibers are usually much thicker and striations are often visible.

(c) Sardines or other fish with tomato sauce - Heat can in boiling H2O and drain off sauce as directed in (b). Mix sauce thoroughly and place portion in centrifuge tube. Discard oil and treat 10 ml of well-mixed sauce with 3 ml of KOH soln. as directed in (a). Use care to differentiate between mold filaments and muscle fibers from the fish. Count as directed in 42.57.


PRODUCTS # 42.65

(a) Tomato juice cocktail - If possible, ascertain formula used in making the dehydrated cocktail, and from it calculate approximate percentage of tomato solids. Multiply this percentage by 34.36. Resulting figure will be ml of solvent to be used to dilute 2 g of the dehydrated cocktail in preparation for mold counting. (Dilution factor is based on tomato solids content of 5.5% for tomato juice).

Weigh 2 g of well-mixed sample into 50 ml beaker and add 50 ml H2O. Determine weight of sample, including beaker and stirring rod, before any moisture is lost by heating. Heat mixture on steam bath 10-15 minutes. Weigh beaker and add H2O to replace moisture lost by heating. Add sufficient 4% pectin soln. to make up the difference between 15 ml and calculated volume of liquid needed for the dilution. Mix thoroughly and proceed as directed in 42.57.

If formula or tomato solids content of product is not available, make dilutions for mold counting by adding a quantity of H2O and the pectin soln. (half and half) equal to amount of liquid specified in directions for preparing product for consumption. Dissolve flakes as directed above and proceed as directed in 42.57.

(b) Tomato flakes and tomato soup flakes - Ascertain if possible formula used in making the flakes, and from it calculate approximate percentage of tomato solids. Multiply this percentage by 21.88. Resulting figure will be ml of chloral hydrate soln. needed to dissolve 2 g of the dehydrated product in preparation for mold counting. (Dilution factor is based on tomato solids content of 8.37% for tomato puree).

If product is tomato flakes containing added starch but no added water-soluble solids, determine approximate total tomato-solids content of dry mixture by testing definite dilution with refractometer and converting reading to tomato solids. If reading is in soluble solids, make correction to insoluble tomato solids, which are ca 1% in cyclone juice. (Soluble solids for cyclone juice average ca 4.7%).

Weigh 2 g of well-mixed sample in 50 ml beaker and add calculated volume of chloral hydrate soln (1X1). Determine weight of sample, including beaker and stirring rod, before any moisture is lost by heating. Boil mixture gently, stirring 5 minutes or until it has gelatinous appearance. (Too much clearing of mixture makes it difficult to see the mold ). Weigh beaker and add H2O to replace lost moisture. Mix thoroughly, and if the mixture remains milky heat almost to boiling, stirring constantly. Proceed as directed in 42.57.

If the formula or tomato-solids content of product cannot be ascertained, dilutions for mold counting may be made on basis of 80% tomato solids for tomato flakes and 40% tomato solids for tomato soup flakes.


Weigh out 10 g of thoroughly mixed sample of ground capsicum and transfer to Waring Blender or equivalent mixer. Add 200 ml of 1% NaOH slon in 3 or 4 successive portions, stirring mixture after each addition, washing down with final portion any material that may stick to walls of blender. Agitate mixture in blender 1 minute. With rubber policeman rub down into mixture any material sticking to walls of blender and repeat blending 2 minutes longer. Add 2 or 3 drops capryl alcohol to break resulting foam. Mix 100 g of this mixture with 50 g of 3% pectin soln, and count with Howard mold-counting cell as directed under 42.57.

Occasionally a blended mixture will contain particles of seed tissue that make it difficult to obtain Newton rings in preparing slide for mold counting. A clamp devised for holding cover slip in place to obviate this difficulty consists of metal plate with circular opening, 2.5 cm in diameter, in center of plate; 2 clips attached to anterior edge of plate fasten cover slip in position when slide is placed on plate.

To clean the counting chamber: After completing the count, remove the cover glass and clean the counting chamber with water or a mild cleaning solution (10% solution of bleach). Dry the counting chamber with a soft cloth or wipe, or rinse with acetone.


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