Proximate Analysis: Moisture

• Principle and Scope
• Sample preparation
• Procedure
• Calculation
• Possible errors

Principle and Scope

The moisture and low volatile materials are removed by heating at 95-100°C under partial vacuum. It is applicable to fish and fish products, including frozen scallops, both IQF and block frozen, and fish by-products.

Sample preparation

1. Sample preparation should take into account the type of product and how it is used and prepared by the consumer
   • for products that are packed in water, brine or similar medium that is normally discarded by the consumer: open the package and drain the product on an appropriate size sieve for 1 to 12 minutes. Comminute the part of the sample retained by the screen until a homogeneous blend is obtained.
   • for products that are packed in a medium that may be or is normally used by the consumer, e.g., fish canned in its own juice or oil: transfer the entire contents of the package into a homogenizer and blend for one minute or until a homogeneous mix is obtained.
   • for smoked fish and smoked fish products: remove the sample from the container with forceps or other suitable instrument, place on a coarse screen and allow to drain for ca 5 minutes. Place the sample on a paper towel or other clean absorbent paper and blot. Repeat blotting using a second and third piece of towel. Comminute the sample until a homogeneous blend is obtained. If the sample texture is too tough for homogenization, pass the sample through a grinder a sufficient number of times to obtain a uniform mix. A food chopper may suffice as an initial step if the sample size is large.
   • for brined or salt-cured fish: scrape off as many loose salt crystals as possible with a spatula, if any are present, then wipe off any remaining salt crystals or moisture with a paper towel. Comminute the sample until a homogeneous blend is obtained.
   • for raw fish, fresh or thawed: pass the sample through a grinder a sufficient number of times to obtain a homogeneous blend.
   • for processed products containing no separable liquid: thaw in the package (if frozen) and pass the sample through a grinder a sufficient number of times to obtain a uniform mix.
   • for frozen scallops: obtain five sample units, remove surface glaze from the scallops under running water until no ice can be felt on the surface of the scallops but it is evident that the ice crystals remain within the product; the interior of the product remains frozen. Block frozen product should be gently broken up to individual scallops or scallop pieces and ice within the block should be removed until the surface of the product is free of ice. Place the scallops on a sieve of appropriate size and drain for 1 to 12 minutes. Obtain a total of approximately 100 g of scallop meat from the five sample units. Comminute the 100 g sample until a homogenous blend is attained.
2. Collect the homogenized sample into a clean, sealable plastic cup or glass bottle.
3. Store the sample in a refrigerator or freezer until required.
4. Ensure that the prepared sample is still homogeneous prior to weighing. If liquid separates from the sample, reblend before use.
5. Since salted samples have a tendency to foam, care should be taken to see that the dish is not over-filled.

Procedure

1. Accurately weigh a moisture dish of appropriate size.
2. Add approximately 10 g of the comminuted sample and reweigh.
3. Place the container in a vacuum oven at 100 oC and less than 100 mm Hg for approximately 5 hours.
4. Remove dish from the oven, cover, cool in desiccator, and weigh.
5. Redry 1 hr and repeat process until constant weight has been achieved, i.e., change in weight between successive dryings at 1 hour intervals is < 5 mg.

Calculation

Calculate the percentage moisture (wet weight basis) as follows:

P = weight in g of sample
a = weight in g of dried sample

Possible errors

• Loss of VOLATILE FATTY ACIDS (VFA's), and LACTIC ACID in SILAGE or any other FERMENTED PRODUCTS
• Most likely UNDERESTIMATE DRY MATTER
• This error is relatively small
• Maillard product formation (protein:CHO complex)