Food Analysis: Moisture and Ash Determination

Importance of Moisture Assay

• Moisture content is a quality factor in the preservation of some products and affects stability.
• Moisture content is important for food processors as water is an inexpensive filler.
• Examples of products where moisture content is important:
  • Dehydrated vegetables and fruits
  • Dried milks
  • Powdered eggs
  • Dehydrated potatoes
  • Spices and herbs
  • Jams and jellies
  • Sugar syrups
  • Prepared cereals
  • Concentrated milks
  • Liquid cane sugar and liquid corn sweetener
  • Dehydrated products
  • Concentrated fruit juices
  • Cheddar cheese
  • Enriched flour
  • Pineapple juice
  • Glucose syrup

Forms of Water in Foods

• Water exists in three states in food products:
  • Free water: retains its physical properties and acts as the dispersing agent for colloids and the solvent for salts.
  • Adsorbed water: held tightly or occluded in cell walls or protoplasm and is held tightly to proteins.
  • Water of hydration: bound chemically, e.g., lactose monohydrate.

Oven Drying Method

• The sample is heated under specified conditions, and the loss of weight is used to calculate the moisture content.
• Various oven methods are approved by AOAC International for determining the amount of moisture in many food products.
• The time required may be from a few minutes to over 24 hours.
• Moisture removal is sometimes best achieved in a two-stage process.
• Particle size, distribution, sample size, and surface area during drying influence the rate and efficiency of moisture removal.

Calculation

• The percentage of moisture is calculated as: % Moisture (wt/wt) = × 100
• Where W1 = initial weight of empty crucible + rod (+ sand), W2 = weight of crucible + food + rod (+ sand) before drying, W3 = final weight of crucible + food + rod (+ sand) after drying.
• % Total solids = 100 - % Moisture

Microwave Analyzer

• Microwave moisture analysis is a precise and rapid technique that allows for in-process adjustments of the moisture content in food products before final packaging.
• A particular microwave moisture/solids analyzer is specified in the AOAC International procedures for total solids analysis of processed tomato products and moisture analysis of meat and poultry products.
• Considerations when using a microwave analyzer:
  • The sample must be of a uniform, appropriate size to provide for complete drying under the conditions specified.
  • The sample must be centrally located and evenly distributed, so some portions are not burned and other areas are underprocessed.
  • The amount of time used to place an appropriate sample weight between the pads must be minimized to prevent moisture loss or gain before weight determination.

Infrared Drying

• No additional information provided.

Physical Methods

• Refractometry: can be used to determine moisture in liquid sugar products and condensed milks.
• Freezing Point: can be used to determine the concentration of solutes in a solution.

How to Select a Method of Analysis

• Understand the principle of different methods.
• Consider the nature of the sample: some foods contain volatiles that are lost at high temperatures, and some foods have constituents that undergo chemical reactions at high temperatures to generate or utilize moisture or other compounds.

Ashing

• Ashing is an important first step in proximate or specific mineral analysis.
• Ash refers to the inorganic (mineral) residue remaining after the combustion or complete acid-facilitated oxidation of organic matter in food.
• Definitions:
  • Dry ashing: uses a muffle furnace capable of maintaining temperatures of 500–600 °C.
  • Wet ashing: uses strong acids, oxidizing agents, or combinations thereof to chemically oxidize and remove organic substances.

Ashing Process

• Suggestions to accelerate incineration:
  • High-fat samples should be extracted prior to ashing.
  • Glycerin, alcohol, and hydrogen can be used to accelerate ashing.
  • Samples such as jellies may require mixing with cotton wool to avoid spattering.
  • Salt-rich foods may require a separate ashing of water-insoluble components and salt-rich water extract.
  • An alcoholic solution of magnesium acetate can be added to accelerate ashing of cereals.

Importance of Moisture Assay

• Moisture content is a quality factor in the preservation of some products and affects stability.
• Moisture content is important for food processors as water is an inexpensive filler.
• Examples of products where moisture content is important:
  • Dehydrated vegetables and fruits
  • Dried milks
  • Powdered eggs
  • Dehydrated potatoes
  • Spices and herbs
  • Jams and jellies
  • Sugar syrups
  • Prepared cereals
  • Concentrated milks
  • Liquid cane sugar and liquid corn sweetener
  • Dehydrated products
  • Concentrated fruit juices
  • Cheddar cheese
  • Enriched flour
  • Pineapple juice
  • Glucose syrup

Forms of Water in Foods

• Water exists in three states in food products:
  • Free water: retains its physical properties and acts as the dispersing agent for colloids and the solvent for salts.
  • Adsorbed water: held tightly or occluded in cell walls or protoplasm and is held tightly to proteins.
  • Water of hydration: bound chemically, e.g., lactose monohydrate.

Oven Drying Method

• The sample is heated under specified conditions, and the loss of weight is used to calculate the moisture content.
• Various oven methods are approved by AOAC International for determining the amount of moisture in many food products.
• The time required may be from a few minutes to over 24 hours.
• Moisture removal is sometimes best achieved in a two-stage process.
• Particle size, distribution, sample size, and surface area during drying influence the rate and efficiency of moisture removal.

Calculation

• The percentage of moisture is calculated as: % Moisture (wt/wt) = × 100
• Where W1 = initial weight of empty crucible + rod (+ sand), W2 = weight of crucible + food + rod (+ sand) before drying, W3 = final weight of crucible + food + rod (+ sand) after drying.
• % Total solids = 100 - % Moisture

Microwave Analyzer

• Microwave moisture analysis is a precise and rapid technique that allows for in-process adjustments of the moisture content in food products before final packaging.
• A particular microwave moisture/solids analyzer is specified in the AOAC International procedures for total solids analysis of processed tomato products and moisture analysis of meat and poultry products.
• Considerations when using a microwave analyzer:
  • The sample must be of a uniform, appropriate size to provide for complete drying under the conditions specified.
  • The sample must be centrally located and evenly distributed, so some portions are not burned and other areas are underprocessed.
  • The amount of time used to place an appropriate sample weight between the pads must be minimized to prevent moisture loss or gain before weight determination.

Infrared Drying

• No additional information provided.

Physical Methods

• Refractometry: can be used to determine moisture in liquid sugar products and condensed milks.
• Freezing Point: can be used to determine the concentration of solutes in a solution.

How to Select a Method of Analysis

• Understand the principle of different methods.
• Consider the nature of the sample: some foods contain volatiles that are lost at high temperatures, and some foods have constituents that undergo chemical reactions at high temperatures to generate or utilize moisture or other compounds.

Ashing

• Ashing is an important first step in proximate or specific mineral analysis.
• Ash refers to the inorganic (mineral) residue remaining after the combustion or complete acid-facilitated oxidation of organic matter in food.
• Definitions:
  • Dry ashing: uses a muffle furnace capable of maintaining temperatures of 500–600 °C.
  • Wet ashing: uses strong acids, oxidizing agents, or combinations thereof to chemically oxidize and remove organic substances.

Ashing Process

• Suggestions to accelerate incineration:
  • High-fat samples should be extracted prior to ashing.
  • Glycerin, alcohol, and hydrogen can be used to accelerate ashing.
  • Samples such as jellies may require mixing with cotton wool to avoid spattering.
  • Salt-rich foods may require a separate ashing of water-insoluble components and salt-rich water extract.
  • An alcoholic solution of magnesium acetate can be added to accelerate ashing of cereals.