Food Dehydration: Methods, Principles, and Moisture Analysis
Dehydration for Food Preservation
Dehydration reduces food’s water content to less than 13%. Advantages include excellent protection against spoilage as microorganisms cannot develop without water, enzymatic activity is inhibited, and chemical reactions slow down. Dehydration is optimal for products stored at elevated temperatures. To achieve maximum protection, remove virtually all water and store the food in tightly sealed containers, isolated from oxygen, light, insects, and rodents. Freeze-drying, a type of dehydration, preserves the original nutritional qualities of the product.
Introduction to Food Humidity
All foods, regardless of processing, contain water in varying proportions, ranging from 60% to 95% in natural foods. Water exists in three forms:
- Free Water: The most abundant form, easily released and measured.
- Adsorbed Water: Physically bound as a monolayer on food constituents’ surfaces.
- Bound Water: Chemically linked, as in hydrates or sucrose molecules.
Reasons for determining humidity include:
- Avoiding excess water in raw materials.
- Preventing microorganism growth.
- Complying with legal maximums in certain products.
- Preventing agglomeration in powders (e.g., sugar).
- Facilitating grinding in some products.
Main Methods for Estimating Moisture and Total Solids:
- Drying Methods (heat application)
- Direct Distillation Methods
- Chemical Methods
- Fast Power Methods
- Instrumental Methods
Methods for Drying
These methods measure weight loss due to water evaporation at or near boiling point. While accurate on a relative basis, they may not reflect the true water content. Volatile oils can be lost at temperatures like 100°C. In some foods (e.g., cereals), only free water is lost at this temperature, while bound water remains. The proportion of free water loss increases with temperature, so consistent drying conditions are crucial. To prevent decomposition in high-sugar foods, use lower temperatures (e.g., 70°C) and vacuum conditions. Rapid moisture determination is possible with high-temperature ovens, but complete moisture removal can be challenging due to food breakdown or loss of volatile substances.
Drying Methods: Oven Drying
The most common methods involve dehydrating a sample to a constant weight at specific temperatures and pressures. Hydrates may form, hindering water removal. Sub-methods include:
Air Oven Method:
Weigh a 1 mg sample in a container, heat it in a ventilated oven at 98-100°C (or 125-130°C) for 2-3 hours, cool in a desiccator, and reweigh. Repeat until the weight variation is less than 2 mg. Express weight loss as % water.
Vacuum Oven Method:
Similar to the air oven method but uses a vacuum pump to maintain a partial vacuum (100 mmHg or less). A thermometer monitors the temperature near the sample. Analysis duration is 4-6 hours.
Dehydration by Desiccator
This method uses a special vacuum-sealed desiccator, often with sulfuric acid as a desiccant for heat-sensitive products. It requires a long time (e.g., 1 week for meat, as observed by Windham) and may not yield reproducible results. AOAC recommends it for plant tissues.
Distillation Methods
These methods distill food with an immiscible solvent (e.g., toluene, heptane, xylene) that has a higher boiling point and lower density than water. Distilled water is collected and measured. A wire or “policeman” can be used to facilitate water drainage. While results may be lower, this method requires minimal attention and does not measure volatile oils, which are caught in the solvent.
Chemical Methods
The Karl Fischer titration method, described by the British Standard, uses the non-stoichiometric reaction of water with iodine and sulfur dioxide in a pyridine-methanol solution. Endpoint detection can be visual or electrometric. The reagent is standardized against a known water solution or a pure salt hydrate. Another method involves ethyl acetate hydrolysis by sodium hydroxide formed from water and excess sodium ethoxide. The remaining sodium ethoxide is determined by electrometric titration. Results for sugar moisture content are consistent with Karl Fischer titrations.
Instrumental Methods
Various instrumental methods based on physical or physicochemical principles are used for rapid moisture determination, especially in quality control. These include methods based on electrical resistance, frequency, dielectric properties, NMR, near-infrared reflectance, microwave, GLC, GCS, refractometry, hydrometry, and thermal gravimetric analysis. Thermal gravimetric analysis provides information about the types of water present.
Expression of Results:
Moisture content is expressed as:
- Water: For powders (e.g., flour, cocoa, sugar) with low water content.
- Moisture-Humidity: When the amount is high (e.g., fresh foods, cold cuts, cheese).
- Total Solids: For liquids (e.g., vinegar, juice, liquor, milk).
The analyst should always indicate the method used when reporting moisture content.