Moisture content is the amount of water held in a material such as food, herbs, extract powders and seeds, and it is usually reported as a percentage of the sample weight. Moisture in materials differs from humidity, which refers to the amount of water vapor in the atmosphere. In Thai, both are called by the same word, “khwam chuen”, so the two are often confused.
For herbal extract powders, powdered dietary supplements, dried herbs and active ingredient raw materials, moisture directly affects shelf life, the stability of active compounds and the risk of microbial growth. Powders whose moisture exceeds the limit in the specification keep absorbing moisture from the air, cake, change color, change flavor and are at risk of mold growth. These problems rarely appear on the day of manufacture; they develop gradually while the product sits at its destination for several months after production.
In Thailand and the ASEAN region, the relative humidity of the air is fairly high for most of the year, so powders produced at low moisture can pick up moisture during filling, transport and storage. For example, a powder with an initial moisture content of around 2% may gain enough moisture to exceed its specification limit within a few months if its packaging is not a good enough moisture barrier. Measuring moisture both at the starting point (T0) and throughout shelf life is therefore an important quality assurance tool.
VISBIO Laboratory analyzes moisture content with two types of instrument that work on different principles. The first is a moisture analyzer that heats the sample with a halogen or infrared lamp and measures the weight lost during drying (loss on drying); it is used to report results for quality control. The second is an electrical moisture meter, which gives instant results and is suited to preliminary screening of raw materials. We also measure moisture by the hot air oven method when clients need results by the reference method. The service covers herbal extract powders, powdered dietary supplements, dried herbs, active ingredient raw materials, protein powders, multivitamin powders and other powdered or dried products in the food, herbal product and pharmaceutical groups.
What is moisture content, and how does it differ from humidity?
In Thai, the word “khwam chuen” has two meanings. The first is humidity, which describes the amount of water vapor in the atmosphere. The second is moisture in materials, which describes the amount of water held in materials such as wood, food, herbs and plant seeds (Phaophang, 2020). Moisture content therefore expresses the amount of water in a sample and is usually reported as a percentage of the sample weight.
Humidity and moisture in materials are related. When the relative humidity of the air rises, materials exposed to that air absorb water vapor until their internal moisture reaches equilibrium with the relative humidity of the surrounding air. For example, dried food stored in humid air keeps absorbing moisture until it reaches equilibrium. Powders and dried raw materials therefore need to be stored in packaging that is a good moisture barrier.
Moisture content also differs from water activity (aw). Moisture content indicates the total amount of water in a sample, whereas aw indicates how available the water in the sample is to microorganisms. Two products with the same moisture content can therefore have different aw values, depending on the sugar, salt and other components that bind the water.
The impact of moisture on product quality
Moisture is one of the most important properties of foods and dried products, because it affects processability, shelf life, functionality and product quality (Pornchaloempong & Rattanapanone, n.d.). The main effects are as follows:
- Microbial spoilage: Foods with a high moisture or water content spoil easily (perishable foods), because they provide suitable conditions for the growth of spoilage microorganisms such as bacteria, yeasts and molds. Moisture therefore directly affects shelf life.
- Food safety: Foods with a high water content favor the growth of pathogens and the production of toxins that cause food poisoning, including mold toxins (mycotoxins) such as aflatoxin and patulin.
- Physical and thermal properties: such as melting point, boiling point, thermal conductivity and specific heat.
- Sensory quality: Moisture affects texture, such as crispness, viscosity and caking, which in turn affects consumer acceptance.
- Chemical reactions during storage: Moisture affects the rate of reactions that reduce quality, such as browning reactions and lipid oxidation.
- Trading price: Some agricultural commodities, such as rice and cereal grains, are priced according to their moisture content.
Moisture standards for powdered or dried products
Accurate control of moisture content plays an important role in quality assurance across many industries, such as food, chemicals, pharmaceuticals, herbal products and dietary supplements. The law also sets maximum moisture levels for some products. The Thai Food and Drug Administration (Thai FDA) compiled these limits in its 2019 manual of analytical tests for specific controlled foods and nine types of standardized-quality foods, for use in licensing applications. Examples of moisture specifications for powdered or dried products are shown below (Thai Food and Drug Administration, 2019).
| Product | Ministry of Public Health Notification | Moisture specification (dried or powdered types only) |
|---|---|---|
| Infant formula and follow-on formula for infants and young children | (No. 156) B.E. 2537 (1994) · (No. 286) B.E. 2547 (2004) · (No. 307) B.E. 2550 (2007) | Not more than 5% by weight |
| Supplementary foods for infants and young children | (No. 158) B.E. 2537 (1994) | Types not requiring cooking before consumption: not more than 5% by weight · Types requiring cooking before consumption: not more than 8% by weight |
| Foods for weight control | (No. 121) B.E. 2532 (1989) and No. 2 | Not more than 8% by weight |
| Beverages in hermetically sealed containers (dried type) | (No. 356) B.E. 2556 (2013) | Not more than 6% by weight |
| Milk powder (full-cream, partly skimmed and skimmed), including fat-modified milk powder (full-fat and partly skimmed) | (No. 350) B.E. 2556 (2013), Cow’s Milk | Not more than 5% by weight |
| Flavored milk, dried and dried tablet types | (No. 351) B.E. 2556 (2013), Flavored Milk | Not more than 5% by weight |
| Dried or powdered ice cream | (No. 354) B.E. 2556 (2013), Ice Cream | Not more than 5% by weight |
| Dehydrated royal jelly | (No. 294) B.E. 2548 (2005), Royal Jelly | Not more than 5% by weight |
This table summarizes the Thai FDA manual of 2019. Some notifications may have been amended or reissued since then, so manufacturers should check the latest notification in force before setting their product specifications. Beyond foods, the Thai Herbal Pharmacopoeia (THP) also sets limits for loss on drying or water content in the monographs of many herbal raw materials (Department of Medical Sciences, 2021).
From a regulatory perspective, moisture is quality data that the manufacturer sets in the product specification under the law for each product type: the Food Act B.E. 2522 (1979), the Herbal Products Act B.E. 2562 (2019) and the Cosmetic Act B.E. 2558 (2015). Some foods also have Ministry of Public Health notifications that set maximum moisture levels, as in the table above. A product whose moisture exceeds the limit fails its quality criteria, and the cause should be investigated before the batch is released.
Why extract powders and dietary supplements need moisture testing for every batch
During formulation, the initial moisture content determines shelf life and the consumer experience. In general, very dry powders, for example below about 1%, tend to keep well, but they require tightly controlled drying processes such as spray drying or freeze drying, which use a lot of energy and cost more. Most products use a moisture range of about 2–4%, which balances cost and quality. Powders above about 5% moisture are at greater risk of caking, and their risk of microbial growth should be assessed together with aw.
When selecting raw material suppliers, the moisture content of the same extract powder from different manufacturers can differ several-fold. For example, an extract powder labeled “98% purity” may count water as part of its weight, so the actual amount of active compound per gram can differ markedly. Moisture measurement lets formulators calculate the active compound content on a dry basis when it is used together with active compound analysis, and compare prices per amount of active compound actually delivered rather than per total weight.
In products filled into capsules or compressed into tablets, moisture affects fill weight uniformity and compression behavior. Powders that are too moist flow poorly and stick to the punches of the tablet press (sticking), so the weight per tablet or capsule varies. Powders that are too dry may build up static electricity, be hard to compress, or form tablets that split into layers (capping). Commercial-scale capsule filling and tableting therefore need powders with consistent moisture within a narrow range.
In export markets, business-to-business (B2B) buyers often ask for moisture data on the certificate of analysis (COA), especially for goods shipped by sea to hot, humid countries, where moisture can increase during transport. Results showing that the initial moisture is low and within specification give buyers confidence in quality.
Methods for analyzing moisture in products
Moisture can be analyzed by several methods, in solids, liquids and gases. Results are usually reported as a percentage and are used as specifications in commercial food production and in quality assurance of the manufacturing process. The commonly used methods are as follows.
1. Drying with an oven or a moisture analyzer
The traditional laboratory method for determining the moisture of solid or semi-solid raw materials is oven drying. The sample is weighed before heating, placed in an oven for the time and at the temperature set by the method so that the water evaporates, and then weighed again. The weight lost is the moisture of the sample. Many standard methods use oven drying as the reference method, for example the air oven method for moisture in flour (AOAC International, 2005).
A moisture analyzer, also called a moisture balance, works on the same principle, measuring the weight lost during drying (loss on drying, LOD), but it heats the sample with a halogen or infrared lamp and weighs it continuously on a built-in balance. It therefore gives results much faster than oven drying and is widely used in research and quality control across many industries. Each measurement requires settings, such as the temperature and the criterion for ending the measurement, that suit the sample type and give results consistent with the reference method for that type of sample. The heating temperature is usually between 100 and 130 °C. One caution is that drying methods measure the weight of all volatile substances, not only water. In samples that contain essential oils or other volatile substances, these evaporate together with the water and make the result higher than the true moisture content.
2. Karl Fischer titration
Karl Fischer titration is a chemical method developed by the German chemist Karl Fischer in 1935. It uses a reaction that is specific to water, so it measures only water and excludes other volatile substances. It comes in volumetric and coulometric forms, and the coulometric form suits samples that contain very small amounts of water. Today, automatic Karl Fischer titrators make the measurement convenient. The method suits work that requires high accuracy, as well as samples that contain volatile substances other than water.
3. Electrical method (moisture meter)
An electrical moisture meter measures the electrical properties of a material, namely electrical resistance or capacitance, which change with water content because water conducts electricity and has a higher dielectric constant than dry material. The meter converts the measured value into percent moisture using an equation calibrated for each material. Its advantages are instant results, no sample preparation, non-destructive measurement, and portability for use at raw material receiving points or on the production line. Its limitations are lower accuracy than the drying and Karl Fischer methods, and results that depend on the type of material, its density and how the water is distributed in the sample. This type of meter is therefore suited to preliminary screening and to urgent decisions on accepting or rejecting raw materials, before samples are sent to the laboratory for confirmation by a drying method.
4. Water activity (aw) measurement
Water activity (aw) is the ratio of the water vapor pressure above a sample, once it has reached equilibrium with the surrounding air, to the vapor pressure of pure water at the same temperature. It ranges from 0 to 1.00. aw does not indicate the amount of water; it indicates how available the water in a sample is to microorganisms and chemical reactions. It is therefore an important value in food product design and food safety assessment. Details of aw measurement are on our water activity (aw) testing page.
Scope of VISBIO Laboratory's moisture analysis service
VISBIO Laboratory’s service covers several sample groups:
- Extract powders and herbs: spray-dried or freeze-dried herbal extract powders, dried herbs (leaves and roots), herbal root powders, fruit peel powders and powdered herbal raw materials
- Dietary supplements and nutraceuticals: protein powders, collagen powders, probiotic powders, multivitamin powders, electrolyte powders and dietary supplement premixes
- Powdered drinks and premixes: coffee powder, cocoa powder, tea powder, herbal drink mixes, bakery premixes and beverage ingredients for industry
- Powdered cosmetics and raw materials: loose powder, powders for tableting, powders for capsule filling and powdered active ingredient raw materials
The report shows percent moisture with the mean and standard deviation of replicate measurements. For stability studies, the report shows the quantitative change over time relative to the starting point (T0). Clients can choose between a single measurement for raw material receiving or batch release and a continuing study throughout shelf life. Results from the moisture analyzer are loss-on-drying values, so please inform our team in advance if your samples contain essential oils or other volatile substances.
Using moisture results
Moisture results are useful at several stages of a product’s life. In research and development, they are used to select spray-drying or freeze-drying conditions that give the lowest-moisture powder within budget, to compare extract powders from several suppliers to confirm that the price paid reflects quality, and to test the effect of changes in milling or filling processes that may affect moisture.
Before launch (pre-launch), moisture serves as the baseline for the specification and is used to set acceptable deviation limits. In commercial production, it is used as a batch release criterion, especially for batches produced in the rainy season, when humidity in the factory is high. In stability studies, it is used together with real-time and accelerated stability studies to confirm that the product’s moisture stays within specification throughout shelf life, under both normal conditions and ASEAN Zone IVb conditions (30 °C, 75% relative humidity), the long-term study condition for countries in the very hot and humid climatic zone.
For export, the results can be attached to the certificate of analysis to support inspections by buyers or by authorities in the destination country when requested.
Frequently asked questions about moisture content
What is moisture?
Moisture, or moisture in materials, is the amount of water held in a material such as food, herbs, extract powders and seeds. Moisture content is usually reported as a percentage of the sample weight.
How does moisture differ from humidity?
Humidity is the amount of water vapor in the air, while moisture is the amount of water in a material. The two are related: materials in humid air absorb water vapor until their internal moisture reaches equilibrium with the relative humidity of the air.
What are the English terms for moisture in materials and in the air?
Moisture in materials is called moisture content, moisture in the air is called humidity, and the relative moisture level of the air is called relative humidity.
What is a moisture analyzer?
A moisture analyzer, or moisture balance, measures moisture by loss on drying: it heats the sample with a halogen or infrared lamp while weighing it continuously. It gives results faster than oven drying, but the result also includes volatile substances other than water.
How many methods are there for measuring moisture?
There are four main methods: drying with an oven or a moisture analyzer; Karl Fischer titration, which measures only water; the electrical method, which gives instant results but is less accurate; and aw measurement, which does not indicate the amount of water but how available the water is to microorganisms.
How does moisture content differ from aw?
Moisture content indicates the total amount of water in a sample, while aw indicates how available that water is to microorganisms. Products with the same moisture content can have different aw values, so microbial risk should be assessed with aw together with moisture content.
References:
- AOAC International. (2005). AOAC Official Method 925.10: Solids (total) and moisture in flour — Air oven method. In Official methods of analysis of AOAC International (18th ed.). AOAC International.
- Department of Medical Sciences. (2021). Thai herbal pharmacopoeia 2021. Ministry of Public Health.
- Phaophang, S. (2020). ความชื้นในวัสดุและอาหาร (Moisture Content) [Moisture in materials and foods (moisture content)]. Neonics. https://www.neonics.co.th/moisture-and-humidity/moisture-content.html
- Pornchaloempong, P., & Rattanapanone, N. (n.d.). Moisture content / ความชื้น [Moisture content]. Food Network Solution. https://www.foodnetworksolution.com/wiki/word/0830/moisture-content-ความชื้น
- Royal Thai Government Gazette. (1979, May 13). Food Act B.E. 2522 (1979). Royal Thai Government Gazette, 96(79 Kor), 1–24.
- Royal Thai Government Gazette. (2015, September 8). Cosmetic Act B.E. 2558 (2015). Royal Thai Government Gazette, 132(86 Kor), 1–22.
- Royal Thai Government Gazette. (2019, April 30). Herbal Products Act B.E. 2562 (2019). Royal Thai Government Gazette, 136(56 Kor), 116–161.
- Thai Food and Drug Administration. (2019). คู่มือรายการตรวจวิเคราะห์อาหารควบคุมเฉพาะและอาหารกำหนดคุณภาพมาตรฐาน 9 ประเภท เพื่อประกอบการขออนุญาต [Manual of analytical tests for specific controlled foods and nine types of standardized-quality foods for licensing applications]. Ministry of Public Health.