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International Journal of Food Properties Volume 9, 2006 - Issue 4
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Original Articles

Density and Porosity: Literature Data Compilation for Foodstuffs

Ch. J. Boukouvalas School of Chemical Engineering, National Technical University of Athens, Athens, Greece
,
M.K. Krokida School of Chemical Engineering, National Technical University of Athens, Athens, GreeceCorrespondence[email protected]
,
Z.B. Maroulis School of Chemical Engineering, National Technical University of Athens, Athens, Greece
&
D. Marinos-Kouris School of Chemical Engineering, National Technical University of Athens, Athens, Greece
Pages 715-746 | Received 18 Mar 2005, Accepted 15 Jan 2006, Published online: 18 Apr 2007

Abstract

Recently published values of food properties, such as true density, apparent density, and porosity, in various foods were retrieved from the literature, classified, and analyzed. Data for more than 130 food materials classified into 11 food categories are presented. These ranges of data are reported as a function of material moisture content and temperature. The relative literature sources are presented for each food material.

INTRODUCTION

The structural properties of food materials are significant for the characterization and prediction of the quality of processed products (e.g., dehydrated products). They are also very important for the developing of new industrial products with desired properties or for quality improvement of already existing ones. Thus, the knowledge of food physical structure is important in the field of Food Material Science.[Citation1–4]

The structure of food material can be characterized by density (apparent and true), porosity, pore size distribution, specific volume, particle density shrinkage, and so on. Among these, density and porosity are the most common structural properties reported in the relevant literature. Apart from product quality characterization, they are vital parameters in the design of process equipment. They provide necessary information for the design of materials handling equipment for drying, storage, aeration and ventilation, etc.

Recent data compilations, found in literature, involve information about drying rate constant,[Citation5] moisture diffusivity,[Citation6] thermal conductivity.[Citation7] The scope of this paper is to compile and analyze the data for the structural properties (true density ρ p , apparent density ρ b , and porosity ε) of various foodstuffs, revealing the range of variation for each food material versus the corresponding ranges of material moisture content Xw and temperature T.

The food materials are separated into 11 major food categories (both animal and plant based), proposed by Saravacos and Maroulis:[Citation4]

  • Baked products

  • Cereal products

  • Diary

  • Fish

  • Fruits

  • Legume

  • Meat

  • Model foods

  • Nuts

  • Others

  • Vegetables

These categories cover the whole range of different food material satisfactorily, as previous data compilations.[Citation5–7] According to foodstuff morphology, the data were also separated into 2 new categories for easy of study:

  1. continuous materials in which porosity develops when water is removed; and

  2. particulate or granular materials, in which porosity is a dependent variable that can be controlled, e.g., by compression, vibration, etc.[Citation4]

Terminologies

Assuming a moist material consisting dry solids, water and air, the following definitions are used:

(1)
where mt , ms and mw are the total mass and the masses of dry solids and water respectively (kg), while the mass of air is neglected.

The total volume of the sample, Vt , is written as:

(2)
where Vs , Vw and Va are the volumes of dry solids, water, and air pores (for continuous materials) or interstitial air phase (for granular materials), respectively (m3). The volume of air is referred to the internal pores only.[Citation4]

  • Apparent density (ρb) concerns powdered and porous materials and it is determined by the mass of the sample and its apparent volume. The terms bulk density and bulk volume are also used for granular materials. Bulk density depends on the packing of granular materials and can be varied by pressure or vibration. On the contrary, apparent density of continuous materials is dependent on the moisture content and on the shrinkage resulted by the water removing method.[Citation1] The apparent density ρ b is defined as:

    (3)

  • True density (ρp) is the density excluding all pores and it is determined by the mass of the sample and its true volume. The terms particle density and particle volume are also used for granular materials.[Citation1] The true density ρ p is defined as:

    (4)
    where Vp  = Vs  + Vw is the true (particle) volume, which is the total volume of the sample, excluding air pores or air phase.

  • Porosity (ε) characterizes the open structure of a material. It is the fraction of the empty volume (void fraction) and it is usually estimated from the apparent density and the true density of the material according to the following equation:[Citation1,Citation4]

    (5)

During processing (e.g, drying, frying, etc.), significant changes in structural properties can be observed as water is removed from the moist material. The main aspect, as far as structural properties are concerned, is to control the product apparent density and porosity in order to produce different physical structures for various uses, choosing an appropriate method, condition, and so on. These properties depend on various factors. Some of these factors are:

  • Pre-treatment

  • Moisture content

  • Processing method

  • Process conditions (pressure, temperature, etc.)[Citation1]

Density and Porosity Measurement Methods

The experimental determination of the structural properties of a material is based on the mass, apparent volume, and true volume determination.[Citation1] Mass is determined by measuring the sample's weight. For apparent and true volume, there are several methods. For apparent volume measurements, the methods most commonly used are:

  1. Volumetric displacement methods: the apparent volume is determined by placing the sample in a container of known liquid volume and measuring the volume displacement;

  2. Dimension methods: the apparent volume is determined by averaging a number of dimension measurements with micrometers, assuming spherical or slab shapes;

  3. Stereopycnometer methods: the sample is covered with silicone grease in order to make it impervious to gases and its apparent volume is measured by a stereopycnometer;

  4. Buoyant force methods: the apparent volume is determined by measuring the buoyant force applied on the sample when coated sample is immersed in a known liquid.

True volume is usually measured by means of a gas (helium) stereopycnometer, which measures the true volume, excluding the interparticle volume.[Citation1,Citation8] This is the case for the data collected in this work. On the other hand, in most cases, porosity is not directly measured, but calculated through measured apparent and true densities and EquationEq. (5). Only in few cases, it is directly measured through porosity measurement methods (direct method, gas expansion method, and optical methods).[Citation8]

DATA COMPILATION

The experimental data, compiled by Rahman,[Citation8] are used as the initial set of data. Apart from these data, an exhaustive literature search was made in the most popular food engineering and food science journals covering the years 1990–2004. The journals used in this research are:

  • Journal of Food Science

  • Biosystems Engineering

  • Drying Technology

  • Journal of Agricultural Engineering Research

  • Journal of Food Engineering

  • Lebensmittel-Wissenschaft und-Technologie

  • International Journal of Food Properties

In this research, most of the available data from review articles published in previous years (prior to 1990) are also included, along with a few available data from some articles from other journals (Chemical Engineering and Processing, Chemical Engineering Science, Food and Bioproducts Processing, Food Chemistry, Food Control, Food Research International, International Journal of Refrigeration, Powder Technology, Journal of Agricultural and Food Chemistry, and International Journal of Food Science and Technology).

RESULTS AND DISCUSSION

A total number of 150 papers were retrieved from the above journals according to the distribution presented in . The accumulation of the papers versus the publishing time is also presented in . The collection of the data resulted in 1419 data concerning properties, such as true density, apparent density, and porosity of food materials.

Figure 1 (a) Number of papers, concerning structural properties data (true density, apparent density and porosity) of various foodstuffs, published in food engineering and food science journals during recent years. (b) Accumulation of published papers, concerning structural properties data (true density, apparent density, and porosity) of various foodstuffs, versus time.

Figure 1 (a) Number of papers, concerning structural properties data (true density, apparent density and porosity) of various foodstuffs, published in food engineering and food science journals during recent years. (b) Accumulation of published papers, concerning structural properties data (true density, apparent density, and porosity) of various foodstuffs, versus time.

These data were obtained and organized into a database. The data for true density are plotted versus moisture content and temperature in and , respectively. The data, presented in , demonstrate an apparent behavior as they approach the water density when the moisture content (in wet basis) tends to unity. In , no apparent scheme can be observed. The majority of data are greater than 800 kg/m3, (only 4% of the 752 data points that involve true density are less than 800 kg/m3).

Figure 2 True density data for all foods at various (a) moisture contents and (b) temperatures.

Figure 2 True density data for all foods at various (a) moisture contents and (b) temperatures.

The data for apparent density are plotted versus moisture content and temperature in and , respectively. The data for true density versus apparent density are presented in . All the data are plotted above the diagonal, as expected since ρb < ρp (EquationEq. (3) & Equation(4)). The total number of data points is approximately 1000.

Figure 3 Apparent density data for all foods at various (a) moisture contents and (b) temperatures.

Figure 3 Apparent density data for all foods at various (a) moisture contents and (b) temperatures.

Figure 4True density data versus apparent density for all foods.

Figure 4True density data versus apparent density for all foods.

The data for porosity are plotted versus moisture content and temperature in and . In , the porosity for granular and continuous materials is plotted separately. It can be seen that the porosity of granular material is usually high, which is not the case for the majority of continuous materials whose porosity is small, especially at higher moisture content. The total number of data points is about 300 and the majority of them (more than 70%) are greater than 0.5.

Figure 5 Porosity data for all foods at various (a) moisture contents and (b) temperatures.

Figure 5 Porosity data for all foods at various (a) moisture contents and (b) temperatures.

These figures give a good picture of the range of variation of true density, apparent density, and porosity versus moisture content and temperature values. Moreover, the histograms in reveal the distribution of the studied properties’ values retrieved from the literature.

Figure 6 Histogram of observed values of true density in food materials.

Figure 6 Histogram of observed values of true density in food materials.

Figure 7 Histogram of observed values of apparent density in food materials.

Figure 7 Histogram of observed values of apparent density in food materials.

Figure 8 (a) Histogram of observed values of porosity in continuous food materials. (b) Histogram of observed values of porosity in granular food materials.

Figure 8 (a) Histogram of observed values of porosity in continuous food materials. (b) Histogram of observed values of porosity in granular food materials.

The range of variation of the structural properties studied in this work is:

  • Apparent density 200–1600 kg/m3 (average 1200)

  • True density 500–2000 kg/m3 (average 850)

  • Porosity 0.00–0.98 (average 0.35)

The ranges of temperature and moisture content are:

  • Temperature −50 to 200 °C (average 50)

  • Moisture content 0.00 to 0.98 kg/kg (wet basis) (average 0.40).

The results are presented in detail in . More than 130 food materials are incorporated and classified into the 11 food categories, previously mentioned. In this table, the range of variation of apparent density, true density, and porosity for each material, the corresponding ranges of moisture and temperature, and the corresponding reference are presented. Due to the large number of the available data points (over 1400), the values are not included in the table. However, these values are readily available upon request, addressed to the corresponding author.

Table 1 Apparent density, true density and porosity of foods versus moisture and temperature. Ranges of variation of available data

CONCLUSIONS

Recently published values of food properties, such as true density, apparent density, and porosity in various foods were retrieved from the literature. Data (over 1400 data points) for more than 130 food materials were classified into 11 food categories. These data were organized in a database, analyzed statistically revealing the range of variation for each food material versus the corresponding ranges of material moisture content Xw and temperature T, and plotted as a function of temperature and moisture content. The histograms of the observed values of the properties, revealing the value distribution for each property, are also plotted.

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