The consumable component of muscle tissue in meat is approximately 75% water, 20% protein, 5% fat, carbohydrate, and minerals. The proportions vary depending upon the type of muscle, the kind of meat, seasonal variability, and the intrinsic pH. The water content is higher for leaner cuts of meat. Fat and water content contribute significantly to the flavor and eating quality of the product and the latter is an indicator of freshness[1]. The breeding growth and development of the animals, their genotype, and diet, as well as stresses in pre-slaughter period (i.e. fasting, and different stunning methods) and in post-slaughter period (i.e. chilling, ageing, injecting non-meat ingredients and tumbling) are factors in the water holding capacity of meat (WHC)[2].

Some initial studies utilizing spectroscopy and hyperspectral imaging in the visible through near-infrared spectral range (VNIR) have shown promise as a means to non-destructively determine not only the moisture but also the fat and protein content as well as marbling, aging, grade and potentially tenderness in meat[3].

Hyperspectral Imaging

Hyperspectral imaging cameras generate ‘hyper-cubes’ of data, whereby the spectrum at each pixel in the image is collected. Subtle reflected color differences that are not observable by the human eye or even by color (RGB) cameras are immediately identifiable by comparison of spectra between pixels. A variety of spectral imaging technologies exist.

The most common type of hyperspectral imager is the push broom system whereby a line on the object plane generates a 2D pattern on an array sensor. The collection of a complete data cube (2D spatial x 1D spectral) requires mechanical scanning. While the dispersing elements can be made small and each spectrum can be collected in as short as 1ms, the mechanical motion makes these instruments somewhat bulky and prone to misalignment. Furthermore, increased spatial resolution comes at the expense of longer collection times. Push-broom grating systems were the earliest forms of hyperspectral cameras, initially developed by NASA, mounted on satellites and airborne platforms for research purposes[4].

Figure 1. Operating Principle Fabry-Perot Filter

Band sequential or front staring imagers do not require mechanical scanning. In this technique, a tunable filter that can sequentially select spectral bands is placed in front of the sensor and generates the hyper-cube by collecting complete images at each spectral band-pass. The acquisition time does not depend on the number of pixels, but rather on the number of spectral bands being acquired. These imagers are especially attractive for applications requiring high spatial and spectral resolutions with tunable spectral ranges and a small form factor.

Often discussed along with hyperspectral imaging technology, are multispectral systems, the most popular of which are based on patterned filter arrays. These are an extension of color cameras where the typical Bayer or RGB filters overlaid on the image sensor are replaced with an array of 16 or even more color filters. While no user alignment is needed, and imagers can be miniaturized, the spectral resolution is quite limited and comes at the expense of spatial resolution, making this technology inadequate for many critical sample analysis applications.

Fabry-Pérot interferometers (FPIs) operate by placing two mirrors parallel to each other. By controlling the reflectivity of the mirrors and their spacing, high-finesse spectral filtering can be achieved (Figure 1). HinaLea Imaging developed the world's first battery-operated, hand-held staring hyperspectral camera based on FPI (Figure 2). This camera captures high-resolution images in 550 spectral bands in as little as two seconds. Moreover, the camera's embedded hardware enables real-time processing, so the user does not need to handle the large data sets typically generated by hyperspectral systems. Rather, the camera can identify features of interest, both in the spectral and spatial domains and classify these features in the image[5].

Figure 2. HinaLea Imaging Model 4100H

The technology can easily be configured into form factors and configurations suitable for laboratory bench-top investigations or production line testing. Such an implementation has not been possible for other band sequential techniques (i.e. AOTFs, liquid crystal tunable filters) due to cost, reproducibility issues, environmental, and power restrictions.

Spectral Image Processing, Unmixing and Classification

To make use of the abundance of data rendered by hyperspectral imaging, various image processing algorithms have been developed over the years. These are essentially mathematical techniques for deconvoluting the multiple spectral emission profiles or species, also referred to as end-members. As with the hardware, these techniques have their origins in satellite remote sensing research.

Figure 3. Hyperspectral imaging camera configuration for meat fat and moisture content measurement

The most basic and common in microscopy is linear spectral unmixing. This method assumes spectra of each pixel is a linear combination (weighted average) of all end-members in the pixel and thus requires a priori knowledge (i.e. reference spectra). Various algorithms such as linear interpolation, are used to solve “n” (number of bands) equations for each pixel where the “n” is greater than the number of end-members pixel fractions.

Another popular technique, spectral angle mapping, involves a vector representation of observed and target spectra to determine closest relationships in a multi-dimensional space proportional to the number of band passes. Spectral angle mapping is widely used due to its insensitivity to brightness differences. The advent of widely accessible machine learning methods has brought a new and powerful set of tools to this endeavor. Among them are Principal Component Analysis or PCA, a dimensionality reduction technique and K-Means Clustering, a type of unsupervised learning algorithm used to find groups in the data based on feature similarity[7].

Figure 4. Images of beef filet at the 630 nm band-pass frame product handling system.

Example of Hyperspectral Imaging Measurement

A time series study of aging and moisture content of red meat using a HinaLea Imaging VNIR hyperspectral camera (Model 4200) configured as per Figure 3 was performed. Data cubes of samples of ground beef and filet were captured over the course of two days at three time intervals with the intent of studying the effects of aging on spectral reflectance properties and consequently moisture content (Figure 4):

  1. Start at 0 hours; 1 sample:

    1. Fresh cut filet and new ground meat sample

  2. At 17 hours; 2 samples:

    1. Fresh cut filet and new ground meat sample

    2. 17 hours old filet and ground meat sample

  3. At 24 hours; 3 samples:

    1. Fresh cut and new ground meat sample and new ground meat sample

    2. 17 hours old filet and ground meat sample

    3. 24 hours old filet and ground meat sample

To study the relationship between spectral reflectance properties and fat content samples of lean ground beef with increasing proportions of added minced pure beef fat or tallow were also captured.