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There is now a peer-verified, microwave drying and NMR analysis AOAC PVM 1:2003 for quantification of the percent-age of moisture and fat in meat products.. The procedure involves determ

Chapter 8

The Rapid Determination of Fat and Moisture in Foods

by Microwave Drying and NMR Analysis

Bobbie McManus and Michelle Horn

CEM Corporation, Matthews, NC 28106–0200

Abstract

Fat and moisture analysis by microwave drying and nuclear magnetic resonance (NMR) is a direct method that yields accurate results in minutes without solvents

or the frequent calibration required by indirect methods such as near infrared (NIR) or Fourier transform infrared (FTIR) In the past, NMR was unreliable for determining fat content in foods because the signal from moisture present in the sample interfered with the fat reading There is now a peer-verified, microwave drying and NMR analysis (AOAC PVM 1:2003) for quantification of the percent-age of moisture and fat in meat products The procedure involves determining the moisture value of meat samples by microwave drying and using the dried sample

to determine the fat value by NMR analysis Five meat products were analyzed using a CEM SMART SystemTM(Moisture) and the SMART TracTMSystem (Fat) The samples, representing a range of products that meat processors commonly analyze in daily plant operations, included the following: (i) fresh ground beef, high fat; (ii) deboned chicken with skins; (iii) fresh pork, low fat; (iv) all beef hot dogs, and (v) NIST standard reference material samples The results were com-pared with moisture and fat values derived from AOAC-approved methods, 950.46 Forced Air Oven Drying and 960.39 Soxhlet Ether Extraction The chapter also discusses the analysis of dairy samples by microwave moisture analysis and NMR fat analysis These samples include cream, ice cream, and milk with various levels

of fat Results are compared with the Mojonnier and Gerber techniques

Introduction

Traditionally, foods have been tested for fat using various wet chemistry methods that are both time consuming and require more than a passing knowledge of chem-istry (1) With the introduction of near infrared (NIR) and Fourier transform infrared (FTIR) instruments, analysis time was reduced to minutes, but the systems required frequent calibration, a time-consuming and expensive process (2) Fat determination by use of a combination of microwave drying and NMR analysis technologies offers the best of both worlds, i.e., the accuracy of traditional wet chemistry methods and the speed of NIR instruments

For many years, laboratories utilized convection or vacuum ovens to dry samples Although accurate, this was a time-intensive procedure, often requiring 3–72 h to dry a sample completely (2) More recently, halogen and infrared moisture analyzers have reduced drying time to minutes; however, they are not as accurate as traditional oven methods Microwave moisture analyzers reduce drying time to minutes, while retain-ing the accuracy of oven dryretain-ing The SMART SystemTM(3), as seen in Figure 8.1, incorporates a unique octagonal cavity, which focuses the microwaves on the sample, reducing drying times (Fig 8.2) It is also the only microwave moisture analyzer that features a temperature-feedback system, which monitors the temperature of the sample and adjusts the power as needed to keep the sample from exceeding the user-defined temperature set point Additionally, the SMART System (3) continuously monitors weight loss of the sample during drying to ensure accuracy Microwave drying has become the standard in a variety of industries, and many methods have been written for it by various organizations The speed and accuracy of microwave drying can greatly help the quality assurance/quality control (QA/QC) of a company and save them money by reducing the amount of out-of-specification product In the 8 h it takes

Fig 8.1 SMART System microwave moisture analyzer (CEM Corporation).

Fig 8.2 Patented SMART System octagonal cavity efficiently

focus-es microwave energy to decrease drying times.

to conduct a single moisture analysis in a standard drying oven, a company can con-duct over 120 analyses in a microwave moisture analyzer

Nuclear magnetic resonance (NMR) imaging is nondestructive and noninvasive, using a magnet and an electromagnetic coil to force the hydrogen atoms in a substance

to align with the magnetic field (Figs 8.3 and 8.4) It then sends a pulse of radio-fre-quency energy through the field, briefly creating a stronger field with which the hydro-gen atoms align (Fig 8.5) When the pulse ends, the atoms “relax” back to their origi-nal alignment and emit a weak electrical sigorigi-nal known as transverse relaxation (T2) decay or free induction decay (FID), which is then measured by the NMR (Fig 8.6)

Fig 8.3 A sample is placed in the magnetic field.

Fig 8.5 An RF pulse forces atoms

to align temporarily.

Fig 8.4 Hydrogen atoms align to the magnetic field.

Fig 8.6 The atoms “relax” and the instrument interprets the signal.

The hydrogen atoms in proteins and carbohydrates relax faster than those in fat; thus, the instrument can distinguish between the signals to determine the amount of fat in a substance (4,5)

The term “nuclear” in NMR refers to the instrument's ability to analyze the nuclei

of a sample (The instrument does not generate any ionizing radiation.) NMR can cause many nuclei to generate a signal, but most research has been performed on the detection of signals from the 1H nucleus, a branch of science commonly known as

“proton NMR.” After a food sample is dried with microwave energy, the remaining components that contain significant amounts of protons are fats, proteins, and carbohy-drates, all of which have different transverse relaxation times The relaxation times for fats are significantly longer than those of proteins and carbohydrates Fats typically have transverse relaxation times of <10 µs or more, whereas proteins and carbohy-drates demonstrate “solid-like” behaviors and generally have relaxation times ≤10 µs Thus, the NMR is able to distinguish between the signals of different components; after exciting the molecules and letting the “solid-like” signals of protein and carbohy-drates decay, the remaining signal from the dried sample will be fat

In the past, NMR was not utilized for determining fat content in many foods because the signal from moisture present in the sample interfered with the fat reading However, by first drying samples using microwave energy, an accurate NMR analysis can be performed The SMART TracTMSystem is a synergy of advanced microwave drying and NMR technologies that culminates in a truly unique fat analyzer that yields accurate results in minutes (Fig 8.7) The SMART Trac is designed to rapidly mea-sure moisture and fat content in all types of foods in <5 min without solvents, chemi-cals, or time-intensive maintenance The system takes a direct measurement of total fat, including both free and chemically bound fat in solids, liquids, and slurries The system is applicable to a full range of food products from low-moisture samples, including powders and snack foods, to high-moisture samples, such as meat and dairy products Unlike wet chemistry methods, the SMART Trac is easy to run and does not require a knowledge of chemistry The system measures fat throughout the entire sam-ple and is not affected by variability in the product (e.g., changes in color, texture, or composition), whereas NIR and FTIR systems measure fat in only a small portion of the sample and must be recalibrated when there are changes in the product

Fig 8.7 SMART Trac fat and moisture analyzer (CEM Corporation).

After first discussing the equipment, this chapter will describe the methodology for analysis of a food sample in general and then discuss the specifics of meat and dairy samples This new technology for fat analysis has generated a great deal of interest and there is now a peer-verified, microwave drying and NMR analysis method (AOAC PVM 1:2003) for the identification of the percentage of moisture and fat in meat products The procedure involves determining the moisture value

of meat samples by microwave drying and using the dried sample to determine the fat value by NMR analysis The samples, which represent a range of products that meat processors commonly analyze in daily plant operations, included the follow-ing: (i) fresh ground beef, high fat; (ii) deboned chicken with skins; (iii) fresh pork, low fat; (iv) all beef hot dogs; and (v) NIST standard reference material samples The results were compared with moisture and fat values derived from AOAC-approved methods, 950.46 Forced Air Oven Drying and 960.39 Soxhlet Ether Extraction (2) A method for dairy samples is currently being evaluated by the AOAC, and approval is expected by mid-2004

Equipment

The microwave drying/NMR method for this experiment uses a SMART Trac System comprised of a SMART SystemTM microwave moisture analyzer and a SMART Trac low-resolution time domain NMR (LR-NMR)

Microwave Moisture Analyzer The SMART System moisture analyzer features

0.2 mg H2O sensitivity, a moisture range of 0.01–99.99% in liquids, solids, and slurries, 0.01% resolution, and an automatic electronic balance (0.1-mg readabili-ty) The system has built-in power management and temperature feedback control for reproducible results Optimization of power flow into the unique octagonal cav-ity provides the fastest possible drying times and ensures that high-carbohydrate samples will not overheat The system can store 100 methods and up to 300 test results, as well as perform a statistical analysis of the process including an average

of results, SD, minimum/maximum range, and graphing in reference to target val-ues

NMR Processor and Magnet The SMART Trac NMR Radio Frequency Pulse

Generator uses a pulse power of 250 W nominal, variable pulse times in 100-ns increments, transmission and reception phases selectable at 0, 90, 180 and 270°C; and, nominal 90° pulse time of 4 µs (18-mm probe) The magnet is permanent and thermally stabilized, 0.47 T (20 MHz); homogeneity is better than 10 ppm Signal detection is accomplished by dual channel (quadrature) detection with programma-ble low-pass filtering and a programmaprogramma-ble data acquisition rate up to 4 MHz per pair of points Figures 8.8–8.10 demonstrate the general method of processing a sample

Meat Applications

Fat is an important component in the meat industry A less expensive ingredient than meat, fat adds flavor, but too much of it renders a product extremely difficult

to market in an ever increasingly health conscious society The SMART Trac enables manufacturers of meat and meat products to maximize yield and meet customer expectations, while simultaneously helping to control production costs

Fig 8.8 A sample is spread or absorbed onto a proton-free medium, typically a glass fiber sample pad, and then continu-ously weighed while it is dried in the microwave system at a controlled temper-ature It is then cooled to 45°C Total time for drying and cooling is 2–3 min.

Fig 8.10 Because the sample is already conditioned at a controlled temperature, the NMR analysis can be immediately performed, typically requiring <1 min.

Fig 8.9 The sample is then placed on a proton-free film (Trac film), rolled up, and pressed into a Teflon tube for the NMR analysis.

Preparation of Meat Samples According to an AOAC-Approved

Peer-Verified Method for the Rapid Determination of Fat and Moisture

To prevent water loss during preparation and subsequent handling, do not use small samples Keep ground material in glass or similar containers with air and watertight covers Prepare samples for analysis as follows:

1 Collect ~5 pounds of freshly processed product and store at ≤4°C until analyzed

2 Dice nonground meat product samples into ~2-inch (5.08 cm) cubes and pass rapidly 2 times through a Hobart grinder (model 4612) or equivalent equipped with a 3/16 inch (0.1875 inch or 0.4763 cm) plate

3 Homogenize ground meat products in a Robot Coupe bowl chopper (model R6

or equivalent) to a paste or paté consistency Place ground material in a chilled bowl chopper (4°C) and chop for 30 s; scrape down the inner walls and bot-tom of the bowl with a spatula (plastic or rubber spatula with ~2 × 4 inch straight-edge blade) and add the material gathered to the body of the test sam-ple Repeat the process for an additional 30 s

4 After homogenization, collect samples and store in plastic containers with screw cap lids If the samples are not to be analyzed immediately, store them

at ≤4°C until needed

Procedure for Fat Determination in a Meat Sample

1 On the SMART System Main Menu screen, select Load Method, then select the appropriate preprogrammed item to be analyzed, (e.g., ground beef) Note: Different types of sample matrices and fat will exhibit different responses on the NMR system To obtain accurate fat readings, two or more samples of the

specif-ic sample type must be analyzed by the AOAC method The samples should cover the entire fat range to be analyzed Preferably, one high-fat reference sam-ple and one low-fat reference samsam-ple should be analyzed The reference values are typed into the SMART Trac system and then replicate runs of each sample are performed to determine the appropriate NMR signal values for that specific sample type After completing the reference scans, the SMART Trac system will establish a linear relationship for fat determination for that type of sample

2 Press the Ready Key to initiate the analysis Place two glass fiber sample pads (square) in the SMART System Moisture/Solids Analyzer microwave chamber on the balance and press Tare on the keypad Tare weight will be automatically recorded

3 With a Teflon-coated spatula, transfer ~3–5 g of sample, from the center of the refrigerated sample container to the center of one of the tared sample pads Spread the meat sample evenly across the square pad (see Figs 8.11 and 8.12)

4 Cover the sample with the other tared square pad as if making a sandwich and place the pads on the balance in the SMART System Moisture Analyzer

5 Dry sample by pressing Start on the keypad A temperature feedback system allows rapid temperature measurement of the sample during drying to adjust the microwave power delivery Percent Moisture will be displayed on the screen (± 0.01%) after the sample has dried to a constant weight Note: Five short beeps will be heard when drying is complete

6 Remove pads and roll both in Trac Film (see Figs 8.13 and 8.14)

7 Compress the rolled sample in the plastic sleeve using the compression tool, insert sample into the NMR chamber for analysis [The sample is placed in the core of an 89-kg magnet and pulsed with radio frequency (RF) energy while in the static magnetic field The resulting signal is recorded and analyzed for the total proton activity of fat present in the sample Proprietary software analyzes the NMR data and provides the moisture and fat results.]

8 Press Ready to continue the fat analysis, then press Start to analyze for fat Percent Fat will be displayed on the screen (± 0.01%)

Fig 8.11 If the sample is

in a paste, semisolid, or crumb form or a raw or skeletal meat product such as fresh pork, ground beef, or chicken, place the sample on the end of a spatula and spread it across one end

of the pad Then spread the sample to a uniform thickness covering ~90%

of the pad surface area.

Fig 8.12 If the sample contains bound water such as an all-meat emul-sion, cooked all-meat sausage, sausage with extenders, semidry sausage, or ham, place the sample on the end of the spatula and apply the sample to the middle of the pad Then spread the sample around the pad in

a circle.

Fat and moisture determinations on several different meat products and NIST standards were performed in laboratories at Texas A&M (TAM) and CEM Corporation The results for beef, all beef hot dogs, and chicken are presented in Tables 8.1, 8.2, and 8.3, respectively

Fig 8.13 Place the two square pads and dried sample in the center of the Trac film Fold the left corner of the film and pads as illustrated Fold the right corner Pull the lower edge

of the film and sample pads toward the top and begin to roll them into a tube.

Fig 8.14 For samples that are rigid after being dried and more difficult to roll into a cylinder, prepare the pads as illustrated.

Dairy Applications

Fat and moisture/solids contents are important components in the dairy industry Cream and milk are bought and sold on the basis of the amount of fat they contain Because fat is an expensive element in dairy products, it is essential that companies are certain of the fat content of the cream obtained from their suppliers and that they tightly control the fat in their products for quality and consistency, cost man-agement, and regulatory requirements

Similarly, effective moisture/solids control maximizes yield Generally, dairies benefit from controlling solids content in all of their products because moisture, of course, is a less expensive ingredient than milk solids Thus, fat and moisture/

TABLE 8.1

AOAC Results from TAM (AOAC Method 950.46 and 960.39) for Fresh Ground, High-Fat Beef

Weight Moisture Weight Fat

SMART Trac results from CEM SMART Trac results from TAM

Weight moisture fat Weight moisture fat