Influence of Microwave Heating on the Formation of - American

Chapter 11 Influence of Microwave Heating on the Formation of N-Nitrosamines in Bacon 1

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Shaun Chenghsiung Chen , Bruce R. Harte , J. Ian Gray , and Alden M. Booren 2

Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: September 13, 1991 | doi: 10.1021/bk-1991-0473.ch011

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School of Packaging and Department of Food Science and Human Nutrition, Michigan State University, East Lansing, MI 48824

Bacon was cooked in a teflon-coated frying pan and in transparent and susceptor packages in a 700 W microwave oven. Bacon slices were prepared to three "degrees of doneness": "undercooked", "overcooked" and "similar" to standard pan frying (3 minutes on each side at 340°F) by controlling the time in the microwave oven. The. package interfacial temperature and bacon temperature were monitored using a Luxtron fluoroptic probe system. Final bacon temperatures of 167°C (330°F) and 143°C (290°F) were observed for samples cooked to the "same degree of doneness" as fried bacon in the susceptor (2.5 minutes) and transparent (3 minutes) packages, respectively. Compared to frying, smaller concentrations of N-nitrosopyrrolidine (NPYR) and N-nitrosodimethylamine (NDMA) were present in bacon cooked either in the transparent or susceptor packages. Greater concentrations of NPYR were found in bacon cooked on susceptors in comparison to cooking in a transparent package. Formation of NPYR in pan-fried bacon was correlated with the concentration of nitrite in the raw product.

Convenience in food preparation makes the microwave oven a necessary component in today's households. With microwave ovens present in approximately 80% of U.S. homes and retail sales of microwave foods at several billion dollars, the development of microwave foods has become an increasingly important research and development focus for many food companies (1,4). The time savings involved in "in-package" cooking is a significant convenience feature for many working people.

0097-6156/91/0473-0118S06.00/0 © 1991 American Chemical Society

Risch and Hotchkiss; Food and Packaging Interactions II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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Microwave Heating and the Formation of N-Nitrosamines

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Insufficient crisping and browning have been the most common negatives associated with microwave cooking (2). Packaging techniques which enhance heating to cause dehydration and browning on the surface of products during microwave cooking are currently being utilized. These energy-absorbing packaging materials, called susceptors, utilize controlled thicknesses of metal vacuum deposited onto paper, paperboard or film carriers. The metal droplets absorb microwave energy and generate localized heating (2-4). Temperatures observed on the surface of the susceptor have been reported in excess of 200°C (5-7). A t these temperatures (>200°C), several safety concerns have been raised including potential migration of package components into food products (6-8). It is also possible that the higher temperatures involved may enhance the formation of N-nitrosamines in microwave-cooked bacon. The formation of N-nitroso compounds in cooked bacon is affected by many factors (9). These include: 1. Cooking method - using a preheated frying pan was found to produce more N-nitrosamines than cooking in an initially cold frying pan (10,11). 2. Cooking time and temperature - increasing cooking time and/or temperature can increase N nitrosamine formation in cooked bacon (10,12). 3. Lean-to adipose tissue ratio - since the formation of N-nitrosopyrrolidine ( N P Y R ) appears to be primarily associated with the adipose tissue, a high adipose-to-lean ratio can enhance N-nitrosamine formation in cooked bacon (13). 4. Inhibitors ascorbic acid, ascorbyl palmitates and a-tocopherol act as "blocking agents" and compete with secondary amines for the nitrosating species, thus less nitrite is available for nitrosation (14,15). 5. Storage period - free proline can be formed by proteolysis during storage prior to cooking, and this can result in greater concentrations of N-nitrosamines after cooking (16). 6. Concentration of nitrite - since the formation of N-nitrosamines depends on the reaction of proline and nitrite, an increased concentration of nitrite induces greater formation of N - P Y R (17). 7. Thickness of slices - slice thickness can influence penetration of heat into bacon, which can affect the formation of N-nitrosamines in bacon (18). 8. Liquid smoke - significant reduction of N P Y R in fried bacon was found when liquid smoke was incorporated into the pork bellies in the curing brine. Use of susceptors in microwave "in-package" cooking may increase the temperature at the surface of the bacon sample while reducing cooking time, both of which may affect the formation of N-nitrosamines. The objectives of this study were to: 1. Measure the temperature of the product and package as a function of microwave cooking in susceptor and non-susceptor (transparent) packages. 2. Develop a correlation between product temperature and formation of N-nitrosamines in microwave cooking in comparison to frying. 3. Determine the effect of product composition on the formation of N nitrosamines as a result of conventional and microwave cooking.


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Methods and Materials Preparation of Bacon Samples. Pork bellies were processed into bacon within 48 hours of slaughter in the Meat Laboratory, Michigan State University. They were injected using a multiple needle system to 110% of their green weight using a brine, to obtain a target concentration of 1.5% sodium chloride, 0.5% sucrose, 0.35% sodium tripolyphosphate, 550 mg/kg sodium ascorbate and 120 mg/kg sodium nitrite. Pumped bellies were placed in plastic bags and allowed to equilibrate overnight at 2°C. Bellies were transferred to an Elec-Trol laboratory smokehouse (Drying System Inc., Chicago, IL) showered with cold water for one min and smoked at 58°C (dry bulb temperature) for 4 hours, followed by three hours at 52°C (dry bulb temperature). The bacon was transferred to a cooler at 2°C overnight prior to slicing and packaging (20). To study the effect of nitrite concentration on the formation of N nitrosamines, pork bellies were injected to 110% of their green weight to obtain a target concentration of 1.5% sodium chloride, 0.5% sucrose, 0.35% sodium tripolyphosphate, 550 mg/kg sodium ascorbate and 200 mg/kg nitrite and processed into bacon as described previously. After smoking, the pork bellies were cut into 032 cm (1/8 in) thick slices. Bacon slices were placed on polystyrene trays, deposited into polyethylene pouches, and sealed under vacuum. The packaged samples were stored for seven days at 2°C prior to cooking. Cooked bacon samples were stored in polyethylene pouches (18 oz. Whirl-Pak, N A S C O , Ft. Atkinson, WI), and placed in a freezer prior to analysis. Two commercial slab bacons, purchased from a local retail store, were sliced (0.32 cm thick) and vacuum packaged in polyethylene pouches immediately after purchasing. Samples were stored at 2°C for seven days prior to cooking. Determination of Residual Nitrite. Residual sodium nitrite ( N a N 0 ) content was determined in a 5 g sample of raw bacon using the A O A C procedure (21). Quantitation was based on a standard curve plotting absorbancy at 540 nm versus concentration. The standard curve was made using 1, 2, 5, 10, 20, 30 and 40 ml of a 1 ppm (fig/ml) N a N 0 solution. The amount of residual nitrite was determined by comparing absorbancy at 540 nm, and calculating relative concentrations from the standard curve. 2

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Thermal Processing of the Bacon Samples. Bacon samples were heated conventionally in a frying pan and in two different microwave packages, to determine the effect of cooking method on the formation of N nitrosamines. Frying was accomplished using a preheated teflon-coated electric pan set at 171°C (340°F) for 3 minutes on each side (6 minutes total). Three slices of bacon (80 - 100 g) were fried simultaneously. After frying, the fried bacon samples were placed on paper towels to permit removal of excess cooked-out fat, and then stored at -20°C until analyzed.



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Microwave "in-package" cooking was achieved using a susceptor (aluminum metallized polyester, adhesive laminated to paperboard) and a microwave transparent tray (polyester coated paperboard) in a 700 watt microwave (model RS458P, Amana Refrigeration Inc., Amana, IA). The output of the microwave oven was determined prior to use, and found to be 680 watts. The bacon samples were cooked in the microwave oven at full power. Three slices of bacon (80 - 100 g) were used in both microwave cooking methods. Cook-out-fat was drained off on paper towels. Heating times were varied to obtain a specific "degree of doneness". Cooking times of 2, 3 and 4 minutes were used for transparent in-package cooking, and 2, 2.S and 3 minutes for the susceptor cooked bacon. Bacon prepared in the transparent package for 3 minutes or in susceptor packages for 2.5 minutes was observed by the authors to have the same degree of doneness as fried. Determination of Temperatures During Heating. A thermocouple (Omega Engineering Inc., Gardiner, N Y ) was used to measure the temperature of bacon during frying. Four probes were placed at the interface between the bacon sample and the frying pan. Temperature readings were recorded every 30 seconds. For bacon cooked in the microwave oven, package material and bacon surface temperatures were measured using a Luxtron 755 Multichannel Fluoroptic Thermometer (Luxtron, Mountain View, C A ) . Two M I W probes were placed onto the surface of the empty microwavable packaging material and/or interface between the samples and the packaging material. Temperatures were measured and recorded every 10 seconds using a data collection computer system. Determination of N-Nitrosamines in Cooked Bacon. The amount of N nitrosamine formed in cooked bacon was quantified using a dry column thermal energy analyzer method (21). The dry column method which was used to extract N-nitrosamines from cooked bacon utilized dichloromethane ( C H C l ) and N-pentane as solvents. A n aliquot (5 fiX) of the concentrated extract was injected into a G C - T E A (Varian Aerograph gas chromatograph, Model 3700, interfaced with a T E A , Model 502). A 2.7 m x 3.2 mm (ID) glass column (Supelco, Bellefonte, C A ) packed with 15% Carbowax 2 0 M - T P A was used for the separation of the N-nitroso compounds. Conditions were set initially at 140°C, then temperature programmed to 180°C at 7°C/minute. Peak areas for the N-nitrosamines detected were calculated using a Hewlett Packard Model 3390A Integrator (Walnut Creek, C A ) . The analytical efficiency was monitored by determining the recovery of the internal standard, N-nitrosoazetidine ( N A Z E T ) . Specific N nitrosamines were identified by comparing their retention times to those of standard N-nitrosamines. Mass spectrometric confirmation of the N nitrosamines was not attempted in this study and hence the N-nitrosamines 2

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can only be referred to as apparent N-nitrosamines. However, previous studies in our laboratories have utilized mass spectrometric techniques for the identification of these N-nitrosamines. The amount of apparent N nitrosamines in the bacon were determined using their relative response area. N-nitrosamine concentrations in the cooked bacon were calculated by:

N-nitrosamine Mkg)

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Influence of Microwave Heating on the Formation of - American

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