Premature browning in cooked ground beef after modifying myoglobin

Some ground beef patties developed an internal, brown cooked color and looked well-done at temperatures as low as 131 ÌŠF, whereas normal patties were re d to pink. The premature brown color was not relate d to percent fat; patty compaction; animal source and maturity; pH (5.5 to 5.8); or concentrations of raw patty heme and nonhemeiron, myoglobin, and total pigment. Because oxidationreduction potential and total reducing activities were higher (P<.05) and TBA numbers were lower (P<.05) in normal than prematurely brown patties, the brown color is apparently related to greater patty oxidation.


Introduction
Interna l color of cooked meat typically chang es from red to pink to tan as endpoint temperature s increase, and these colors are often used t o assess doneness.This assessment is not reliable for ground beef that develops a well-done appearance at final internal temperature s lower than expected.
This prematur e brown color could result in consumptio n of undercooked ground beef, causing serious concerns for food safety.
In an earlier study we found that heme and nonhem e iron, total pigment, percentage fat, patty compaction, animal source or maturity, and pH (5.5 to 5.8) were not different between patties that developed a normal or premature brown cooked color.The present study was done to determine if chemically oxidizing or reducin g treatments of normal patties or those that turn brown at low temperatures could influence cooked color when heated to only 131EF.

Experimental Procedures
Patties exhibiting normal and premature brow n cooked color were obtained from the quadricep s muscle of A-and E-maturity, British-bee f and dairy breeds and from frozen beef trimmings.Patty pH was 5.5 to 5.8, and fat was 3 to 18%.Quarter pound patties were formed , crust frozen (-4 0EF), vacuum packaged, and stored a t -4EF for 2 to 11 month.
Patties exhibiting normal and premature brown cooked color received one of three chemical modifications: 1) no chemical modificatio n (NO); 2) reduced (RD ; 10 mL of .05mM sodium hydrosulfite); or 3) oxidized (OX; 10 mL of .04 mM po t assium ferricyanide) for a total of six treatment combinations.
Prior to coo king, external and internal patty colors were assessed visuall y to the nearest halfpoint using a 5-point descriptiv e scale (1=purple red, 2=dark reddish purple, 3=bright red, 4=brownish red, 5=very brown).Patties were cooked to 13 1EF on an ele ctric griddle (32 5EF).Internal temperature was mo nitored by intermittently inserting a needle thermo-probe into the patty.Patties were cooled for 5 minutes and sliced for internal color e valuation to the nearest half-point using a 5-point descriptive scale (1=very dark red to purple, 2=bright red, 3=very pink, 4=slightly pink, 5 = tan, no evidence of pink).A Hunter Labscan 600 0 was used to instrume ntally evaluate color.Saturation index was calculated.Expres sible juice was squeezed from patties and its color was evaluated to the nearest half-point using a five-point scale (1=dark, dull red; 2=red; 3=p i nk; 4=pinkish tan; 5=yellow, no pink).
Data were analyzed as a completely randomized design where t reatments were a 2 × 3 factorial with cooked color group and chemica l modification as variables.SAS Genera l Linear Models procedures and least square me ans separation techniques were used.

Results and Discussion
Because patties from the normal group had higher total reducing activity than those in the prematurel y brown group, we examined the effects of modifying the myoglobin state before cooking.Patties with NO and R D modifications did not differ in total reducing activity (P>.05) and both were considerably higher (P<.05) in reducin g activity than patties with OX modificatio n (Table 1).
The potassium ferricyanide in OX patties may h ave confounded determination of total re ducing activity, because it is the compound monitored for reducing activity.Thus, t he extremely low total reducing abilities of OX patties were unexpected.
Raw patty external and i nternal appearances were altered by chemical modification (Table 1).
Visually, patties from normal-RD, prematurely brown-RD, a nd normal-NO groups were the most (P<.05) purplish red.Patties from normal-OX, prematurely brown-OX, and prematurel y brown-NO groups were the most oxidize d and brown in appearance.OX modificatio n resulted in patties with a brown externa l and internal appearance, typical of a metmyoglobin.RD resulted in a purple-red external surface and purple-red interna l surface typical of deoxymyoglobin.Instrumental color (not all data shown in Table 1) followed a pattern similar to visual evaluations , with normal patties having higher (P<.05 ) a* values (more red) than prematurely brown patties.RD patties had the highest (P<.05) a* values and saturation indices, NO was intermediate, and those with OX had instrumental values indicative of being brown.
Patties with normal-RD treatment had the most (P<.05) red internal visual cooked color (Table 1).NO-normal and RD-prematurely brown patties were intermediate and were scored very pink.Prematurel y brown-NO and -OX modif ications yielded patties that were the least red.Instrumentally (not all data shown), patties from normal-RD and prematurely brown-RD groups had the highest (P<.05) a* values and saturation indices (Table 1).Althoug h differences occurred in the internal patty app earance between cooked color groups and from the chemical modification, no differences in color of expressible juice were found.The juice from patties f rom all treatment groups was very red in color.The reason for the disparity in patt y color (red vs. brown) and juice color is unknown, but we had found previousl y that expressible juice color and internal patty color were not highly related.Endpoint temperature was more related to expressible juice color than to internal patty color, especially at low endpoint temperatures.
The relationship be t ween pigment oxidative state and internal cooked color indicates the need for rapid and conscientious handling of raw materials to ensure sufficient reductive capacity in meat that w i ll lead to normal cooked color.B ecause oxidiation of ground beef leads to the development of prematurely brown internal color, factors inf l uencing metmyoglobin formatio n also may influence premature browning .
Higher storage temperatures promot e greater oxygen uptake and more rapid reduction of reducing enzyme activity.Inc reasing length of time postmor-tem and mechanic al manipulation, such as grinding, drastically decrease a muscle's reducing ability.Rapid pH decline during chilling while carcass temperature is still high might promote premature browning by decreasing myoglobin stability.

Table 1 . Characteristics of Ground Beef with Normal and Prematurel y Brown Cooked Internal
4Means within a trait without a common superscript letter differ (P<.05).a,b,c