A bacteriological study of ham souring

by Charles Neil McBryde

Fig. 4.—Diagrammatic views showing construction of ham thermometer. A, front view, showing open space between metal point and mercury bulb, which becomes filled with particles of meat, grease, and dirt; B, side view.

The packing-house method of taking the temperatures of hams by means of a pointed, metal-capped thermometer which is thrust deep into the bodies of the hams has already been referred to, but deserves to be described somewhat more in detail, as it will be at once apparent that this manipulation furnishes a ready means whereby hams may become infected with putrefactive bacteria. The construction of a ham thermometer is shown in figure 4.

The instrument consists of a glass thermometer inclosed in a metal case, the front portions of the case being cut away so as to expose the scale above and the mercury bulb below. As was explained before, the thermometer is thrust deep into the body of the ham so that the pointed end containing the mercury bulb rests beside or a little behind the upper portion of the femur, the bone being used as a guide in introducing the thermometer.

Ham temperatures are taken at three stages in the preparation for cure—(1) on the hanging floor, just before the hams go to the chill rooms, in order to determine the amount of heat lost prior to chilling; (2) on leaving the chill rooms, in order to determine the thoroughness of the chill;(3) on the packing floor, just before the hams are placed in pickle, as a further check on the thoroughness of the chilling.

In taking the temperatures of hams which have been chilled—and most of the temperatures are taken subsequent to chilling—it is customary for the packing-house attendant who has this matter in charge to warm the thermometer by holding the pointed or bulb end in his hand, so as to force the mercury column up to about 60° F., or well above the temperature of hams. The thermometer is then thrust into the ham and allowed to remain for several minutes, by which time the mercury column will have fallen to the temperature of the ham. The thermometer is then slowly withdrawn so as to expose the top of the mercury column, and an accurate reading is thus obtained of the inside temperature of the ham. The thermometer is warmed by the hand before each ham is tested, and this undoubtedly insures more accurate readings than would result were the thermometer removed from one ham and plunged immediately into another, but the procedure is open to certain objections, for the open space between the metal point of the thermometer and the mercury bulb soon becomes filled with particles of meat and with grease and dirt from the attendant’s hands, and it is at once apparent that a thermometer in this condition would furnish a ready means whereby extraneous matter might be introduced into the bodies of the hams. In other words, a contaminated thermometer would furnish an excellent means whereby hams could be inoculated with putrefactive bacteria.

In order to determine whether hams actually become inoculated in this manner, the following experiment was carried out:

EXPERIMENT TO SHOW WHETHER HAMS BECOME INFECTED FROM HAM THERMOMETERS.

This experiment was designed to show (1) whether the usual packing-house method of taking ham temperatures was apt to induce souring in the hams thus tested, and (2) whether souring would result in hams which were tested with a thermometer purposely contaminated with the bacillus isolated from sour hams.

The experiment was carried out as follows: Thirty hog carcasses were selected as they entered the hanging floor from the killing floor. They had been cleaned, eviscerated, and split, and were of the same average weight and of sufficient size to yield hams weighing from 12 to 14 pounds. They were divided into three lots of 10 each and were allowed to remain on the hanging floor for two hours, after which they were given the usual 48-hour chilling.

Lot 1.—The hams in this lot were tested with an ordinary ham thermometer as they entered the hanging floor, as they left the hanging floor, and as they left the coolers. The thermometer used was borrowed from one of the plant attendants and was used in the condition in which it was received from him; that is, it was not cleaned or disinfected prior to use.