Pomegranate physics for Passover

(Wood anatomy, heat transfer, and the Paschal lamb; July 2008, revised Aug. 2008 and Aug. 2009)

One of the highlights of the Exodus from Egypt and its commemoration in ancient times was the sacrifice and consumption of the Paschal lamb, described in Exodus 12:3-11. A lamb was to be roasted whole and consumed in its entirety by each household. There were to be no leftovers: If the lamb provided more meat than the family could consume at one sitting, then the party was expanded to include friends and neighbors. Although there are no animal sacrifices in Judaism today, the Passover seder meal is still a time for a large, convivial feast with one’s extended family and friends.

Exodus 12:8-9 describes the cooking of the lamb. It is to be thoroughly roasted directly over a fire; it may not be eaten braised, stewed, or raw in part or whole. In view of the great importance of this sacrifice, it is puzzling that the Torah warns us twice in these verses against preparing it in any way but fire roasting. The importance of this requirement is underscored by the fact that after verse 8 creates a positive commandment that the lamb be roasted, verse 9 follows with a negative commandment—not to eat the lamb after cooking it in any other way. Negative commandments are normally considered stronger than positive ones: A prohibition overrides a conflicting positive commandment, and the penalty for violation of a prohibition is generally stronger than the penalty for failing to carry out a prescribed positive action.

The Talmud (Pesachim 74a) discusses how to ensure that the Paschal lamb is cooked properly. These deliberations reveal some fascinating insights into plant anatomy and the physics of a barbecue. The discussants agree near the outset that the lamb should be roasted on a spit. However, the suitability of various materials for this spit is the subject of some debate. Metals are rejected on the grounds that “if part of a metal object is hot, then all of it is hot”. Heating of a metal spit would cause the part of the meat in contact with the spit to cook internally due to contact with the hot metal. The meat that was cooked by thermal conduction from the metal would then be wasted, because it had not been fire-roasted and therefore may not be eaten. The rabbis apparently recognized the great thermal conductivity of metals, and turned the discussion toward the relative merits of various species of wood. However, Rabbi Yehudah offers a minority opinion. Arguing from the observation that the part of a wooden spit that is inside the meat is not charred during cooking, he points out that the spit’s exposed ends are warmed directly by the heat radiating from the fire. Therefore, he reasons, the part of a metal spit that is within the meat is insulated from the fire, and will remain cool. It is certainly true that most of the heating of the meat and spit that occurs during fire-roasting is due to the thermal radiation (infrared radiation) and upward convection of hot smoke and gases from the fire. However, objects heated by exposure to a fire also conduct heat to their interiors, and as Rabbi Yehudah’s companions reminded him, “When part of this [metal] is hot, all of it becomes hot; when part of this [wood] becomes hot, not all of it becomes hot”. Wood is less conductive than metal.

What kind of wood should be used for the spit? Does it matter? The Mishna that is the basis of this discussion begins with the presumption that one should use pomegranate wood. After rejecting metal as a material, the Gemara (discussion and analysis of the Mishna) considers and rejects the wood of the date palm, because it contains water-filled channels. The concern is that when heated, the water contained in these channels would cook the internal meat, invalidating the sacrifice. Rashi, the great medieval French Jewish commentator on Bible and Talmud offers an explanation of this statement that suggests the Gemara is referring to the fine tubes known to botanists as xylem vessels. In a live tree or other plant, these vessels form a network that carries water from the roots throughout the entire plant. However, this is problematic: If the presence of xylem would disqualify a tree species for this use, then no wood would be acceptable, for all wood contains xylem. It is possible that the Gemara’s objection to palm wood is based on the fact that in palm trunks, small clusters of xylem vessels form visible fibers scattered throughout a matrix of softer, moister “ground tissue”. Unlike other trees, palms do not grow stouter by forming rings of hard wood. They simply produce more ground tissue and scattered xylem bundles. When a palm-wood spit is heated, moisture from the ground tissue can easily pass through the xylem into the surrounding meat. However, the scattered xylem bundles in a palm trunk are very thin, and so appear to the naked eye like solid fibers. A microscope is needed to see that they are hollow, making it unlikely that Rashi or the Gemara were aware of their structure. It is more likely that the Gemara is referring to the midrib and petiole, or leaf stalk, of the palm frond, which contains numerous xylem vessels that are large enough to see with the unaided eye. This spear-like structure is also ideally shaped for use as a spit, and when dry, it is suitably hard and woody.

The wood of a fig tree or a young sapling of any species is also found to be unacceptable, because their stems are not completely woody. Instead, their centers consist of soft, moist pith. Again, heating such wood would lead to invalid moist cooking of the internal meat. The carob tree and sycomore fig (Ficus sycomorus, no relation to the American sycamore) are considered next. These trees produce dense, highly branched canopies. Reflecting this fact, the Gemara expresses concern that a spit cut from either of these trees would contain knots where the branches had been cut off. The severed xylem tubes at the knots could exude sap into the meat and like the fig, cause the meat to be cooked in liquid. This objection is not raised against the use of palm wood, probably because most species of palms are usually unbranched.

Readers who have seen pomegranate trees will be able to anticipate the Gemara’s next question at this point: Pomegranate trees also have many branches. Wouldn’t their wood be as likely as that of a carob or a sycomore to contain knots? The Gemara first suggests sealing any knots in a pomegranate branch with clay or plaster. Perhaps recognizing that this begs the question (Why not do so with carob or sycomore wood?), it then recommends the use of wood from a young (one year-old) pomegranate tree. Presumably, such a young tree would have developed enough woody tissue to obliterate the pithy center of the juvenile trunk, but would not yet have produced branches of such a size that the knots where they had been cut off would pose a risk of sap leakage. Rather, the only exposed cut ends of xylem vessels would be at the ends of the spit, outside of the meat.

The laws of the Paschal lamb are discussed by the Rambam (Maimonides) in his magnum opus, the Mishna Torah, an encyclopedic code of Jewish law. In the section devoted to sacrifices, he summarizes the results of the Talmudic discussion described above (Hilchot Korban Pesach 10:8). There, he departs from the usual catalog of prescriptions (“one must…) and prohibitions (“One must not…”) to note simply that “they used a spit of pomegranate wood”. Some commentators have suggested that the shift to plain descriptive language here implies that while the Rambam considers pomegranate wood preferable (“min hamuvchar”), he does not regard it as absolutely necessary, and any hard, dry, knot-free wood may be used.

Julia F. Morton has written about the pomegranate, including the uses of its wood, in her book “Fruits of Warm Climates”. Her chapter on pomegranates (Morton, J. 1987. Pomegranate. p. 352–355 in Fruits of Warm Climates. Julia F. Morton, Miami, FL) has been reproduced on-line at:
http://www.hort.purdue.edu/newcrop/morton/pomegranate.html

There, readers will find a description of the pomegranate as a source of small-diameter lumber, perhaps suitable as a spit for a lamb roast:

The pale-yellow wood is very hard and, while available only in small dimensions, is used for walking-sticks and in woodcrafts.

It is also interesting to note some of Morton’s other comments on the pomegranate. The bark contains a number of bitter medicinal compounds. One of these is isopelletierine, which is used to kill tapeworms. The bark has also been found to be toxic to parasitic flatworms. In view of the biblical requirement to roast the entire lamb, including its intestines, these properties of pomegranate bark may have been important to public health in Temple times. In fact, the Talmud’s discussion of the roasting of the Paschal lamb describes the placement of the intestines outside of the carcass on the pomegranate wood spit, where any worm-infested intestines would roast in contact with the medicinal bark. Presumably, the worm-killing compounds in the bark helped to ensure that the roasted intestines were free of parasites.

Surprisingly, neither the Gemara’s nor the Rambam’s discussion of the pomegranate spit considers the problem of the biblical prohibition of destroying fruit trees for the sake of their wood. (Deuteronomy 20:19-29). The Talmud does accept the possibility of a cost-benefit analysis in the application of this commandment. For example, it permits the uprooting of a vineyard or orchard when replacing it with another crop would be more profitable (Baba Kama 91b). Whether this principle was understood implicitly to exempt the cutting of pomegranate saplings for the purpose of the Paschal sacrifice, or whether the issue was simply allowed to rest after the destruction of the second Temple and cessation of animal sacrifices is unclear to me. Certainly a great deal of wood must have been cut each year to enable each family to enjoy the Paschal feast. I would welcome hearing from any reader who has an answer to this question.

Why does the Torah give such importance to the roasting of the Paschal lamb? The medieval Spanish Jewish commentator Avraham ibn Ezra suggests an explanation. In his commentary on Exodus 12:8-9, ibn Ezra reports (on the authority of “the sages of Spain”) that the most elegant preparation of meat, favored by epicurean royalty, is a sort of gentle poaching or steaming in a double boiler or bain-marie. Perhaps, just as we eat matzah, “the bread of poverty,” thumbing our noses at Egyptian leavened bread, and our ancestors ate the Paschal lamb, turning their backs on Egyptian polytheism, ibn Ezra is suggesting that the roasting of this lamb is meant to remind us that the simple barbecue we ate as desert nomads is preferable to the delicacies of the aristocracy, consumed at the expense of an enslaved people.

NOTE: This essay is an excerpt from the popular Torah Flora live program “Three Mitzvot of Pesach.” For information about live Torah Flora programs, see Events or e-mail jon@torahflora.org.