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A Brief Comment on Estimating Caloric
Yields from
Cultivated Agave in Southern Arizona
Jeff D. Leach
Paleobiotics Lab
jeff@paleobioticslab.com
Abstract
Archaeological research in the northern Tucson Basin over the last two
decades has confirmed that species of the genus Agave were
cultivated in extensive agricultural fields marked by the presence of rock
piles, terraces, and check dams. Researchers estimate that ~ 10,000 agaves
were harvested annually from a standing population of greater than 100,000
cultivated plants in the larger fields, potentially providing the annual
caloric requirements for as many as 155 persons. However, the annual
caloric return from harvested agave has been overestimated by ~55% when
you consider that inulin-type fructans are the major storage carbohydrate
in agave. As a nondigestible carbohydrate, inulin and its subgroup
oligofructose are not absorbed in the small intestine, but are fermented
in the large bowel and thus have a lower net energy value than traditional
carbohydrates such as starch.
Introduction
Over 300 species of agave have been reported throughout the American
Southwest and northern Mexico (Gentry 1982). Early travelers and
ethnographers in the region noted the importance of agave and other desert
succulents as a food source among indigenous populations (e.g., Castetter
et al. 1938), while ongoing archaeological research throughout the
southern US continues to expand our knowledge of the importance and
antiquity of agave exploitation (e.g., Dering 1999; Fish et al.
1982, 1985; Leach 2005; Leach and Lopez, in press). Perhaps one of the
more fascinating and important aspects of this research has been the
identification of agave as cultivated plant among the desert Hohokam of
southern Arizona (Fish et al. 1982, 1985). Along the mountains
slopes and bajadas of the lowlands north of Tucson, extensive fields of
rock piles and stone borders, along with charred remains of agave plant
remains in rock-lined roasting pits, confirm the importance of agave in a
mixed economy among these desert farmers.
This paper reports that the caloric yields from agave
harvested from these extensive fields have been overestimated as a result
of not considering the type of carbohydrate stored in agave plants. When
the appropriate conversion factors are applied to the carbohydrate portion
of the plant tissue, the overall calories derived from a given amount of
agave is reduced by more than half, thus reducing the contribution of
cultivated agave in the subsistence economy for inhabitants of this
semi-arid region.
The Non Digestible Carbohydrates Inulin and Oligofructose in Agave
The three principal reserve carbohydrates in plants are starch, fructan,
and sucrose, with fructan being present as the major storage carbohydrate
in at least 36,000 species of plants (Hendry
1987). Agaves utilize crassulacean acid metabolism (CAM) for CO2
fixation and fructans are the principal photosynthetic products generated
(Wang and Nobel 1998; Leach and Lopez, in press; Lopez et al.
2003).
From a chemical point of view, fructans can be
divided into inulin, levan, phlein, graminan, and kestoses based on their
respective fructosyl-fructose linkage structure. Of interest here is the
fructan inulin and its subgroup oligofructose. Inulin is a polydisperse
set of predominately linear molecules made up of
D-fructose residues linked to a terminal
glucose residue by β(2→1) osidic bond (Van Loo et al.
1995). The degree of polymerization (DP) of inulin varies between three
and sixty-five (Van Loo 2004). Oligofructose is
a subgroup of inulin, consisting of polymers with a DP ≤10. While the
fructan structure may vary within the genus Agave, inulin-type
fructans are the most predominant (Leach and Lopez in press; Lopez et
al. 2003).
The unique linkage between the fructose
molecules of inulin and oligofructose distinguish them from typical
carbohydrates in that they resist digestion by human alimentary enzymes
and absorption in the small intestine but are hydrolyzed and fermented by
colonic microflora (Roberfroid 1993). This fermentation produces gases (H2,
CO2, CH4) and short chain fatty acids, such as
butyrate, that are subsequently absorbed and utilized for energy. The
intestinal physiological effects demonstrated by the carbohydrates inulin
and oligofructose therefore meet the basic and essential definition of a
dietary fiber (Cherbut 2002; Roberfroid 1993, 2004), and thus a net energy
content lower than digested carbohydrates such as starch.
Energy Contribution of the Nondigestible
Carbohydrate Fraction of Agave
As a non digestible dietary fiber, inulin and
oligofructose are the major storage carbohydrate in the genus Agave
and should be treated differently than traditional carbohydrates when
calculating caloric yield. To estimate the calories from a given amount of
food, internationally accepted conversion factors for the protein, fat and
carbohydrate present in a given food item are utilized. In the US, the
Atwater factor (Atwater 1910) is typically utilized. Traditionally, the
energy value of foods is given in kilocalories (kcal), a unit of heat, as
measured by the amount of heat obtained by burning any food in a
calorimeter (Livesey et al. 2000). In a mixed food diet energy is
calculated by chemically analyzing the amount of protein, fat, and
carbohydrates present, then multiplying those amounts by appropriate
energy conversion factors – “the average amounts of energy provided to
the body by 1 g of typical food fat, protein or carbohydrate (Livesey
et al. 2000).
When calculating energy contribution by the Atwater factor, all fats
provide 9 kcal per gram, protein provides 4 kcal per gram, and all
carbohydrates provide 4 kcal per gram.
These
‘catch-all’ conversion factors for energy contribution of protein, fat and
carbohydrates are applied by archaeologosts, anthropologists, and
ethno-botanists when calculating the potential kcal yield of a given plant
resource to the overall diet and when ranking resources for the purposes
of optimal foraging models (e.g, Dering 1999). However, advances in food
science, as demonstrated by in vitro and in vivo studies (Livesey
et al. 2000), demonstrate that the declared caloric
contribution of a food item such as a carbohydrate, and the
scientifically determined energy value, may differ, depending on the
type of carbohydrate and its linkage structure.
Of interest here is the conversion factor of 4
kcal g for all carbohydrates. As already mentioned, the fructans inulin
and oligofructose present in the genus Agave are non digestible
dietary fiber, not subject to hydrolysis and absorption in the
small intestine. Research conducted in the realm of food science,
specifically on inulin and oligofructose derived from chicory (Cichorium
intybus), have demonstrated that the selective anaerobic hydrolysis
and fermentation of these substrates produces short-chain fatty acids
(acetate, propionate, butyrate) and lactic acids that are subsequently
absorbed by the cells to produce energy, thus salvaging a portion
of the originally ingested food ingredient (for an extensive review see
Roberfroid 1993). Therefore, their contribution to the metabolic energy of
the host is reduced and indirect (Roberfroid 1999: 1436S), and the
conversion factor of 1.5 kcal g, rather than the ‘catch-all’ of 4 kcal g,
should be utilized in determining caloric yield from the carbohydrate
portion of the genus Agave.
Discussion
Researchers working in the Tucson
Basin estimate that within a well-studied five square kilometer area along
a series of bajadas, project as many as 42,000 rockpiles and 120,000
meters of terraces and checkdams were utilized for the cultivation of
agave (Fish et al. 1985). Based on these data, the researchers estimate
that as many 102,000 agave plants could have been grown at one time,
producing an annual harvest of 10,200 plants. With an approximate weight
of 4 kilograms per agave heart, the annual yield would result in 40.8
metric tons of edible agave. Citing nutritional data by Ross (1944), the
researchers further suggest that agave provides 347 calories per 100
grams, potentially providing the annual caloric requirements of 155
persons.
From the nutritional data cited by
Fish et al (1985) it is not clear what conversion factors were used
to estimate the 347 calories per 100 g of edible agave. It is safe to
assume that given the 1944 date for the source of the nutritional data,
that a conversion factor of 4 kcal g for the carbohydrate portion was
utilized. Working east of the Tucson Basin, Dering (1999: Table 3)
provides recent nutritional analysis of pit baked samples of Agave
lechuguilla. The nutritional composition (adjusted for dry weight) of
100 g sample is as follows: 6.18 g protein, 3.5 g fat, and 65.6 g
carbohydrates. The calculated caloric value of cooked agave is thus the
following:
6.18 g protein x 4 kcal/g + 3.5 g fat x 9
kcal/g + 65.6 g carbohydrate x 1.5 kcal/g = 154.62 kcal/100 g
On the basis of the nutritional data from
Agave lechuguilla, adjusted for the conversion factor of 1.5 kcal / g
for non digestible carbohydrates, the caloric contribution of agave
harvested from the extensive fields in the Tucson Basin are reduced by ~
55% from 347 calories per 100 g to 154.62 calories per 100 g. The original
estimate that the annual caloric requirements of 155 persons were
potentially met by the harvested agave is accordingly reduced to ~ 70
persons.
Further, Fish et al (1985:112) suggest
that “agave hearts of small Southwestern species approximate four
kilograms,” but do not specify if this is harvested and uncooked plants,
or plants that have been harvested, cooked, and dried to reduce spoilage.
This variable is critical as the scale of the agricultural fields and the
size of the roasting pits recorded among the fields suggest that large
quantities of agave were processed. This bulk processing of agave in the
roasting pits was no doubt followed by pounding (kneading) of the cooked
agave into cakes or loaves that were subsequently air-dried for transport
and possible storage (Note the reduction of water via air-drying reduces
the chance of spoilage). This final stage of processing is consistent with
ethnographic observations of agave processing (Ferg 2003), and greatly
reduces the water content and weight of the processed plant tissue, thus
the potential annual caloric yield of the fields.
In his study of the small species Agave
lechuguilla, Dering (1999: Table 5) reports an average cooked and
dried weight of 0.086 kilograms per agave heart, which is well below the 4
kilograms per agave heart reported by Fish et al. (1995:112). In a
series of unpublished experimental studies with Agave murpheyi, a
candidate species that may have been cultivated in the agricultural fields
of the Tucson Basin (Fish et al. 1982, 1985), this author found
that cooked and air-dried agave hearts (n=7) weighed on average about 1.2
kilograms. While this is higher than reported for the smaller species
studied by Dering, it is considerably less than that reported by Fish
et al (1985). If we assume that the 4 kilogram per harvested agave
heart cited by Fish et al (1985:112) is for uncooked and undried
agave, and subsistitute the 1.2 kilograms per cooked and air-dried agave
arrived at through the experimental studies, then the annual yield of
agave from the large fields is reduced from 40.8 metric tons to 12.24
metric tons, accordingly.
The overall reduction in annual yield
calculated for cooked and subsequently dried agave estimated from
experimental study, coupled with the calories per 100 g sample adjusted
for the conversion factor for non digestible carbohydrates, suggest the
annual caloric requirements of less than two dozen persons would have been
met with an annual harvest of 10,200 plants.
Conclusion
The identification of agave as a
cultivated crop in southern Arizona is a significant contribution to our
understanding of human adaptation and paleonutrition in this semi-arid
region. The presence of inulin and oligofructose as the dominant
carbohydrate stored in the genus Agave allows for refinement in the
net energy contribution of this plant. The adjusted energy yield of agave
has resulted, along with the adjusted cooked and dried weight of agave
hearts, in an overall reduction in potential caloric contribution of agave
to the diet and economy of these desert farmers.
The following discussion raises the question of
why so much effort (labor) was invested in the cultivation of agave, when
one realizes the low caloric yields from such agricultural practices and
the additional labor required to harvest, cook, and dry the plant tissue
for transport and possible storage. Clearly, the inclusion of cultivated
agave in the diet by these desert farmers serves as a reminder as to the
difficulties of subsisting in a semi-arid region, where ever increasing
demographic pressure and unpredictable environmental realities required
cultivation of such marginal resources an unavoidable necessity.
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