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90 D. Lemos et al.rAquaculture 186 (2000 89–105
of postharvest shrimp may also be affected by increased digestive proteinase activity that seems to
be stimulated by low nutritional quality feeds. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Protein digestion; Digestive enzymes; Proteinases; Digestibility; In vitro; Shrimp

1. Introduction

Feed is a major concern for shrimp farmers, representing up to 60% of the total
variable production costs ŽAkiyama et al., 1992; Sarac et al., 1993 . It must fulfill
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several characteristics including organoleptic properties such as odor, texture, and flavor,
and physical properties such as particle size. In addition, the feed should contain all the
essential nutrients for the cultured organism, and be readily available at low cost, highly
digestible with the nutrients available for assimilation, and devoid of antinutritional
factors ŽSudaryono et al., 1995 . In both natural and prepared diets, protein is the most
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abundant ingredient for shrimp playing a critical role for growth and development
ŽSmith et al., 1992; Sudaryono et al., 1995 . Protein ingredients are the most expensive
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constituents in aqua feeds. Besides the price, availability of protein ingredients is also a
problem for feed producers. The search for alternative protein sources with high
nutritional quality at reasonable cost is a current concern among shrimp farmers
ŽAkiyama, 1991; Sarac et al., 1993; Sudaryono et al., 1995, 1999 . Locally available
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feedstuffs such as fish or plant seed meals are normally used for formulation of low cost
feeds. Fish meals naturally contain a well-balanced mixture of essential amino acids and
other nutrients which are readily digested. However, the raw material can be technically
abused causing a loss in its nutritional and functional properties ŽGarcia-Carreno, 1998 .
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and the world supply of fish meal, mainly derived from capture fisheries, is heavily
influenced by climatic events such as the El Nino effect ŽCushing, 1975 . On the other
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hand, plant derived ingredients may contain antinutritional factors that impair the
efficiency of digestion. The most common are enzyme inhibitors that decrease the ability
of digestive enzymes to hydrolyze dietary protein ŽOlli et al., 1994 . A reduced digestion
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limits the bioavailability of amino acids needed for protein synthesis and growth. Some
of these inhibitors are in fact proteins that occur in the protein source used to formulate
the diet. As an example, some plant seed meals contain proteinase inhibitors which
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affect the degree of protein hydrolysis by shrimp digestive enzymes ŽGarcia-Carreno,
1996 ., leading to poor growth.
Understanding protein digestion is a relevant issue for the many steps in the shrimp
production process. Penaeid life history is marked by changes in morphology and
behavior, with a shift from planktonic herbivory to omnivory in late protozoea and the
adoption of a benthic existence as postlarvae. These ontogenetic events are accompanied
by significant changes in metabolic rates and digestive enzyme activities ŽLaubier-
Bonichon et al., 1977; Lovett and Felder, 1990; Chu and Ovsianico-Koulikowsky, 1994;
Lemos et al., 1999 .. The characterization and quantification of proteolytic enzyme
activity at different stages of development may contribute in improving feeding condi-
tions during the entire production process. Moreover, the penaeid digestive gland is
known to contain high proteolytic activity ŽTsai et al., 1986; Van Wormhoudt et al.,