  We analyzed the spectroscopic and photometric database of the 5th data
release of the Sloan Digital Sky Survey (SDSS) to search for evidence of the
quasar reddening produced by dust embedded in intervening damped Ly alpha (DLA)
systems. From a list of 5164 quasars in the interval of emission redshift 2.25
</= z_e </= 3.5 and SDSS spectra with signal-to-noise ratio SNR >/= 4, we built
up an "absorption sample" of 248 QSOs with a single DLA system in the interval
of absorption redshift 2.2 < z_a </= 3.5 and a "pool" of 1959 control QSOs
without DLA systems or strong metal systems. For each QSO of the absorption
sample we extracted from the pool a subset of control QSOs that are closest in
redshift and magnitude. The mean color of this subset was used as a zero point
to measure the "deviation from the mean color" of individual DLA-QSOs, Delta_i.
The colors were measured using "BEST" ugriz SDSS imaging data. The mean color
excess of the absorption sample, <E>, was estimated by averaging the individual
color deviations Delta_i. We find <E(r-z)> = 27 +/- 9 x 10**(-3) mag and
<E(g-z)> = 54 +/- 12 x 10**(-3) mag. These values are representative of the
reddening of DLA systems at z_a ~ 2.7 in SDSS QSOs with limiting magnitude r
=/~ 20.2. The detection of the mean reddening is confirmed by several
statistical tests. Analysis of the results suggests an origin of the reddening
in dust embedded in the DLA systems, with an SMC-type extinction curve. By
converting the reddening into rest-frame extinction, we derive a mean
dust-to-gas ratio <A_V/N(HI)> ~ 2 to 4 x 10**(-23) mag cm^2. This value is ~
-1.25 dex lower than the mean dust-to-gas ratio of the Milky Way, in line with
the lower level of metallicity in the present DLA sample.
